Investigation of antioxidant systems in human meibomian gland and conjunctival tissues

Oxidative stress associated with direct contact with the environment and light exposure is one very potent and continuous stressor of the ocular surface and more internal structures of the eye that should cope with it. Permanent replenishment of tears together with its superficial lipid layer produced by the meibomian glands (MG) is one protective mechanism. The lipid-rich fraction of the tears is there to prevent evaporation of the more aqueous fractions, however lipids are particularly sensitive to oxidative damage that could thus alter tear film quality. To counteract oxidative damage MG as well as the other structures of the ocular surface use primary antioxidant (AO) systems to limit lipid peroxidation. Because very little is known as to the human MG equipment in primary enzymatic AO, we presently investigated it. Using a different approaches (RT-PCR, enzymatic activity) and immuno-fluorescent microscopy we report thereafter the presence, distribution and subcellular locations of the major AO enzymes belonging to the classical catalytic triad (superoxide dismutase, catalase and glutathione peroxidases) in adult human MG and conjunctiva. We show that both tissues are well equipped especially in term of glutathione peroxidase coverage. Beside the ubiquitous cytosolic GPx1 protein, MG and conjunctiva show significant expression of GPx2, GPx4 and GPx7, all isoforms known to preferentially scavenge phospholipid-hydroperoxide compounds. This characterization of the primary AO equipment of human MG and conjunctiva paves the road for the development of diagnostic approaches in cases of MG dysfunction (MGD) such as the very common dry eye syndrome.