Differential regulation of rat aquaporin-5 promoter/enhancer activities in lung and salivary epithelial cells

A Cardioplegic Solution with an Understanding of a Cardiochannelopathy

Cardiac surgeries have been improved by accompanying developing cardioplegia solutions. However, the cardioplegia application presents an ongoing challenge with a view of a sufficiently restored cardiac function. In this review, we focus on the cardioplegia-induced mechanism and summarize the findings of studies undertaken to improve cardioprotective strategies. Currently, and somewhat surprisingly, relatively little is known about cardiac electrolyte regulation through channel physiology. We hope that an improved understanding of the electrolyte transport through ion channels/transporters and modulations of water channel aquaporins will provide an insight into cardiac channel physiology and a channel-based cardiac pathology of a cardiochannelopathy.

In vitro and ex vivo models in inhalation biopharmaceutical research - advances, challenges and future perspectives

Oral inhalation results in pulmonary drug targeting and thereby reduces systemic side effects, making it the preferred means of drug delivery for the treatment of respiratory disorders such as asthma, chronic obstructive pulmonary disease or cystic fibrosis. In addition, the high alveolar surface area, relatively low enzymatic activity and rich blood supply of the distal airspaces offer a promising pathway to the systemic circulation. This is particularly advantageous when a rapid onset of pharmacological action is desired or when the drug is suffering from stability issues or poor biopharmaceutical performance following oral administration. Several cell and tissue-based in vitro and ex vivo models have been developed over the years, with the intention to realistically mimic pulmonary biological barriers. It is the aim of this review to critically discuss the available models regarding their advantages and limitations and to elaborate further which biopharmaceutical questions can and cannot be answered using the existing models.

Integrated Single-Cell RNA-Sequencing Analysis of Aquaporin 5-Expressing Mouse Lung Epithelial Cells Identifies GPRC5A as a Novel Validated Type I Cell Surface Marker

Molecular and functional characterization of alveolar epithelial type I (AT1) cells has been challenging due to difficulty in isolating sufficient numbers of viable cells. Here we performed single-cell RNA-sequencing (scRNA-seq) of tdTomato⁺ cells from lungs of AT1 cell-specific Aqp5-Cre-IRES-DsRed (ACID);R26tdTomato reporter mice. Following enzymatic digestion, CD31^-CD45^-E-cadherin⁺tdTomato⁺ cells were subjected to fluorescence-activated cell sorting (FACS) followed by scRNA-seq. Cell identity was confirmed by immunofluorescence using cell type-specific antibodies. After quality control, 92 cells were analyzed. Most cells expressed ‘conventional’ AT1 cell markers (Aqp5, Pdpn, Hopx, Ager), with heterogeneous expression within this population. The remaining cells expressed AT2, club, basal or ciliated cell markers. Integration with public datasets identified three robust AT1 cell- and lung-enriched genes, Ager, Rtkn2 and Gprc5a, that were conserved across species. GPRC5A co-localized with HOPX and was not expressed in AT2 or airway cells in mouse, rat and human lung. GPRC5A co-localized with AQP5 but not pro-SPC or CC10 in mouse lung epithelial cell cytospins. We enriched mouse AT1 cells to perform molecular phenotyping using scRNA-seq. Further characterization of putative AT1 cell-enriched genes revealed GPRC5A as a conserved AT1 cell surface marker that may be useful for AT1 cell isolation.

Cross-contamination of the human salivary gland HSG cell line with HeLa cells: A STR analysis study

OBJECTIVES

The human salivary gland (HSG) cell line, labeled as a submandibular ductal cell line, is commonly used as in vitro models to study radiation therapy, Sjögren's syndrome, pleomorphic adenoma, mucocele, epithelial-to-mesenchymal transition, and epigenetics. However, the American Type Culture Collection (ATCC) has recently released a list of cross-contaminated cell lines that included HSG. Despite this notice, some research laboratories still use HSG as a salivary cell model. Therefore, this study examined the authenticity of HSG sampled from three different laboratories.

METHODS

DNA was extracted from HSG and additional salivary cell lines (NS-SV-AC, NS-SV-DC, A253, HSY) and submitted for cell line authentication with short tandem repeat (STR) analysis.

RESULTS

All HSG samples had STR profiles indicating >80% match with HeLa in both the ATCC and Deutsche Sammlung von Mikroorganismen und Zellkulturen (DSMZ) databases. This confirmed that HSG sampled from three different laboratories and HSY shared a common ancestry (host) with HeLa, whereas NS-SV-AC, NS-SV-DC, and A253 had unique STR profiles.

CONCLUSION

Short tandem repeat analysis revealed that HSG was contaminated by the HeLa cell line. Furthermore, because genotyping of the original HSG cell line was not performed during its establishment, it will be difficult to authenticate an uncontaminated sample of HSG.

Effects of hypercapnia in acute respiratory distress syndrome

In patients with acute respiratory distress syndrome (ARDS) hypercapnia is a marker of poor prognosis, however there is controversial information regarding the effect of hypercapnia on outcomes. Recently two studies in a large population of mechanical ventilation patients showed higher mortality associated independently to hypercapnia. Key roles responsible for the poor clinical outcomes observed in critically ill patients exposed to hypercapnia are not well known, two possible mechanisms involved are the effect of CO2 on the muscle and the alveolar epithelium. Hypercapnia frequently coexists with muscle atrophy and dysfunction, moreover patients surviving ARDS present reduced muscle strength and decreased physical quality of life. One of the possible mechanisms responsible for these abnormalities could be the effects of hypercapnia during the course of ARDS. More over controversy persists about the hypercapnia role in the alveolar space, in the last years there is abundant experimental information on its deleterious effects on essential functions of the alveolar epithelium.

