When the World is Colourless
Jülich scientists are investigating the causes of colour blindness
[30. Januar 2004]
Jülich, 30 January 2004 - All cats are grey by night, but they take on their colour again at daybreak. However, people born colour-blind - suffering from achromatopsia - always see the world in shades of grey as in a black-and-white film. It was already known that colour blindess results from genetic mutations. These lead to a dysfunction of certain light-sensitive cells - the cones. Scientists from Research Centre Jülich and the University Eye Hospital in Tübingen have now discovered that an ion channel in cones displays altered properties in people with colour blindness. The researchers suspect that the influx of calcium ions through this channel into the cell is disturbed so that the cones are less light-sensitive. Light stimuli therefore cannot be optimally processed. The findings have been published in the January edition of the "Journal of Neuroscience".
Two groups of photoreceptors are found in the retina of the eye - cones and rods. The cones are responsible for vision in daylight and for colour vision. The rods enable us to see in the twilight. In the case of people who are colour-blind - which should not be confused with red-green colour deficiency - only the rods function properly. The genetically conditioned visual dysfunction is revealed in daylight. Those affected cannot distinguish any, or hardly any colours, suffer from insufficient visual acuity and are extremely sensitive to glare. Intensive light is often painful for these people.
In the eye, the photoreceptors convert light stimuli into electrical signals. To this end, ion channels are located in the cell membranes of the cones and when open admit sodium and calcium ions into the interior of the cells. The channel is controlled by a cellular messenger molecule. In the dark, the concentration of this messenger is high and the ion channels are open. In the light, the messenger concentration is reduced, the ion channels close and the influx of charged particles ceases. The electrical voltage at the membrane thus also changes. Finally, the cones convert this voltage change into a chemical signal which is further processed by other cells in the retina.
The Jülich scientists Dr Dimitri Tränkner, Dr Reinhard Seifert and Prof. U. Benjamin Kaupp investigated how the properties of the ion channel in persons suffering from colour blindness differs from that in healthy people. The biophysicists from the Institute of Biological Information Processing (IBI 1) are concerned with a special form of achromatopsia that occurs in two sisters. Both sisters can readily recognize and distinguish strong, saturated colours, but they confuse pale colours (pastel shades).
The ion channels in the cones are composed of A and B subunits. Two genes are responsible for the structural plan. Achromatopsia results from mutations in these genes. In cooperation with scientists from the University Eye Hospital in Tübingen, the Jülich researchers have succeeded in modelling the sisters' ion channels. "To this end, we inserted the genes of the healthy and mutated channels in nurse cells and we were then able to investigate the human channels very precisely there", is how Tränkner explains the procedure. "Using a fine glass pipette, I isolated the membrane patches containing the channels, added messenger molecules and measured the electrical currents." The scientists first constructed functional ion channels consisting exclusively of mutated A subunits. The lA-subunit channels had greatly modified properties in comparison to healthy channels. Next, the scientists added the B subunit. With one exception, all themodified properties were neutralized by this building block. "This result surprised us," said Dimitri Tränkner. "All the more so since the channel is composed of three mutated A subunits and only one healthy B subunit. However, this explains why the two sisters only suffer from a mild form of colour blindness."
The outstanding feature of the ion channels from these patients is found in the affinity for calcium ions. Calcium ions are able to pass through the ion channels more rapidly so that the ion current is greater than in normal channels. "We then discussed what effects this property could have on the cones", the biophysicist explained. Calcium is important for photoreceptors since it regulates their sensitivity to light. The calcium balance in the cones is disturbed by this modified property of the ion channel. Furthermore, considerable current fluctuations in the patient's cones also result. In order to overcome this current noise stronger light stimuli are necessary to excite the visual cells. The cones require more light and a greater colour intensity in order to provide colour contrasts. "This could explain why the sisters can only distinguish and identify saturated colours", Dimitri Tränkner summarized the scientists' ideas.
|Coloured tepees - how a healthy person sees them (left) and how they probably appear to the colour-blind sisters: blurred and in pale colours (right).|
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