Scientists have figured out how to confer a superpower, like those wielded by the mythical X-Men, at least to mice. Using nanoparticles that convert infrared (IR) light to visible light, researchers have given mice the ability to see in the dark. If the same technique works in humans, it could offer soldiers night vision without the need for goggles and possibly counter ailments that cause patients to gradually lose their sight.
“This paper is astonishing,” says Michael Do, a neuroscientist at Harvard Medical School in Boston, who was not involved with the work. “To think that you can inject these nanoparticles and have them work is incredible.”
When injected into the eye, the nanoparticles deliver visible light to the light-sensitive pigments that vertebrates use to see. The pigments are in specialized cells called photoreceptors, located in the retina at the back of the eye. A combination of pigments in these photoreceptors absorb different colors of light, triggering nerve impulses to flow through the optic nerve to the visual centers of the brain. Humans have three pigments that give us color vision and another pigment that helps us see black and white, especially in dim light. Mice and some primates have just two color pigments and one for dim light.
Researchers have previously added genes for a third pigment to mice and primates to give them a humanlike range of sensitivity to visible light. But until now, no mammals have been able to see IR light under normal conditions.
To change that, Xue Tian, a vision physiology expert at the University of Science and Technology of China in Hefei, teamed up with Gang Han, a nanoparticle expert at the University of Massachusetts Medical School in Worcester. Han had previously developed nanoparticles that can convert IR to blue light. Because blue light carries more energy than IR, these so-called up-converting nanoparticles (UCNPs) must absorb multiple IR photons before they release a single blue photon. That led Han and Xue to wonder whether such nanoparticles on photoreceptors would convert enough IR to visible light to enable mice to see in the dark.
To find out, Han and his colleagues first improved the animals’ chances: They tweaked the UCNPs to emit green light. (Green photopigments in animals are more sensitive than blue.) Then they coated their UCNPs with a protein that binds to specific sugar molecules on the membranes of photoreceptors. After injecting these behind the retinas of mice, they found that the UCNPs bound tightly to the photoreceptors and stayed there for up to 10 weeks with no obvious lasting side effects.
And the nanoparticle injections seemed to have the desired effect. Mice that received them showed physical signs of detecting IR light and converting it to visible light: their pupils constricted, for example, while mice injected with only a buffer solution showed no response. Electrophysiology recordings also showed the IR light triggered nerve responses in the retina and visual cortex only in the animals with nanoparticles.
Lastly, Xue, Han, and their colleagues ran the mice through behavioral tests to determine whether the animals with nanoparticles were seeing a diffuse haze or able to recognize distinctive shapes and patterns. In one test, animals swam in a water maze without an exit. On the wall above one route researchers projected a triangle and on another, a circle; below the triangle, the researchers placed a submerged platform onto which the animals could climb to get out of the water.
When the shapes were illuminated by visible light, all the animals quickly learned to associate the comfort of the platform with the triangle and swam immediately toward it, even when the researchers swapped the position of the triangle and circle. When the patterns were under IR light, only the animals injected with the UCNPs swam consistently toward the triangle, the researchers report today in Cell. “The students couldn’t see which [pathway] showed the triangle, but the mice would go to the correct side,” Xue says with a chuckle.
Given the similarities between mice and humans in vision physiology, Xue says, “I definitely think it will work in humans.” If it does, future versions of nanoparticles could give first responders and military personnel temporarily enhanced night vision. Nanoparticles could also be tailored to absorb and re-emit visible light. These particles might intensify color sensation to treat patients with macular degeneration, a leading cause of age-related vision loss, in which the photoreceptor cells gradually die over time.
Creating other X-Men powers, such as telekinesis or manipulating the weather, will take more time.