Chlorpheniramine attenuates histamine-mediated aquaporin 5 downregulation in human nasal epithelial cells via suppression of NF-κB activation

Background: Aquaporin 5 (AQP5) is most likely the primary water channel in the human nasal mucosa and acts as a key tight junction protein. The signaling cascades responsible for AQP5 regulation are still works in progress. Objective: This study sought to determine the effects of histamine and chlorpheniramine on AQP5 expression in human nasal epithelial cells (HNEpC) and to detect the signaling cascades responsible for these effects. Methods: HNEpC were cultured with four concentrations of histamine or chlorpheniramine in vitro. The sub-cellular distribution of AQP5 was explored using immunocytochemistry. The pharmacologic effects of histamine and chlorpheniramine on the expression of the phosphorylation of cyclic adenosine monophosphate-responsive element binding protein (p-CREB), the AQP5 and the NF-κB protein were examined using Western blotting. Results: AQP5 was found to be located in cell membrane and cytoplasm and present in every group without significant difference. Histamine inhibits the expression of AQP5 and p-CREB in HNEpC, while chlorpheniramine dose-dependently increases these protein levels with statistical significance. HNEpC treated with histamine and chlorpheniramine in turn showed the same trends as those intervened separately with these two drugs. Moreover, chlorpheniramine had the ability to reverse the inhibitory effect of histamine. Western blotting analysis revealed that after incubation with 10-4 M histamine, NF-κB protein was significantly heightened by 165% compared with the untreated control group. Again, such increase can be significantly reversed after chlorpheniramine treatment. Conclusions: The current study demonstrated that histamine inhibits CREB phosphorylation in HNEpC, which results in decreased AQP5 expression via activation of NF-κB pathway. Chlorpheniramine attenuates the inhibitory effect of histamine in p-CREB/AQP5 expression via suppression of NF-κB signal cascades. This observation could provide additional insight into the anti-inflammatory effects of H1-antihistamines that contribute to maintain airway surface liquid and mucosal defense.

An Optimised Human Cell Culture Model for Alveolar Epithelial Transport

Robust and reproducible in vitro models are required for investigating the pathways involved in fluid homeostasis in the human alveolar epithelium. We performed functional and phenotypic characterisation of ion transport in the human pulmonary epithelial cell lines NCI-H441 and A549 to determine their similarity to primary human alveolar type II cells. NCI-H441 cells exhibited high expression of junctional proteins ZO-1, and E-cadherin, seal-forming claudin-3, -4, -5 and Na⁺-K⁺-ATPase while A549 cells exhibited high expression of pore-forming claudin-2. Consistent with this phenotype NCI-H441, but not A549, cells formed a functional barrier with active ion transport characterised by higher electrical resistance (529 ± 178 Ω cm² vs 28 ± 4 Ω cm²), lower paracellular permeability ((176 ± 42) ×10⁻⁸ cm/s vs (738 ± 190) ×10⁻⁸ cm/s) and higher transepithelial potential difference (11.9 ± 4 mV vs 0 mV). Phenotypic and functional properties of NCI-H441 cells were tuned by varying cell seeding density and supplement concentrations. The cells formed a polarised monolayer typical of in vivo epithelium at seeding densities of 100,000 cells per 12-well insert while higher densities resulted in multiple cell layers. Dexamethasone and insulin-transferrin-selenium supplements were required for the development of high levels of electrical resistance, potential difference and expression of claudin-3 and Na⁺-K⁺-ATPase. Treatment of NCI-H441 cells with inhibitors and agonists of sodium and chloride channels indicated sodium absorption through ENaC under baseline and forskolin-stimulated conditions. Chloride transport was not sensitive to inhibitors of the cystic fibrosis transmembrane conductance regulator (CFTR) under either condition. Channels inhibited by 5-nitro-1-(3-phenylpropylamino) benzoic acid (NPPB) contributed to chloride secretion following forskolin stimulation, but not at baseline. These data precisely define experimental conditions for the application of NCI-H441 cells as a model for investigating ion and water transport in the human alveolar epithelium and also identify the pathways of sodium and chloride transport.

Aquaporin-5: from structure to function and dysfunction in cancer

Aquaporins, a highly conserved group of membrane proteins, are involved in the bidirectional transfer of water and small solutes across cell membranes taking part in many biological functions all over the human body. In view of the wide range of cancer malignancies in which aquaporin-5 (AQP5) has been detected, an increasing interest in its implication in carcinogenesis has emerged. Recent publications suggest that this isoform may enhance cancer cell proliferation, migration and survival in a variety of malignancies, with strong evidences pointing to AQP5 as a promising drug target and as a novel biomarker for cancer aggressiveness with high translational potential for therapeutics and diagnostics. This review addresses the structural and functional features of AQP5, detailing its tissue distribution and functions in human body, its expression pattern in a variety of tumors, and highlighting the underlying mechanisms involved in carcinogenesis. Finally, the actual progress of AQP5 research, implications in cancer biology and potential for cancer detection and prognosis are discussed.

p300/β-Catenin Interactions Regulate Adult Progenitor Cell Differentiation Downstream of WNT5a/Protein Kinase C (PKC)

Maintenance of stem/progenitor cell-progeny relationships is required for tissue homeostasis during normal turnover and repair. Wnt signaling is implicated in both maintenance and differentiation of adult stem/progenitor cells, yet how this pathway serves these dichotomous roles remains enigmatic. We previously proposed a model suggesting that specific interaction of β-catenin with either of the homologous Kat3 co-activators, p300 or CREB-binding protein, differentially regulates maintenance versus differentiation of embryonic stem cells. Limited knowledge of endogenous mechanisms driving differential β-catenin/co-activator interactions and their role in adult somatic stem/progenitor cell maintenance versus differentiation led us to explore this process in defined models of adult progenitor cell differentiation. We focused primarily on alveolar epithelial type II (AT2) cells, progenitors of distal lung epithelium, and identified a novel axis whereby WNT5a/protein kinase C (PKC) signaling regulates specific β-catenin/co-activator interactions to promote adult progenitor cell differentiation. p300/β-catenin but not CBP/β-catenin interaction increases as AT2 cells differentiate to a type I (AT1) cell-like phenotype. Additionally, p300 transcriptionally activates AT1 cell-specific gene Aqp-5. IQ-1, a specific inhibitor of p300/β-catenin interaction, prevents differentiation of not only primary AT2 cells, but also tracheal epithelial cells, and C2C12 myoblasts. p300 phosphorylation at Ser-89 enhances p300/β-catenin interaction, concurrent with alveolar epithelial cell differentiation. WNT5a, a traditionally non-canonical WNT ligand regulates Ser-89 phosphorylation and p300/β-catenin interactions in a PKC-dependent manner, likely involving PKCζ. These studies identify a novel intersection of canonical and non-canonical Wnt signaling in adult progenitor cell differentiation that has important implications for targeting β-catenin to modulate adult progenitor cell behavior in disease.

