Oct. 23, 2003 Johns Hopkins scientists have discovered that the eye's vision-producing rods and cones cannot tell the difference between their respective light-detecting molecules. The findings appeared in a recent issue of Nature.
At the heart of the researchers' side-by-side comparison is the quest to solve a fundamental mystery of vision: how rods and cones have such different sensitivities to light despite using very similar processes to detect it.
Rods function in near darkness, while rarer cones function in bright light, providing vibrant color vision. In each cell type, the process of forming vision begins when light activates a cell-specific molecule, called a visual pigment, and ends when the cell emits an electrical signal.
To set up the "taste test," the Hopkins researchers created frogs whose rods contained, in addition to their usual pigment, a pigment found only in cones. The researchers expected the rods to treat the two pigments differently -- picking up signals only from its native pigment and spurning the other -- or to behave a little like cones.
"Surprisingly, the cell's response to light was identical regardless of which pigment was activated," says King Wai Yau, Ph.D., professor of neuroscience in Johns Hopkins' Institute for Basic Biomedical Sciences. "It's as though the label of 'rod' pigment and 'cone' pigment is gone. The pigments alone do not explain the cells' functional differences."
Some scientists had speculated that the pigment defines a cell's role in vision, making a rod, a rod or a cone, a cone. Until now, however, no experiments have measured whether starting the process with the "wrong" pigment affects the cell's critical characteristics -- the size and shape of the electrical signals it produces.
Studying individual rods containing both the rod pigment, called rhodopsin, and a cone pigment (called human red cone pigment), the Johns Hopkins scientists discovered for the first time that rod machinery treats both pigments the same. The findings prove that functional differences between rods and cones stem in part from the cellular environments they offer, rather than inherent differences in their pigments, says Yau, who is also a Howard Hughes Medical Institute investigator.
Both pigments detect light by absorbing it and changing their structures in specific ways (called isomerization), thereby triggering events that generate an electrical signal. The pigment molecules then relax and eventually return to their original forms, ready to start the process anew.
Cone pigment relaxes 10 times faster than rod pigment, which led many scientists to assume that this timing difference would explain rods' and cones' different sensitivities. However, the Hopkins team showed that both pigments were "turned off" at the same time when in the same cell, well before either pigment relaxed, says Vladimir Kefalov, Ph.D., a postdoctoral fellow in neuroscience.
The real off-switch turns out to be addition of a phosphate group to the activated pigment, and subsequent binding by a protein called arrestin, says Yingbin Fu, Ph.D., a Howard Hughes postdoctoral fellow in neuroscience. Even though rods and cones each have their own phosphate-adding enzyme, the rod version recognizes the cone pigment as an equally appropriate target, says Yau. In separate experiments using a mutant version of the cone pigment that couldn't be phosphorylated, the rod did in fact produce a longer signal.
Only one inherent characteristic of the cone pigment -- its instability -- seemed to contribute to rods' and cones' sensitivity differences. Unlike rod pigment, cone pigment spontaneously changes its shape even without exposure to light, causing cones to generate false signals that reduces their sensitivity. Through a number of calculations, Kafelov determined that, in primates, this cone pigment "noise" could account for roughly half of the normal sensitivity difference between cones and rods.
The experiments were funded by the National Institutes of Health and the Howard Hughes Medical Institute. Authors on the paper are Kefalov, Fu, Yau and Nicholas Marsh-Armstrong, all of Johns Hopkins. Marsh-Armstrong is also affiliated with the Kennedy Krieger Institute.
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