The purinergic receptor P2X7 represents an ATP-gated ionotropic receptor with a selective localization in alveolar epithelial type I cells of the lung. Despite the involvement of the receptor in inflammatory processes of the lung, it is not established whether this receptor plays a specific role in the alveolar epithelial cell biology. There is evidence that P2X7 receptor influences Wnt/β-catenin signalling pathways in alveolar epithelial cells under conditions of injury. Here, we investigated the expression of GSK-3β, a potent protein kinase involved in alveolar epithelial barrier functions, and of tight junction molecules occludin, claudin-4 and claudin-18 in wild-type and P2X7-/- mice. Western blot analysis, immunohistochemistry and quantitative real-time RT-PCR revealed a remarkable increase in claudin-18 mRNA and protein in lungs of P2X7-/- mice animals. Furthermore, alveolar epithelial cells from P2X7-/- animals showed decreased levels of GSK-3β protein and its inactive form GSK-3β (pS9). Conversely, claudin-18 knockout mice exhibited decreased P2X7 mRNA transcript abundance as measured by mRNA expression microarray and quantitative PCR. Our data are consistent with the hypothesis that P2X7R contributes to alveolar epithelial barrier function through effects on GSK-3β. Furthermore, these data suggest a potential reciprocal regulation of claudin-18 and P2X7R in the alveolar epithelium.
Keywords: GSK-3β; Knockout; Mouse lung; P2X7 receptor; claudin-18; β-catenin.