Alveolar epithelium: beyond the barrier

I am deeply honored to have been awarded an American Thoracic Society Recognition Award for Scientific Accomplishment for 2014. Over the last 20 years, it has become clear that the alveolar epithelium, my area of research focus, is not simply a gas exchange surface and barrier to leakage of fluid and protein into the alveoli, but is an active participant in the pathogenesis of a number of lung diseases, including pulmonary fibrosis. Recognition by this Award stimulates a review of the awardee's contributions to the field, as summarized in this perspective.

The expression and role of Aquaporin 5 in esophageal squamous cell carcinoma

BACKGROUND

Aquaporins (AQPs) are water channel proteins that facilitate transcellular water movements. Recent studies have shown that AQP5 is expressed in various cancers, and plays a role in tumor progression. However, its expression and role in esophageal squamous cell carcinoma (ESCC) have not been investigated. We examined the pathophysiologic role of AQP5 in cell proliferation and survival, and also investigated its expression and effects on the prognosis of ESCC patients.

METHODS

AQP5 expression in human ESCC cell lines was analyzed by Western blot testing. Knockdown experiments with AQP5 siRNA were conducted, and the effects on cell proliferation, cell cycle progression, and cell survival were analyzed. The cells' gene expression profiles were analyzed by microarray analysis. Immunohistochemistry of AQP5 for 68 primary tumor samples obtained from ESCC patients undergoing esophagectomy was performed.

RESULTS

AQP5 expression was high in TE2 and TE5 cells. In these cells, the knockdown of AQP5 using siRNA inhibited cell proliferation and G1-S phase progression, and induced apoptosis. The AQP5 siRNA transfected TE5 cells showed significant increase in p21 and decrease in CCND1 mRNA expression, respectively. The expression pattern of AQP5 and p21 protein was sharply contrasted, but AQP5 and CCND1 protein expression showed a similar pattern in ESCC tissue. These findings agree with the microarray results. Immunohistochemical staining of 68 ESCC patients showed the AQP5 expression is associated with tumor size, histological type, and tumor recurrence.

CONCLUSION

The AQP5 expression in ESCC cells may affect cell proliferation and survival, and impact on the prognosis of ESCC patients.

Primary oocyte transcriptional activation of aqp1ab by the nuclear progestin receptor determines the pelagic egg phenotype of marine teleosts

In marine teleosts, the aqp1ab water channel plays a vital role in the development of the pelagic egg phenotype. However, the developmental control of aqp1ab activation during oogenesis remains to be established. Here, we report the isolation of the 5'-flanking region of the teleost gilthead seabream aqp1ab gene, in which we identify conserved cis-regulatory elements for the binding of the nuclear progestin receptor (Pgr) and members of the Sox family of transcription factors. Subcellular localization studies indicated that the Pgr, as well as sox3 and -8b transcripts, are co-expressed in seabream oogonia, whereas in meiosis-arrested primary growth (pre-vitellogenic) oocytes, when aqp1ab mRNA and protein are first synthesized, the Pgr appears to be completely translocated from the ooplasm into the nucleus. By contrast, sox9b is highly expressed in more advanced oocytes, coinciding with a strong depletion of aqp1ab transcripts in the oocyte. Functional characterization of wild-type and mutated aqp1ab promoter constructs, using mammalian cells and Xenopus laevis oocytes, demonstrated that aqp1ab transcription is initiated by the Pgr, which is activated by the progestin 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-P), the natural ligand of the seabream Pgr. In vitro incubation of seabream primary ovarian explants with the follicle-stimulating hormone or 17,20β-P confirmed that progestin-activated Pgr enhanced Aqp1ab synthesis via the aqp1ab promoter. However, transactivation assays in heterologous systems showed that Sox transcription factors can potentially modulate this mechanism. These data uncover the existence of an endocrine pathway involved in the early activation of a water channel necessary for egg formation in marine teleosts.

Aqp5 is a new transcriptional target of Dot1a and a regulator of Aqp2

Dot1l encodes histone H3 K79 methyltransferase Dot1a. Mice with Dot1l deficiency in renal Aqp2-expressing cells (Dot1l(AC)) develop polyuria by unknown mechanisms. Here, we report that Aqp5 links Dot1l deletion to polyuria through Aqp2. cDNA array analysis revealed and real-time RT-qPCR validated Aqp5 as the most upregulated gene in Dot1l(AC) vs. control mice. Aqp5 protein is barely detectable in controls, but robustly expressed in the Dot1l(AC) kidneys, where it colocalizes with Aqp2. The upregulation of Aqp5 is coupled with reduced association of Dot1a and H3 dimethyl K79 with specific subregions in Aqp5 5' flanking region in Dot1l(AC) vs. control mice. In vitro studies in IMCD3, MLE-15 and 293Tcells using multiple approaches including real-time RT-qPCR, luciferase reporter assay, cell surface biotinylation assay, colocalization, and co-immunoprecipitation uncovered that Dot1a represses Aqp5. Human AQP5 interacts with AQP2 and impairs its cell surface localization. The AQP5/AQP2 complex partially resides in the ER/Golgi. Consistently, AQP5 is expressed in none of 15 normal controls, but in all of 17 kidney biopsies from patients with diabetic nephropathy. In the patients with diabetic nephropathy, AQP5 colocalizes with AQP2 in the perinuclear region and AQP5 expression is associated with impaired cellular H3 dimethyl K79. Taken together, these data for the first time identify Aqp5 as a Dot1a potential transcriptional target, and an Aqp2 binding partner and regulator, and suggest that the upregulated Aqp5 may contribute to polyuria, possibly by impairing Aqp2 membrane localization, in Dot1l(AC) mice and in patients with diabetic nephropathy.

Abnormal subcellular localization of AQP5 and downregulated AQP5 protein in parotid glands of streptozotocin-induced diabetic rats

BACKGROUND

The mechanisms underlying diabetic xerostomia have not been clarified in relation with aquaporin-5 (AQP5) subcellular localization in salivary glands.

METHODS

Western blotting, real-time PCR, and immunocytochemistry were used to analyse AQP5 protein levels and mRNA expression. AQP5 protein levels were measured in the apical plasma membrane (APM) and detergent-insoluble fraction prepared from streptozotocin-diabetic rat parotid glands.

