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 NYSCF researchers develop novel bioengineering technique for personalized bone grafts
18.07.2018 | New York Stem Cell Foundation

nachricht Pollen taxi for bacteria
18.07.2018 | Technische Universität München

All articles from Life Sciences >>>

The most recent press releases about innovation >>>

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

Im Focus: First evidence on the source of extragalactic particles

For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.

To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...

Im Focus: Magnetic vortices: Two independent magnetic skyrmion phases discovered in a single material

For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.

Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...

Im Focus: Breaking the bond: To take part or not?

Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.

A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...

Im Focus: New 2D Spectroscopy Methods

Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.

"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....

Im Focus: Chemical reactions in the light of ultrashort X-ray pulses from free-electron lasers

Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.

Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Leading experts in Diabetes, Metabolism and Biomedical Engineering discuss Precision Medicine

13.07.2018 | Event News

Conference on Laser Polishing – LaP: Fine Tuning for Surfaces

12.07.2018 | Event News

11th European Wood-based Panel Symposium 2018: Meeting point for the wood-based materials industry

03.07.2018 | Event News

 
Latest News

Machine-learning predicted a superhard and high-energy-density tungsten nitride

18.07.2018 | Materials Sciences

NYSCF researchers develop novel bioengineering technique for personalized bone grafts

18.07.2018 | Life Sciences

Why might reading make myopic?

18.07.2018 | Health and Medicine

VideoLinks
Science & Research
Overview of more VideoLinks >>>