Research: SEEING THE LIGHT

Professor Mark Hankins and colleagues at Imperial College

London and the University of Manchester have shown that melanopsin, a protein present in ganglion cells in the eye, is able to make mouse cells sensitive to light. Melanopsin works in a similar way to invertebrate photoreceptors, prompting questions about how ganglion cells evolved.

For a while it has been clear that a light-sensing system exists in the eye, separate from the rod and cone cells needed for vision. A small number of non-vision ganglion cells at the back of the eye respond to light and are required for setting the body’s internal clock (its circadian rhythms).

The retinal ganglion cells contain melanopsin, which has the hallmarks of a light-sensitive protein. To see if melanopsin could make cells responsive to light, Professor Hankins and colleagues inserted the gene for melanopsin into cultured mouse cells. The modified cells expressed the protein and generated electric currents when illuminated, confirming that melanopsin is a genuine photoreceptor protein.

The induced currents were dependent on the presence of particular forms of retinaldehyde, a chemical needed for light detection in the rods and cones. As with these photoreceptors, melanopsin requires cis-isomers of retinaldehyde to function. However, there was also some light-sensitivity when all-trans-retinaldehyde was added, suggesting that melanopsin is able to convert it into the 11-cis form – something seen only in invertebrates before.

External links

• Melyan Z et al. Addition of human melanopsin renders mammalian cells photoresponsive. Nature 2005;433(7027):741–5.