RESULTS

Despite an increase in AQP5 mRNA, AQP5 protein levels were decreased in diabetic parotid glands compared with controls. Immunohistochemical studies indicated that AQP5, under unstimulated conditions, colocalised with flotillin-2 and GM1 with a diffuse pattern in the apical cytoplasm of acinar and duct cells in both control and diabetic rats. Ten minutes after intravenous injection of muscarinic agonist cevimeline, AQP5 was dramatically increased together with flotillin-2 and GM1 in the APM of parotid acinar and duct cells of control but not diabetic rats. Sixty minutes after injection, AQP5 was located in a diffuse pattern in the apical cytoplasm in both rats. Treatment of the parotid tissues with cevimeline for 10min increased the Triton X-100 solubility of AQP5 in control but not diabetic rats. Administration of insulin to diabetic rats tended to restore the cevimeline-induced translocation of AQP5.

CONCLUSION

Lack of AQP5 translocation in the salivary gland in response to a muscarinic agonist and downregulation of AQP5 protein might lead to diabetic xerostomia.

GENERAL SIGNIFICANCE

Cevimeline is useful to cure diabetic xerostomia under insulin administration.

Directed expression of Cre in alveolar epithelial type 1 cells

Pulmonary alveolar epithelium is comprised of two morphologically and functionally distinct cell types, alveolar epithelial type (AT) I and AT2 cells. Genetically modified mice with cell-specific Cre/loxP-mediated knockouts of relevant genes in each respective cell type would be useful to help elucidate the relative contributions of AT1 versus AT2 cells to alveolar homeostasis. Cre has previously been efficiently expressed in AT2 cells in mouse lung with the surfactant protein (SP)-C promoter; however, no transgenic mouse expressing Cre in AT1 cells has so far been available. To develop an AT1 cell-specific transgenic Cre mouse, we generated a knockin of a Cre-IRES-DsRed cassette into exon 1 of the endogenous aquaporin 5 (Aqp5) gene, a gene expressed specifically in AT1 cells in the distal lung epithelium, resulting in the mouse line, Aqp5-Cre-IRES-DsRed (ACID). Endogenous Aqp5 and transgenic Cre in ACID mice showed a very similar pattern of tissue distribution by RT-PCR. To analyze Cre activity, ACID was crossed to two Cre reporter strains, R26LacZ and mT/mG. Double-transgenic offspring demonstrated reporter gene expression in a very high fraction of AT1 cells in the distal lung, whereas AT2 cells were negative. As expected, variable reporter expression was detected in several other tissues where endogenous Aqp5 is expressed (e.g., submandibular salivary gland and stomach). ACID mice should be of major utility in analyzing the functional contribution of AT1 cells to alveolar epithelial properties in vivo with Cre/loxP-mediated gene deletion technology.

GATA-6 mediates transcriptional activation of aquaporin-5 through interactions with Sp1

We investigated mechanisms underlying GATA-6-mediated transcriptional activation of the alveolar epithelial type I cell-enriched gene aquaporin-5 (AQP5). GATA-6 expression increases in alveolar epithelial cells in primary culture, concurrent with upregulation of AQP5 and transition to a type I cell-like phenotype. Cotransfections in MLE-15 and NIH 3T3 cells demonstrated trans-activation by GATA-6 of a rat 1,716-bp-AQP5-luciferase (-1716-AQP5-Luc) reporter. Electrophoretic mobility shift assay and chromatin immunoprecipitation identified an interaction between GATA-6 and putative binding sites in the AQP5 promoter. However, mutation of these sites did not reduce GATA-6-mediated activation, implicating mechanisms in addition to direct binding of GATA-6 to DNA. A 5'-deletion construct, -358-AQP5-Luc, that does not encompass GATA motifs was still activated by GATA-6 by as much as 50% relative to -1716-AQP5-Luc. Internal deletion of the -358/-173 GC-rich domain, which includes several putative Sp1 consensus sites, reduced trans-activation by approximately 60%, suggesting importance of this region for GATA-mediated activity. -358-AQP5-Luc was similarly activated by both GATA-6 and a GATA DNA-binding defective mutant, whereas cotransfections in Schneider S2 cells demonstrated dose-dependent trans-activation of -358-AQP5-Luc by Sp1. Activation of -358-AQP5-Luc by GATA-6 was dramatically reduced by Sp1 small-interfering RNA, and -358-AQP5-Luc was activated synergistically by GATA-6 and Sp1 in NIH 3T3 cells. Furthermore, association between endogenous GATA-6 and Sp1 was demonstrated by coimmunoprecipitation. These results suggest that transcriptional activation of AQP5 by GATA-6 is mediated at least in part through cooperative interactions with Sp1 occurring at the proximal promoter.

Rat aquaporin-5 4.3-kb 5'-flanking region differentially regulates expression in salivary gland and lung in vivo

We previously cloned a 4.3-kb genomic fragment encompassing 5'-flanking regulatory elements of rat aquaporin-5 (Aqp5) that demonstrated preferential transcriptional activity in lung and salivary cells in vitro. To investigate the ability of Aqp5 regulatory elements to direct transgene expression in vivo, transgenic (TG) mice and rats were generated in which the 4.3-kb Aqp5 fragment directed the expression of enhanced green fluorescent protein (EGFP). RT-PCR revealed relative promoter specificity for the lung and salivary glands in TG mice. Immunofluorescence microscopy showed strong EGFP expression in salivary acinar cells but not in lung type I (AT1) cells, both known sites of endogenous AQP5 expression. Similar results were obtained in TG rats generated by lentiviral transgenesis. EGFP mRNA was detected in both salivary glands and lung. Robust EGFP fluorescence was observed in frozen sections of the rat salivary gland but not in the lung or other tested tissues. The percentage of EGFP-positive acinar cells was increased in parotid and submandibular glands of TG rats receiving a chronic injection of the beta-adrenergic receptor agonist isoproterenol. EGFP-positive cells in the lung that were also reactive with the AT1-cell specific monoclonal antibody VIIIB2 were identified by flow cytometry. These findings demonstrate that the 4.3-kb Aqp5 promoter/enhancer directs strong cell-specific transgene expression in salivary gland and low-level AT1 cell-specific expression in the lung. While these Aqp5 regulatory elements should be useful for functional studies in salivary glands, additional upstream or intronic cis-active elements are likely required for robust expression in the lung.

