Lung injury due to required mechanical ventilation poses a significant clinical risk to septic patients, however the mechanisms through which ventilation or sepsis injures the alveolar epithelium, specifically, have yet to be fully elucidated. In three distinct aims, we examine mitogen activated protein kinase (MAPk) signaling, tight junction protein expression, and epithelial permeability following either stretch or septic insults, or a combination of the two using an epithelial model of cultured primary alveolar epithelial cells. We find that independently, both high magnitude cyclic stretch and sepsis result in activation of MAPk signaling, altered expression of tight junction proteins, and increased epithelial permeability to either uncharged molecular tracers (stretch) or ions (sepsis). Inhibition of MAPk signaling significantly improved epithelial barrier function following either stretch or septic injuries. Following both stretch and sepsis, we find increases in epithelial permeability occur at lower stretch magnitudes in septic compared to healthy monolayers, however differences in MAPk activation were not observed between septic and healthy epithelia following stertch, and stretch induced alterations in tight junction protein expression were only observed in healthy epithelia. Inhibition of MAPk signaling did significantly improve epithelial barrier function, but in healthy monolayers only. Stabilizing the actin cytoskeleton prevented increases in septic epithelial permeability following low magnitude stretch. In summary, we find that both stretch and sepsis alter epithelial permeability through activation of MAPk signaling and alteration of the tight junction, however increased susceptibility of septic epithelial to stretch induced barrier dysfunction is due to alterations of the actin cyctoskeleton. Future studies, and clinical treatments during ventilation, should focus on pathways through which sepsis modulates cytoskeletal integrity.
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