Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Vision-producing cells fail ’taste-test,’ treat key light-detecting molecules identically

23.10.2003


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.

On the Web:
Nature, Oct. 2, 2003
http://www.nature.com/nature

Joanna Downer | EurekAlert!
Further information:
http://www.hopkinsmedicine.org/
http://www.nature.com/nature

More articles from Life Sciences:

nachricht New catalyst controls activation of a carbon-hydrogen bond
21.11.2017 | Emory Health Sciences

nachricht The main switch
21.11.2017 | Albert-Ludwigs-Universität Freiburg im Breisgau

All articles from Life Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Nanoparticles help with malaria diagnosis – new rapid test in development

The WHO reports an estimated 429,000 malaria deaths each year. The disease mostly affects tropical and subtropical regions and in particular the African continent. The Fraunhofer Institute for Silicate Research ISC teamed up with the Fraunhofer Institute for Molecular Biology and Applied Ecology IME and the Institute of Tropical Medicine at the University of Tübingen for a new test method to detect malaria parasites in blood. The idea of the research project “NanoFRET” is to develop a highly sensitive and reliable rapid diagnostic test so that patient treatment can begin as early as possible.

Malaria is caused by parasites transmitted by mosquito bite. The most dangerous form of malaria is malaria tropica. Left untreated, it is fatal in most cases....

Im Focus: A “cosmic snake” reveals the structure of remote galaxies

The formation of stars in distant galaxies is still largely unexplored. For the first time, astron-omers at the University of Geneva have now been able to closely observe a star system six billion light-years away. In doing so, they are confirming earlier simulations made by the University of Zurich. One special effect is made possible by the multiple reflections of images that run through the cosmos like a snake.

Today, astronomers have a pretty accurate idea of how stars were formed in the recent cosmic past. But do these laws also apply to older galaxies? For around a...

Im Focus: Visual intelligence is not the same as IQ

Just because someone is smart and well-motivated doesn't mean he or she can learn the visual skills needed to excel at tasks like matching fingerprints, interpreting medical X-rays, keeping track of aircraft on radar displays or forensic face matching.

That is the implication of a new study which shows for the first time that there is a broad range of differences in people's visual ability and that these...

Im Focus: Novel Nano-CT device creates high-resolution 3D-X-rays of tiny velvet worm legs

Computer Tomography (CT) is a standard procedure in hospitals, but so far, the technology has not been suitable for imaging extremely small objects. In PNAS, a team from the Technical University of Munich (TUM) describes a Nano-CT device that creates three-dimensional x-ray images at resolutions up to 100 nanometers. The first test application: Together with colleagues from the University of Kassel and Helmholtz-Zentrum Geesthacht the researchers analyzed the locomotory system of a velvet worm.

During a CT analysis, the object under investigation is x-rayed and a detector measures the respective amount of radiation absorbed from various angles....

Im Focus: Researchers Develop Data Bus for Quantum Computer

The quantum world is fragile; error correction codes are needed to protect the information stored in a quantum object from the deteriorating effects of noise. Quantum physicists in Innsbruck have developed a protocol to pass quantum information between differently encoded building blocks of a future quantum computer, such as processors and memories. Scientists may use this protocol in the future to build a data bus for quantum computers. The researchers have published their work in the journal Nature Communications.

Future quantum computers will be able to solve problems where conventional computers fail today. We are still far away from any large-scale implementation,...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Ecology Across Borders: International conference brings together 1,500 ecologists

15.11.2017 | Event News

Road into laboratory: Users discuss biaxial fatigue-testing for car and truck wheel

15.11.2017 | Event News

#Berlin5GWeek: The right network for Industry 4.0

30.10.2017 | Event News

 
Latest News

Previous evidence of water on mars now identified as grainflows

21.11.2017 | Physics and Astronomy

NASA's James Webb Space Telescope completes final cryogenic testing

21.11.2017 | Physics and Astronomy

New catalyst controls activation of a carbon-hydrogen bond

21.11.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>