Evaluation of promoters for use in tissue-specific gene delivery

Vectors used in gene therapy require an expression cassette. The expression cassette consists of three important components: promoter, therapeutic gene and polyadenylation signal. The promoter is essential to control expression of the therapeutic gene. A tissue-specific promoter is a promoter that has activity in only certain cell types. Use of a tissue-specific promoter in the expression cassette can restrict unwanted transgene expression as well as facilitate persistent transgene expression. Therefore, choosing the correct promoter, especially a tissue-specific promoter, is a major step toward achieving successful therapeutic transgene expression. Ideally, the elements of the natural promoter region, necessary for obtaining the required level of the gene expression while retaining tissue-specificity, should be known. Also, it is important to understand whether interactions occur between the promoter region and the rest of the vector genome that could affect promoter activity and specificity. To assess this, it is helpful to select a suitable vector system that will be used in further gene therapy studies. Second, have one or several candidate tissue-specific promoters available for use. Third, ideally have an in vitro cell model suitable to evaluate tissue-specificity. Fourth, have a convenient in vivo animal model to use. Fifth, select a good reporter gene system. Next, using conventional recombinant DNA techniques create different promoter constructs with the selected vector system. Lastly, have a suitable transfection method to test the plasmid constructs in both the in vitro and the in vivo models.

SMAD3 prevents binding of NKX2.1 and FOXA1 to the SpB promoter through its MH1 and MH2 domains

Mechanisms of gene repression by transforming growth factor-beta (TGF-beta) are not well understood. TGF-beta represses transcription of pulmonary surfactant protein-B gene in lung epithelial cells. Repression is mediated by SMAD3 through interactions with NKX2.1 and FOXA1, two key transcription factors that are positive regulators of SpB transcription. In this study, we found that SMAD3 interacts through its MAD domains, MH1 and MH2 with NKX2.1 and FOXA1 proteins. The sites of interaction on NKX2.1 are located within the NH2 and COOH domains, known to be involved in transactivation function. In comparison, weaker interaction of FOXA1 winged helix, and the NH(2)-terminal domains was documented with SMAD3. Both in vitro studies and in vivo ChIP assays show that interaction of SMAD3 MH1 and MH2 domains with NKX2.1 and FOXA1 results in reduced binding of NKX2.1 and FOXA1 to their cognate DNA-binding sites, and diminished promoter occupancy within the SpB promoter. Thus, these studies reveal for the first time a mechanism of TGF-beta-induced SpB gene repression that involves interactions between specific SMAD3 domains and the corresponding functional sites on NKX2.1 and FOXA1 transcription factors.

Conserved elements within first intron of aquaporin-5 (Aqp5) function as transcriptional enhancers

A 4.3 kb rat aquaporin-5 (Aqp5) promoter that directs lung and salivary cell-specific expression in vitro directs low level expression of a GFP reporter in lungs of transgenic mice. Alignment of rat, mouse, and human AQP5 genomic sequences identified a highly conserved region in the 3' portion of intron 1, here termed ci1. To investigate the role of ci1 in Aqp5 expression, transient transfections were undertaken in AQP5-expressing mouse lung epithelial (MLE-15) and rat salivary (Pa-4) cells and AQP5-non-expressing NIH/3T3 cells. A 536 bp ci1 fragment enhanced transcriptional activity of the rat Aqp5 minimal promoter specifically in MLE-15 cells in an orientation-independent manner. Enhancer activity was Aqp5 promoter-specific, since no increase in activity was detected with the TK promoter. These results suggest that expression of transgenes in mouse lungs under direction of the 4.3 kb rat Aqp5 promoter may be augmented by inclusion of ci1 in transgenic constructs.

Transcriptional regulation by targeted expression of architectural transcription factor high mobility group A2 in salivary glands of transgenic mice

High mobility group A2 (HMGA2) protein is a non-histone architectural transcription factor. Numerous studies have demonstrated that HMGA2 is exclusively expressed in the nucleus of embryonic, but not of terminally differentiated, cells, and aberrant expression of HMGA2 is associated with various benign tumors, including pleomorphic salivary adenoma. Herein, we report the use of a 4.5-kb enhancer/promoter region of the aquaporin-5 (AQP-5) gene to target HMGA2 transgene expression in the mouse salivary acinar cells as a model to investigate the biochemical and biological role of ectopic HMGA2 expression. The expression pattern was analyzed by microarray analyses to profile HMGA2-dependent salivary gene regulation. By using quantitative reverse transcription-polymerase chain reaction (RT-PCR) assays, the expression of a cluster of genes involved in cytokine signaling, including Il7r, Il2rg, and Ptprc, was verified to be up-regulated in the salivary glands of AQP-5/HMGA2 mice. In concert, the expression of a cluster of genes, namely Ppara, Phyh, and Cidea, governing fatty acid and lipid metabolism, was confirmed to be down-regulated by HMGA2. Additionally, squamous carcinoma-like salivary tumors were observed in the AQP-5/HMGA2 transgenic mice, albeit at a low incidence. Our findings indicate that the AQP-5 promoter/enhancer-containing region is sufficient to target salivary-specific transgene expression and suggest novel roles for HMGA2 in salivary epithelial cells.

Physical and functional interactions between homeodomain NKX2.1 and winged helix/forkhead FOXA1 in lung epithelial cells

NKX2.1 is a homeodomain transcription factor that controls development of the brain, lung, and thyroid. In the lung, Nkx2.1 is expressed in a proximo-distal gradient and activates specific genes in phenotypically distinct epithelial cells located along this axis. The mechanisms by which NKX2.1 controls its target genes may involve interactions with other transcription factors. We examined whether NKX2.1 interacts with members of the winged-helix/forkhead family of FOXA transcription factors to regulate two spatially and cell type-specific genes, SpC and Ccsp. The results show that NKX2.1 interacts physically and functionally with FOXA1. The nature of the interaction is inhibitory and occurs through the NKX2.1 homeodomain in a DNA-independent manner. On SpC, which lacks a FOXA1 binding site, FOXA1 attenuates NKX2.1-dependent transcription. Inhibition of FOXA1 by small interfering RNA increased SpC mRNA, demonstrating the in vivo relevance of this finding. In contrast, FOXA1 and NKX2.1 additively activate transcription from Ccsp, which includes both NKX2.1 and FOXA1 binding sites. In electrophoretic mobility shift assays, the GST-FOXA1 fusion protein interferes with the formation of NKX2.1 transcriptional complexes by potentially masking the latter's homeodomain DNA binding function. These findings suggest a novel mode of selective gene regulation by proximo-distal gradient distribution of and functional interactions between forkhead and homeodomain transcription factors.

