The expression of aquaporin 5 in alveolar epithelial type I cells under conditions of cadmium-induced injury has not yet been discovered. We investigated the effect of the P2X7R agonist BzATP under this condition, since P2X7R is involved in altered regulation of aquaporin 5 in pulmonary fibrosis. CdCl2/TGF-β1 treatment of lung epithelial MLE-12 cells was leading to increasing P2X7R, and aquaporin 5 protein levels. The aquaporin 5 expression was P2X7R-independent in MLE-12 cells under cadmium, as was shown in blocking experiments with oxATP. Further, the expression of both proteins increased after 24 h CdCl2/TGF-β1 treatment of precision-cut lung slices, but decreased after 72 h. Using immunohistochemistry, the activation of the P2X7R with the agonist BzATP modulated the aquaporin 5 immunoreactivity in the alveolar epithelium of precision-cut lung slices from wild-type but not from P2X7R knockout mice. Similarly, aquaporin 5 protein was reduced in BzATP-treated immortal lung epithelial E10 cells. Surprisingly, untreated alveolar epithelial type II cells of P2X7R knockouts exhibited a pronounced apical immunoreactivity in addition to the remaining alveolar epithelial type I cells. BzATP exposure did not alter this distribution pattern, but increased the number of apoptotic alveolar epithelial type II cells in wild-type lung slices.

Several lung diseases are associated with extensive damage of pulmonary epithelia, and the regulatory mechanisms involved in their regeneration are not clearly defined. P2X receptors are cation selective ion channels activated by extracellular ATP. Recent work indicates that alveolar epithelial type I cells express P2X4 and P2X7 receptor subtypes in addition to a large number of other ion channels present in the alveolar epithelium. Up- or downregulation of their expression is associated with several disease states. For example, P2X7R activation inhibited the Wnt/beta-catenin signaling leading to alveolar epithelial type I (ATI) cell death. An important step is the activation ofglycogen synthase kinase 3beta (GSK-3beta) by phosphorylation of Tyr- 216 via P2X7R. Our previous studies have shown that the N-glycosylated P2X7R is localized in the plasma membrane and is partly associated with caveolae, containing the structure protein caveolin-1. We have shown that the P2X7R interacts with caveolin-1 in ATI cells. An injury induced ATP release, initiates via P2X7R a protein kinase C (PKC)-beta1 translocation from the cytoplasm to the lipid rafts. The expression of PKC-beta1 was repressed by the P2X7R inhibitor oxATP, suggesting that PKC-beta1 is downstream of P2X7R activation. In the present study using different injury models we investigated a possible involvement of thePKC-beta1 in the Wnt/β-catenin pathway and characterized subsequent ATI cell survival.

There is growing evidence that amorphous silica nanoparticles cause toxic effects on lung cells in vivo as well as in vitro and induce inflammatory processes. The phagocytosis of silica by alveolar macrophages potentiates these effects. To understand the underlying molecular mechanisms of silica toxicity, we applied a co-culture system including the immortal alveolar epithelial mouse cell line E10 and the macrophage cell line AMJ2-C11. In parallel we exposed precision-cut lung slices (lacking any blood cells as well as residual alveolar macrophages) of wild type and P2rx7-/- mice with or without AMJ2-C11 cells to silica nanoparticles. Exposure of E10 cells as well as slices of wild type mice resulted in an increase of typical alveolar epithelial type 1 cell proteins like T1α, caveolin-1 and -2 and PKC-β1, whereas the co-culture with AMJ2-C11 showed mostly a slightly lesser increase of these proteins. In P2rx7-/- mice most of these proteins were slightly decreased. ELISA analysis of the supernatant of wild type and P2rx7-/- mice precision-cut lung slices showed decreased amounts of IL-6 and TNF-α when incubated with nano-silica. Our findings indicate that alveolar macrophages influence the early inflammation of the lung and also that cell damaging reagents e.g. silica have a smaller impact on P2rx7-/- mice than on wild type mice. The co-culture system with an organotypic lung slice is a useful tool to study the role of alveolar macrophages during lung injury at the organoid level.

