The authors are looking at “dry eye disease” (keratoconjunctivitis sicca), which can have a rather complex biological background. There are quite a few contributing causes (contact lens use, aging, clogged oil glands, decreased tear production overall, and more). But a common feature that most of these lead to is a chronic inflammation/oxidative stress condition in the eye tissue, and like most of these situations, it can be a self-worsening condition over time as damage leads to more damage.
The authors are hoping that extra NADPH would help to interrupt this cycle. That’s a universal reducing agent in a whole range of cellular processes, and the authors suggest that its supplementation in eye tissue and the resulting antioxidant effect could restore redox balance and reduce the presence of reactive oxygen species in the inflamed tissues. That’s a reasonable idea, but how do you get this on site where it’s needed? Here’s their answer, which I’m pretty sure will not have occurred to anyone in the audience: engineer a thylakoid-like system (based on what plants use as their organ of photosynthesis) and allow photosynthetic production of extra NADPH and ATP in the eye tissue (which is being bathed in light anyway, right?)
A bold strategy! The closest thing in the animal kingdom is probably a group of sea slugs that latch onto the chloroplasts found in the algae that they eat and sequester them in their own intestinal cells, where they are used as backup ATP food producers when normal prey are running low. That’s rather distant from the human eyeball, but at the same time it does make one think that running some sort of cross-kingdom collaboration between animals and plants is not prima facie crazy, either. In fact, it’s been tried a few times for inflammatory conditions like arthritis, but those attempts have been understandably hampered by the need to get around the fact that there’s not much light inside (say) the knee joint. The eye seems to be a bit more straightforward shot on goal.
The paper describes isolation of a stripped-down thylakoid-like unit from spinach leaves, which nanostructures have lost most of their light-independent (and NADPH-consuming) co-enzymes such as the famous RuBisCO. They name this LEAF (light-reaction enriched thylakoid NADPH-foundry). In experiments with macrophage surrogate cells, which took these LEAFs up by endocytosis, these seem to be able to provide sufficient NADPH (by conversion from NADP+) to mammalian cells when their own NADPH-generating systems are deliberately blocked. These effects were also seen in immortalized human corneal epithelium cells, which also internalized the LEAF structures. When this was done on deliberately stressed cells, NADPH increased and reactive oxygen species went down along with other markers of inflammation. The LEAFs persisted intracellularly for unexpected long periods, and are apparently not just automatically routed to lysosomal compartments for destruction.
When these two types of cells were grown in a“transwell” setup (physically separated by a porous membrane but in the same culture fluid), it appeared that introducing LEAFs into the macrophage-cell side showed normalizing effects on the corneal epithelium side as well, which is encouraging. It appears that the LEAF structures have useful effects both inside and outside the cells, and that releasing more NADPH in general into either environment is beneficial. And these effects translated all the way to topical administration in the eyes of rats (and the benefits were light-dependent, as you would figure).
So all this makes you wonder if some sort of “LEAF eyedrop) could alleviate dry eye to an extent that other therapies can’t realize. As it stands, it looks like things start dropping off on a time scale of hours, which is inconvenient but not a show-stopper. But this are early results: can such structures be made to persist longer, even within cells? If they do, where will they go during mitosis? Are there ways to make them associate with particular organelles like the mitochondria or the ER? These are rather odd things to be thinking about, but I’m glad that someone is.