Downregulation of aquaporins 1 and 5 in nasal gland by osmotic stress in ducklings, Anas platyrhynchos: implications for the production of hypertonic fluid

Using primers against highly conserved regions of mammalian and bird aquaporins in RT-PCR experiments, we amplified products derived from duck (Anas platyrhynchos) nasal gland RNA that were identified as homologues of mammalian and chicken aquaporin 1 and aquaporin 5 cDNAs by sequencing. Using digoxigenin-labelled probes derived from these PCR products in northern blot analyses of mRNA isolated from nasal glands of untreated (naïve) or osmotically stressed ducklings (replacement of drinking water with a 1% NaCl solution), we observed a decrease in aquaporin 1 (AQP1) and aquaporin 5 (AQP5) mRNA abundance (by approximately 40%) during saline adaptation in the animals. Western blot analysis of AQP1 and AQP5 expression in the glands revealed that protein abundance decreased in a similar fashion. Immunohistochemical analysis of AQP1 distribution in cryosections of nasal gland indicated that AQP1 is mainly expressed in endothelial cells of the capillaries, but definitely not in the secretory or ductal cells of the gland. AQP5 distribution in the gland, however, seems to be different, since staining was exclusively observed in apical and basolateral plasma membranes of individual epithelial cells of the primary and central ducts, which collect fluid from the secretory tubules. The observations are consistent with the hypothesis that strongly hyperosmotic fluid is produced by the secretory cells at very low (unstimulated gland) or high (activated gland) rates. In the unstimulated gland, secretions may be diluted by aquaporin-mediated transcellular water flux while passing through the ductal system flushing the glandular ducts, thereby potentially preventing ascending infections. In the activated gland, however, downregulation of aquaporins in capillaries and duct cells may prevent dilution of the initially secreted fluid, enabling the animals to excrete large volumes of a highly concentrated salt solution.

Hypertonic induction of aquaporin-5: novel role of hypoxia-inducible factor-1alpha

Aquaporin-5 (AQP5) is a water channel protein expressed on the apical surface of alveolar epithelial type I cells in distal rat lung, suggesting a role for AQP5 in regulating alveolar surface liquid tonicity and/or cell volume. We investigated the molecular mechanisms underlying hypertonic induction of AQP5 in primary rat alveolar epithelial cells (AEC). Steady-state levels of AQP5 mRNA and protein were increased by exposure to sorbitol (200 mM in culture fluid) for 24 h. The increase in AQP5 was not accompanied by changes in mRNA half-life. Transduction of mouse lung epithelial (MLE-15) cells and primary rat AEC with lentivirus vectors encoding AQP5-luciferase demonstrated transcriptional activation of the reporter by exposure to hypertonic sorbitol solution. Hybridization of proteins from sorbitol-treated cells to a transcription factor DNA array demonstrated induction of hypoxia-inducible factor-1alpha (HIF-1alpha) by hypertonicity, which was confirmed by quantitative RT-PCR. Cotransfections of AQP5-luciferase with HIF-1alpha and HIF-1beta expression plasmids in MLE-15 cells led to dose-dependent transcriptional enhancement, which was partially abrogated by mutagenesis of putative HIF-1alpha binding sites in the proximal AQP5 promoter. Importantly, hypertonic induction of AQP5 was significantly inhibited by preventing HIF-1alpha induction with small interfering RNA. Hypertonicity induced activation of a transiently transfected vascular endothelial growth factor (VEGF) hypoxia response element-driven luciferase construct and increased expression of endogenous VEGF. These results demonstrate that hypertonic induction of both AQP5 and VEGF is transcriptionally regulated and mediated, at least in part, by HIF-1alpha, suggesting a novel role for HIF-1alpha in modulating cellular adaptive responses to osmotic stress.

Shedding of soluble ICAM-1 into the alveolar space in murine models of acute lung injury

Intercellular adhesion molecule-1 (ICAM-1; CD54) is an adhesion molecule constitutively expressed in abundance on the cell surface of type I alveolar epithelial cells (AEC) in the normal lung and is a critical participant in pulmonary innate immunity. At many sites, ICAM-1 is shed from the cell surface as a soluble molecule (sICAM-1). Limited information is available regarding the presence, source, or significance of sICAM-1 in the alveolar lining fluid of normal or injured lungs. We found sICAM-1 in the bronchoalveolar lavage (BAL) fluid of normal mice (386 +/- 50 ng/ml). Additionally, sICAM-1 was spontaneously released by murine AEC in primary culture as type II cells spread and assumed characteristics of type I cells. Shedding of sICAM-1 increased significantly at later points in culture (5-7 days) compared with earlier time points (3-5 days). In contrast, treatment of AEC with inflammatory cytokines had limited effect on sICAM-1 shedding. BAL sICAM-1 was evaluated in in vivo models of acute lung injury. In hyperoxic lung injury, a reversible process with a major component of leak across the alveolar wall, BAL fluid sICAM-1 only increased in parallel with increased alveolar protein. However, in lung injury due to FITC, there were increased levels of sICAM-1 in BAL that were independent of changes in BAL total protein concentration. We speculate that after lung injury, changes in sICAM-1 in BAL fluid are associated with progressive injury and may be a reflection of type I cell differentiation during reepithelialization of the injured lung.

Evaluation of viral and mammalian promoters for use in gene delivery to salivary glands

To optimize vectors for salivary gland gene transfer, we screened viral [cytomegalovirus (CMV; human immediate early), Rous sarcoma virus (RSV), simian virus 40, and Moloney murine leukemia virus long terminal repeat] and mammalian [elongation factor 1alpha (EF1alpha), cytokeratin 18 (K18), cytokeratin 19 (K19), kallikrein (Kall), and amylase (AMY), all human, and rat aquaporin-5 (rAQP5), and derivative elements] promoters driving luciferase activity in vitro and in vivo. In adenoviral vectors, the CMV promoter showed highest activity, with the EF1alpha and RSV promoters slightly less powerful, in rat submandibular glands (SMGs). The K18 2.5-kb, K19 3.0-kb, and rAQP5 0.4-kb and Kall promoters had intermediate activity, while the AMY promoter exhibited lowest activity. To localize transgene expression, enhanced green fluorescence protein was used. The CMV, RSV, EF1alpha, K18 2.5-kb, K19 3.0-kb, rAQP5 0.4-kb, and AMY promoters were not cell-type specific in SMGs; however, the Kall promoter was primarily active in ductal cells. These data will facilitate optimal expression cassette design for salivary gland gene transfer.

