Inner Workings of Rare Eye Cells Explained

Spidery cells, controlling our
circadian body clock: ipRGCs
© Brown University

The Brown University team led by neuroscientist David Berson has found that a protein called melanopsin plays a key role in the inner workings of mysterious, spidery cells in the eye called intrinsically photosensitive retinal ganglion cells, or ipRGCs.

Like rods and cones, ipRGCs turn light energy into electrical signals. But while rods and cones aid sight by detecting objects, colours and movement, ipRGCs gauge overall light intensity. Numbering only about 1,000 to 2,000 out of millions of eyes cells, ipRGCs are different in another way: They have a direct link to brain, sending a message to the tiny region that controls the body clock about how light or dark the environment is.

Through these signals, ipRGCs synchronize the body's daily rhythms to the rising and setting of the sun. This circadian rhythm controls alertness, sleep, hormone production, body temperature and organ function, say the researchers. "It's a general brightness detection system in the eye,” said Berson. "What we've done now is provide more details about how this system works.”

The research provides the first evidence that melanopsin is a functional sensory photopigment. In other words, this protein absorbs light and sets off a chain of chemical reactions in a cell that triggers an electrical response. The study also showed that melanopsin plays this role in ganglion-cell photoreceptors, helping them send a powerful signal to the brain.

The team made the discovery by inserting melanopsin into cells taken from the kidneys and grown in culture. These cells, which are not normally sensitive to light, were transformed into photoreceptors when flooded with melanopsin. In fact, the kidney cells responded to light almost exactly the way ipRGCs do, confirming that melanopsin is the photopigment for ganglion-cell photoreceptors.; Source: Brown University Providence