The Na(+)/H(+) exchanger NHE3 colocalizes with beta-actin at the leading edge of directionally migrating cells. Using human osteosarcoma cells (SaOS-2), rat osteoblasts (calvaria), and human embryonic kidney (HEK) cells, we identified a novel role for NHE3 via beta-actin in anode and cathode directed motility, during electrotaxis. NHE3 knockdown by RNAi revealed that NHE3 expression is required to achieve constant directionality and polarity in migrating cells. Phosphorylated NHE3 (pNHE3) and beta-actin complex formation was impaired by the NHE3 inhibitor S3226 (IC50 0.02 µM). Fluorescence cross-correlation spectroscopy (FCCS) revealed that the molecular interactions between NHE3 and beta-actin in membrane protrusions increased 1.7-fold in the presence of a directional cue and decreased 3.3-fold in the presence of cytochalasin D. Data from flow cytometric analysis showed that membrane potential of cells (Vmem) decreases in directionally migrating, NHE3-deficient osteoblasts and osteosarcoma cells whereas only Vmem of wild type osteoblasts is affected during directional migration. These findings suggest that pNHE3 has a mechanical function via beta-actin that is dependent on its physiological activity and Vmem. Furthermore, phosphatidylinositol 3,4,5-trisphosphate (PIP3) levels increase while PIP2 remains stable when cells have persistent directionality. Both PI3 kinase (PI3K) and Akt expression levels change proportionally to NHE3 levels. Interestingly, however, the content of pNHE3 level does not change when PI3K/Akt is inhibited. Therefore, we conclude that NHE3 can act as a direction sensor for cells and that NHE3 phosphorylation in persistent directional cell migration does not involve PI3K/Akt during electrotaxis.

P2X7 receptors, ATP-gated cation channels, are specifically expressed in alveolar epithelial cells. The pathophysiological function of this lung cell type, except a recently reported putative involvement in surfactant secretion, is unknown. In addition, P2X7 receptor-deficient mice show reduced inflammation and lung fibrosis after exposure with bleomycin. To elucidate the role of the P2X7 receptor in alveolar epithelial type I cells we characterized the pulmonary phenotype of P2X7 receptor knockout mice by using immunohistochemistry, western blot analysis and real-time RT PCR. No pathomorphological signs of fibrosis were found. Results revealed, however, a remarkable loss of aquaporin-5 protein and mRNA in young knockout animals. Additional in vitro experiments with bleomycin treated precision cut lung slices showed a greater sensitivity of the P2X7 receptor knockout mice in terms of aquaporin-5 reduction as wild type animals. Finally, P2X7 receptor function was examined by using the alveolar epithelial cell lines E10 and MLE-12 for stimulation experiments with bleomycin. The in vitro activation of P2X7 receptor was connected with an increase of aquaporin-5, whereas the inhibition of the receptor with oxidized ATP resulted in down regulation of aquaporin-5. The early loss of aquaporin-5 which can be found in different pulmonary fibrosis models does not implicate a specific pathogenetic role during fibrogenesis.

Caveolae and caveolins, structural components of caveolae, are associated with specific ion channels in cardiac myocytes. We have previously shown that P2X purinoceptor 7 (P2X7R), a ligand-gated ion channel, is increased in atrial cardiomyocytes of caveolin-1 knockout mice; however, the specific biochemical relationship of P2X7R with caveolins in the heart is not clear. The aim of this work was to study the presence of the P2X7R in atrial cardiomyocytes and its biochemical relationship to caveolin-1 and caveolin-3. Caveolin isoforms and P2X7R were predominantly localized in buoyant membrane fractions (lipid rafts/caveolae) prepared from hearts using detergent-free sucrose gradient centrifugation. Caveolin-1 knockout mice showed normal distribution of caveolin-3 and P2X7R to buoyant membranes indicating the importance of caveolin-3 to formation of caveolae. Using clear native-PAGE, we showed that caveolin-1, -3 and P2X7R contribute to the same protein complex in the membranes of murine cardiomyocytes and in the immortal cardiomyocyte cell line HL-1. Western blot analysis revealed increased caveolin-1 and -3 proteins in tissue homogenates of P2X7R knockout mice. Finally, tissue homogenates of atrial tissues from caveolin-3 knockout mice showed elevated mRNA for P2X7R in atria. The colocalization of caveolins with P2X7R in a biochemical complex and compensated upregulation of P2X7R or caveolins in the absence of any component of the complex suggests P2X7R and caveolins may serve an important regulatory control point for disease pathology in the heart.

