Scientists got mouse eyes to perform photosynthesis ‪—‬ and no, they didn't turn green

Scientists have developed new eye drops that allow mouse eyes to perform certain steps of photosynthesis.

The drops, which contain photosynthetic machinery extracted from spinach leaves, use light-driven reactions to improve symptoms of dry eye disease, according to a study published May 15 in the journal Cell. Although the researchers tested the drops in mice, the hope is that with further testing, the therapy could be used in people someday.

The study is a "cool application" of engineering inspired by symbiotic relationships in nature, said Corey Allard, a cell biologist at Harvard Medical School who was not involved in the work.

Plants rely on photosynthesis to make energy, in the form of glucose, from sunlight. Organelles called chloroplasts conduct photosynthesis and give plants their green color. While no animals photosynthesize naturally on their own, a few have developed symbiotic relationships with photosynthetic algae that let them harness solar power. Some species of sea slug, including leaf sheep (Costasiella kuroshimae) and the mangrove leaf slug (Elysia bangtawaensis), even steal chloroplasts from algae they eat.

In the new study, David Tai Leong, a chemical engineer at the National University of Singapore, and colleagues tested whether mammal eyes could tolerate similar behavior, in hopes of pointing to a way to treat dry eye disease, which affects the film of tears covering the eye and produces oxidants and inflammation that can impair vision.

To create the eye drops, the team first removed stacked compartments called thylakoid grana from chloroplasts in spinach. Thylakoid grana are the chlorophyll-bearing structures inside chloroplasts where the first, light-dependent steps of photosynthesis occur. Then, the team encapsulated those thylakoid stacks in tiny packages to create a system they dubbed "light- reaction enriched thylakoid NADPH-foundry," or LEAF.

When incorporated into eye drops, LEAF reduced eye inflammation in mice induced to have dry eye disease. Along the path to making glucose, chloroplasts produce a chemical called NADPH at the thylakoid grana. NADPH acts as an antioxidant, and it helped to eliminate compounds that were exacerbating eye inflammation in the mice.

After five days, the mice treated with the LEAF eye drops were comparable to mice treated with an existing, commercially available medicine to treat dry eye disease. They showed greater tear production and reduced cornea damage compared with the mice that received only saline eye drops.

"The eye is uniquely suited for this type of strategy, since light is already an intrinsic component of its normal physiological function," Dr. Xianfeng Lin, an orthopedic surgeon at Zhejiang University School of Medicine in China, wrote in an email to Live Science. Although Lin wasn't involved with the new work, he and colleagues published a 2022 study in the journal Nature using a similar photosynthetic system to combat inflammation from arthritis in mouse knees.

"The work extends the role of light in the eye from being purely sensory to potentially contributing to local metabolic support and tissue repair," Lin said.

Even though the eye drops contain chlorophyll, the concentration is very low, and the drops appear transparent.

"We are able to use a highly optimized photosynthetic machine, so we do not need huge amounts of the LEAF system," Leong said. "Because it's so low concentration, you can't see the green color. So we won't have green eyes like the Incredible Hulk."

The eye drops aren't ready for human use yet; they still need to undergo extensive testing for safety and long-term efficacy. But the team is working to set up a clinical trial that will first look at safety, Leong told Live Science.

If approved for human use, the medicine would take advantage of how our eyes naturally work, since it only requires ambient light to activate. A patient would be "receiving a therapy that is aligned with how we normally go about our day," Leong said.