Abstract Body (Do not enter title and authors here): Background. Lung endothelial cell (EC) barrier disruption is a hallmark of acute lung injury (ALI), which is commonly caused by bacterial pneumonia. Alveolar epithelial cells (AEC) are the first line of defence against invading respiratory pathogens in the lungs and can mediate inflammatory signaling to the adjacent EC contributing to barrier disruption. We have previously demonstrated that pneumolysin (PLY), a major Streptococcus pneumoniae virulence factor, induces both mitochondrial dysfunction in lung AEC and release of large extracellular vesicles (EVs). These EVs, which are enriched with mitochondrial cargo, contribute to increased permeability of the lung EC barrier. Here, we investigate strategies to inhibit mitochondrial dysfunction in AEC and examine how these interventions impact EC barrier function.
Methods. Human alveolar epithelial cells (A549) or immortalized AEC (Cell Biologics) were pre-treated with mitoTEMPO (mtROS scavenger), P110 (excessive fission inhibitor), or ensifentrine [dual phosphodiesterase (PDE) 3/4 inhibitor] prior to PLY treatment (100-300 ng/ml, 4 hrs). Mitochondrial function was assessed by the JC-1 assay. EVs were isolated from cell media and analyzed for their mitochondrial content (Tom20 and Tim23 expression). To assess their functional role, EVs were added to TNF-α treated human lung microvascular EC, and barrier function was then determined using the ECIS assay to measure transendothelial electrical resistance (TER).
Results. PLY-induced mitochondrial dysfunction was reduced in the presence of mitoTempo or P110. EVs from PLY-treated AEC (EV^PLY; enriched mitochondrial content) exacerbated TNF-α-induced EC barrier permeability as indicated by the significant decrease in TER. EV^PLY derived from MitoTEMPO or P110-pretreated AEC contained less mitochondrial cargo and resulted in less EC permeability after TNF-α. Compared to mitoTEMPO and P110, ensifentrine exhibited superior protection against mitochondrial dysfunction after PLY exposure. In addition, in the presence of ensifentrine, the quantity of EV release and their extracelllular mitochondrial content were both dramatically reduced. In contrast to control EV^PLY, EV^PLY from ensifentrine-pretreated AEC demonstrated no barrier disruptive properties.
Conclusions. Mitochondrial dysfunction in AEC contributes to vascular endothelial injury in ALI through mechanisms involving the release of barrier-disruptive EVs, which can be effectively targeted by inhibiting PDE3/4.