Changes in intracellular calcium concentration [Ca(2+)](i) are believed to influence the proliferation and differentiation of airway epithelial cells both in vivo and in vitro. In the present study, using mouse alveolar epithelial E10 cells, we demonstrated that the treatment of lung epithelial cells with BLM resulted in elevated intracellular Ca(2+) levels. BLM further increased P2rx7 mRNA expression and P2X7R protein levels, paralleled by increased PKC-β1 levels. BLM treatment or stimulation of the P2X7R with the P2X7R agonist BzATP induced translocation of PKC-β1 from the cytoplasm to the membrane. The expression of PKC-β1 was repressed by the P2X7R inhibitor oxATP, suggesting that PKC-β1 is downstream of P2X7R activation. Furthermore, cells exposed to BLM contained increased amounts of P2X7R and PKC-β1 in Cav-1 containing lipid raft fractions. The comparison of lung tissues from wild-type and P2rx7(-/-) mice revealed decreased protein and mRNA levels of PKC-β1 and CaM as well as decreased immunoreactivity for PKC-β1. The knockdown of P2X7R in alveolar epithelial cells resulted also in a loss of PKC-β1. These data suggest that the effect of P2X7R on expression of PKC-β1 detected in alveolar epithelial cells is also functioning in the animal model. Immunohistochemical evaluation of fibrotic lungs derived from a BLM-induced mouse model revealed a strong increase in PKC-β1 immunoreactivity. The present experiments demonstrated that the increased expression of P2X7R influences PKC-β1. We predict that increased Ca(2+) concentration stimulates PKC-β1, whereas the prerequisite for activating PKC-β1 after P2X7R increase remained to be determined. Our findings suggest that PKC-β1 is important in the pathogenesis of pulmonary fibrosis.

Aims: T1α/(podoplanin) is abundantly expressed in the alveolar epithelial type I cells (ATI) of rodent and human lungs. Caveolin-1 is a classical primary structural protein of plasmalemal invaginations, so-called caveolae, which represent specialized lipid rafts, and which are particularly abundant in ATI cells. The biological functions of T1α in the alveolar epithelium are unknown. Here we report on the characteristics of raft domains in the microplicae/microvillar protrusions of ATI cells, which contain T1α. Methods: Detergent resistant membranes (DRMs) from cell lysates of the mouse epithelial ATI-like cell line E10 were prepared using different detergents followed by flotation in a sucrose gradient and tested by Western and dot blots with raft markers (caveolin-1, GM1) and nonraft markers (transferrin receptor, PDI and β-Cop). Immunocytochemistry was employed for the localization of T1α in E10 cells and in situ in rat lungs. Results: Our biochemical results showed that the solubility or insolubility of T1α and caveolin-1 differs in Triton X-100 and Lubrol WX, two distinct non-ionic detergents. Caveolin-1 was unsoluble in both detergents, whereas T1α was Triton X-100 soluble but Lubrol WX insoluble. Immunofluorescence double stainings revealed that both proteins were colocalized with GM1, while caveolin-1 and T1α were not colocalized in the plasma membrane. Cholesterol depletion modified the segregation of T1α in Lubrol WX DRMs. Cellular processes in ultrathin sections of cultured mouse E10 cells were immunogold positive. Immunoelectron microscopy (postembedding) of rat lung tissue revealed the preferential localization of T1α on apical microvillar protrusions of ATI cells. Conclusion: We conclude that T1α and caveolin-1 are located in distinct plasma membrane microdomains, which differ in their protein-lipid interactions. The raft-associated distribution of T1α may have an impact on a specific, not yet clarified function of this protein in the alveolar epithelium.