Inhibition of Unc-51-like-kinase is mitoprotective during Pseudomonas aeruginosa infection in corneal epithelial cells.

Pseudomonas aeruginosa (PA) is an opportunistic gram-negative pathogen that can infect the cornea, leading to permanent vision loss. Autophagy is a cannibalistic process that drives cytoplasmic components to the lysosome for degradation and/or recycling. Autophagy has been shown to play a key role in the removal of intracellular pathogens and, as such, is an important component of the innate immune response. Autophagy is intimately linked to mitochondria, organelles that mediate energy homeostasis, immune signaling, and cell death. Using a combination of biochemical and imaging approaches, we investigated the effects of PA on autophagy and host cell mitochondria in relation to pro-inflammatory cytokine expression. Using a standard invasive test strain of PA, we show that PA infection triggers dephosphorylation of the mechanistic target of rapamycin in corneal epithelial cells, leading to the induction of autophagy through ULK1/2. This was associated with robust mitochondrial depolarization, changes in mitochondrial ultrastructure, and an increase in IL-6 and IL-8 secretion. PA infection was also associated with an increase in purine metabolism by host cells. Treatment with the ULK1/2 inhibitor, MRT68921, which blocks phagophore formation, attenuated levels of intracellular PA in corneal epithelial cells. Unexpectedly, treatment of cells with MRT68921 blocked PA-induced mitochondrial depolarization and downregulated purine and pyrimidine metabolism. While MRT68921 attenuated the PA-induced increase in IL-6, it further increased IL-8 and neutrophil chemotaxis. This was associated with the nuclear internalization of NF_κB. Taken together, these findings highlight a novel mechanism whereby the inhibition of ULK1/2 activity confers mitoprotection during PA infection in corneal epithelial cells.IMPORTANCEPseudomonas aeruginosa (PA) is a common pathogen that can cause severe disease in man. In the eye, PA infection can lead to blindness. In this study, we show that PA induces autophagy, a mechanism whereby cells recycle damaged proteins and organelles. PA infection further depolarizes mitochondria, leading to the release of pro-inflammatory mediators. Unexpectedly, the inhibition of ULK1/2, an enzyme involved in the early stages of autophagy, not only inhibits autophagy but enhances mitochondrial polarization. This leads to a reduction in intracellular levels of PA and changes in the inflammatory milieu. Together, these data suggest that the inhibition of ULK1/2 may be mitoprotective in corneal epithelial cells during PA infection.