Pneumocystis carinii activates the NF-kappaB signaling pathway in alveolar epithelial cells

Pneumocystis carinii pneumonia (PcP) is a clinically important infection of immunocompromised patients. Although the interaction of Pneumocystis with the alveolar epithelium has been well documented, very little information regarding the epithelial response to Pneumocystis is currently available. In order to study Pneumocystis-epithelium interactions, a murine cell line derived specifically from an alveolar epithelial cell (AEC) was utilized. The coculture of murine AECs with mouse Pneumocystis induced a dose- and time-dependent release of the CXC chemokine MIP-2. Importantly, the specific removal of Pneumocystis from the preparation, or the pretreatment of AECs with sulfasalazine, a potent and specific inhibitor of NF-kappaB, nearly completely abrogated the chemokine response to Pneumocystis. Since the murine MIP-2 promoter contains consensus kappaB binding sequences, the ability of Pneumocystis to stimulate NF-kappaB signaling in AECs was examined. Pneumocystis stimulation of an AEC line stably transfected with a kappaB-dependent reporter construct triggered the NF-kappaB signaling pathway and reporter production. These data were confirmed in gel shift assays, providing direct evidence that Pneumocystis induced the nuclear translocation of the p50/p65 heterodimeric form of NF-kappaB. Maximal NF-kappaB activation was dependent upon direct contact with viable Pneumocystis organisms. These data demonstrate that Pneumocystis activates NF-kappaB signaling in AECs and establish a reporter cell line for studying NF-kappaB activation in AECs. Given the global regulatory functions of the NF-kappaB family, these findings suggest that Pneumocystis directly alters AEC gene expression in a manner that promotes pulmonary immune and inflammatory responses.

Evidence of a functional CFTR Cl(-) channel in adult alveolar epithelial cells

The cystic fibrosis transmembrane conductance regulator (CFTR) is expressed in the fetal lung, but during lung development it gradually disappears in cells of future alveolar spaces. Recent studies have implicated the CFTR in fluid transport by the adult alveolar epithelium, but its presence has not been demonstrated directly. This study re-evaluated CFTR expression and activity in the adult pulmonary epithelium by using freshly isolated rat alveolar type II (ATII) cells. CFTR mRNA was detected by semiquantitative polymerase chain reaction on the day of cell isolation but was rapidly reduced by 60% after 24 h of cell culture. This was paralleled by a similar decrease of surfactant protein A expression and alkaline phosphatase staining, markers of the ATII cell phenotype. CFTR expression increased significantly on day 4 in cells grown on filters at the air-liquid interface compared with cells submerged or grown on plastic. Significantly higher CFTR expression was detected in distal lung tissue compared with the trachea. The CFTR was also found at the protein level in Western blot experiments employing lysates of freshly isolated alveolar cells. Whole cell patch-clamp experiments revealed cAMP-stimulated, 5-nitro-2-(3-phenylpropylamino)-benzoate-sensitive Cl(-) conductance with a linear current-voltage relationship. In cell-attached membrane patches with 100 microM amiloride in pipette solution, forskolin stimulated channels of approximately 4 pS conductance. Our results indicate that 50-250 of functional CFTR Cl(-) channels occur in adult alveolar cells and could contribute to alveolar liquid homeostasis.

Expression and localization of aquaporins, members of the water channel family, during development of the rat submandibular gland

The expression and localization of aquaporins (AQP1-AQP5), members of the water channel family, in the developing rat submandibular gland were analysed using RT-PCR, Northern blotting and immunohistochemistry to explore their relation to the development of this salivary gland. RT-PCR analysis revealed unique expression patterns of each AQP. AQP1 was expressed constitutively during prenatal development, whereas the expression of AQP5 became more intense in the course of development from embryonic day 16.5 (E16) to E20. These expression patterns concurred with the results of Northern blot analysis. AQP3 and AQP4 mRNAs in the prenatal development were not detected in Northern blots, although they were detected by RT-PCR. During postnatal development, AQP5 and AQP1 mRNAs were expressed continuously, but no message for AQP3 or AQP4 was detected. AQP2 mRNA was not detected during either prenatal or postnatal development in this tissue. Immunohistochemical studies revealed that AQP5 was first localized at the apical membrane of proacinar cells at E18, and then became clearly distributed at the apical membrane of acinar cells in accordance with the differentiation and establishment of the mature acini. In addition, some vasculature also showed immunoreactivity for AQP5. AQP1 was immunolocalized in the blood vessels, including capillaries, of the gland throughout development. These observations suggest the existence of transcriptional regulation of rat AQP5, which is one of the most probable regulators of saliva production and secretion, during the establishment of the functional submandibular salivary gland.

Enhanced binding of Sp1/Sp3 transcription factors mediates the hyperoxia-induced increased expression of the lung type I cell gene T1alpha

The transcription factor Sp1 plays an important regulatory role in transactivation of the lung type I cell differentiation gene T1alpha. Like other lung cells, type I cells may encounter changes in oxygen concentration during the lifetime of the organism. We found that exposure of mice to hyperoxia rapidly increases expression of T1alpha and other type I cell genes, and that returning the mice to normoxia quickly decreases expression. Likewise hyperoxia increases both endogenous T1alpha expression in lung epithelial cell lines and transcription of luciferase (Luc) from T1alpha promoter deletion constructs. Using wild-type promoter fragments and gel shift assays, we determined that Sp1/Sp3 and a key Sp cis-element in the proximal promoter mediate the hyperoxic response. Mutations of this element and inhibition of Sp-DNA binding by mithramycin block the hyperoxic response. Western analyses of cell homogenates show that the overall abundance of Sp1 and Sp3 proteins is not altered by hyperoxia. However, the abundance of nuclear Sp1 increases after short hyperoxic exposures, suggesting that signaling pathways activated by hyperoxia lead to Sp protein translocation, perhaps as a result of increased Sp phosphorylation.

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.

