The brain’s ability to sense light is about more than vision.© PunchstockThanks to the rod and cone cells in our eyes, our brains can use light to build images. Recent studies identified a third type of cell that responds to light and dark. Three research groups have now confirmed that melanopsin is the pigment that this cell-type uses, opening possible avenues for treating blind people.
In the classic model, mammals have two types of light-detecting cells, called photoreceptors, in the retina at the back of their eye. Rod cells use the rhodopsin pigment to pick up dim light, and cone cells use related pigments to discriminate colour.
But three years ago, scientists found a third type of light-sensitive cell. In such cells, a pigment called melanopsin is used to tell night from day. But apparently the visual parts of the brain do not use this information. Instead, these cells communicate with the neurons at the base of the brain that set the daily body cycle.
For example, mice without working rods or cones cannot see images. But researchers showed that they can still use a small set of melanopsin-containing cells in the retina to adjust their biological clocks. Exactly how melanopsin worked, however, remained a mystery.
Convincing conversion
Now researchers have proved that melanopsin is a light-sensitive pigment, by activating the gene for it inside non-vision cells, and converting them into photoreceptors. The results of their work appear this week in the journals Science1 and Nature2,3.
Previous studies had shown that a small number of ganglion cells need melanopsin to respond to light. "But that doesn't prove it's a photopigment, it just shows that it's crucial," says neuroscientist Mark Hankins of Imperial College in London.
By making embryonic mouse neurons produce melanopsin, he and his team made them sensitive to light. "This shows that it's melanopsin that functions as a photopigment," says Hankins.
Likewise, another group demonstrated that frog eggs also became light-sensitive when injected with the genes for melanopsin. The third team converted human embryonic kidney cells using this pigment.
"It was fantastic," says Satchidananda Panda, a biologist at the Salk Institute for Biological Studies in La Jolla, California, who investigated the frog eggs. He explains that very few light-sensitive proteins still work in the cells of different species.
Illuminations
Melanopsin resembles pigments in invertebrates' eyes, in that light makes the cells containing it more active. Pigments in vertebrates' rods and cones have the opposite effect, inhibiting their cells. This may help biologists understand the evolution of the circadian rhythm system in humans, says Panda.
The results also underline the possibility of conferring visual powers on unlikely cells. "If you could put the melanopsin gene into cells then you could make the normally non-sensitive ones become light-sensitive," says Ron Douglas, a vision researcher at City University in London.
"It's quite important, because there are some forms of blindness where the rods and cones are lost," says Hankins. In the future, converting other cells in these people's eyes with melanopsin could help them to form images.
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