Protein transport across the lung epithelial barrier

Alveolar lining fluid normally contains proteins of important physiological, antioxidant, and mucosal defense functions [such as albumin, immunoglobulin G (IgG), secretory IgA, transferrin, and ceruloplasmin]. Because concentrations of plasma proteins in alveolar fluid can increase in injured lungs (such as with permeability edema and inflammation), understanding how alveolar epithelium handles protein transport is needed to develop therapeutic measures to restore alveolar homeostasis. This review provides an update on recent findings on protein transport across the alveolar epithelial barrier. The use of primary cultured rat alveolar epithelial cell monolayers (that exhibit phenotypic and morphological traits of in vivo alveolar epithelial type I cells) has shown that albumin and IgG are absorbed via saturable processes at rates greater than those predicted by passive diffusional mechanisms. In contrast, secretory component, the extracellular portion of the polymeric immunoglobulin receptor, is secreted into alveolar fluid. Transcytosis involving caveolae and clathrin-coated pits is likely the main route of alveolar epithelial protein transport, although relative contributions of these internalization steps to overall protein handling of alveolar epithelium remain to be determined. The specific pathways and regulatory mechanisms responsible for translocation of proteins across lung alveolar epithelium and regulation of the cognate receptors (e.g., 60-kDa albumin binding protein and IgG binding FcRn) expressed in alveolar epithelium need to be elucidated.

Lung edema clearance: 20 years of progress: invited review: role of aquaporin water channels in fluid transport in lung and airways

Water transport across epithelial and endothelial barriers in bronchopulmonary tissues occurs during airway hydration, alveolar fluid transport, and submucosal gland secretion. Many of the tissues involved in these processes are highly water permeable and express aquaporin (AQP) water channels. AQP1 is expressed in microvascular endothelia throughout the lung and airways, AQP3 in epithelia in large airways, AQP4 in epithelia throughout the airways, and AQP5 in type I alveolar epithelial cells and submucosal gland acinar cells. The expression of some of these AQPs increases near the time of birth and is regulated by growth factors, inflammation, and osmotic stress. Transgenic mouse models of AQP deletion have provided information about their physiological role. In lung, AQP1 and AQP5 provide the principal route for osmotically driven water transport; however, alveolar fluid clearance in the neonatal and adult lung is not affected by AQP deletion nor is lung CO(2) transport or fluid accumulation in experimental models of lung injury. In the airways, AQP3 and AQP4 facilitate water transport; however, airway hydration, regulation of the airway surface liquid layer, and isosmolar fluid absorption are not impaired by AQP deletion. In contrast to these negative findings, AQP5 deletion in submucosal glands in upper airways reduced fluid secretion and increased protein content by greater than twofold. Thus, although AQPs play a major physiological role outside of the airways and lung, AQPs appear to be important mainly in airway submucosal gland function. The substantially slower rates of fluid transport in airways, pleura, and lung compared with renal and some secretory epithelia may account for the apparent lack of functional significance of AQPs at these sites. However, the possibility remains that AQPs may play a role in lung physiology under conditions of stress and/or injury not yet tested or in functions unrelated to transepithelial fluid transport.

Localization of aquaporin-5 in sweat glands and functional analysis using knockout mice

Sweat secretion involves the transport of salt and water into the lumen of the secretory coil of the sweat gland. By analogy to salivary and submucosal glands, where fluid secretion is aquaporin-5 (AQP5) dependent, we postulated that aquaporin water channels might facilitate sweat secretion. Immunolocalization with specific antibodies revealed strong expression of AQP5 at the luminal membrane of secretory epithelial cells in sweat glands in mouse paw skin. Novel quantitative methods were developed to compare sweat secretion in wild-type mice and mice lacking AQP5. Total hindpaw sweat secretion was measured by proton nuclear magnetic resonance of sweat-derived (1)H(2)O in (2)H(2)O solvent, and sweat secretion from individual glands was measured by real-time video imaging of sweat droplet formation under oil. Sweat secretion rates after pilocarpine stimulation did not differ in wild-type mice (0.21 +/- 0.03 nl min(-1) gland(-1)) vs. mice lacking AQP5 (0.19 +/- 0.04 nl min(-1) gland(-1)). The lack of effect of AQP5 on sweat secretion rate was confirmed by microcapillary collections of sweat from defined regions of mouse paws. Also, as by direct counting of droplets, the number of functional sweat glands was not affected by AQP5 deletion. Sweat gland morphology was similar in wild-type and AQP5 null mice. From sweat coil geometry and gland secretion rate, the rate of fluid secretion was estimated to be 130 nl min(-1) cm(-2) of secretory epithelium, substantially lower than that of > 500 nl min(-1) cm(-2) in kidney proximal tubules and salivary glands, where active fluid absorption or secretion is aquaporin dependent. These results indicate the expression of AQP5 in sweat gland secretory epithelium, but provide direct evidence against its physiological involvement in sweat fluid secretion in mice.

Na transport proteins are expressed by rat alveolar epithelial type I cells

Despite a presumptive role for type I (AT1) cells in alveolar epithelial transport, specific Na transporters have not previously been localized to these cells. To evaluate expression of Na transporters in AT1 cells, double labeling immunofluorescence microscopy was utilized in whole lung and in cytocentrifuged preparations of partially purified alveolar epithelial cells (AEC). Expression of Na pump subunit isoforms and the alpha-subunit of the rat (r) epithelial Na channel (alpha-ENaC) was evaluated in isolated AT1 cells identified by their immunoreactivity with AT1 cell-specific antibody markers (VIIIB2 and/or anti-aquaporin-5) and lack of reactivity with antibodies specific for AT2 cells (anti-surfactant protein A) or leukocytes (anti-leukocyte common antigen). Expression of the Na pump alpha(1)-subunit in AEC was assessed in situ. Na pump subunit isoform and alpha-rENaC expression was also evaluated by RT-PCR in highly purified (approximately 95%) AT1 cell preparations. Labeling of isolated AT1 cells with anti-alpha(1) and anti-beta(1) Na pump subunit and anti-alpha-rENaC antibodies was detected, while reactivity with anti-alpha(2) Na pump subunit antibody was absent. AT1 cells in situ were reactive with anti-alpha(1) Na pump subunit antibody. Na pump alpha(1)- and beta(1)- (but not alpha(2)-) subunits and alpha-rENaC were detected in highly purified AT1 cells by RT-PCR. These data demonstrate that AT1 cells express Na pump and Na channel proteins, supporting a role for AT1 cells in active transalveolar epithelial Na transport.