Rods and cones, the major light receptor cells in the mammalian eye, are densely packed across the retina where they convert light into the image-forming signals that allow the brain to create vision. Rods are low-light photoreceptors, allowing us to perceive shapes in black and white and to navigate in dimly lit conditions. Cones, although they account for only 3 - 5% of retinal photoreceptors, are responsible for daylight sight, colour vision and visual acuity.
Scientists have long known that cones depend on rods for survival, and that when rods die from retinal degenerative diseases such as retinitis pigmentosa, cones next to them die, too, resulting in a gradual but ultimately complete loss of human vision. Now, RPB-supported scientists have shown that dying cone cells can trigger the death of healthy rod cells, the first time that this has been shown to happen the other way.
The study, published in the journal PLOS Genetics, was conducted by Kyoung-in Cho, PhD, and colleagues working in the lab of Paulo Ferreira, PhD, an RPB Jules & Doris Stein Professor. They showed that when a multifunctional protein known as 'Ran-binding protein 2' (RanBP2) is removed from cone cells it causes not only those cells to die but also results in the demise of the healthy rod cells in the eye. The authors also showed that the rod and cone cells undergo a cascade of distinct and unusual events as they are dying. According to the researchers, the demise of these cells can play a part in diseases such as retinitis pigmentosa and age-related macular degeneration (AMD).
"Our studies provide a paradigm-shift in our thinking about the development of retinal diseases," says Ferreira. "Ranbp2 clearly plays a role in mediating the viability or function of retinal neurons and so becomes a prospect for therapeutic interventions."
With support from RPB, other investigators are developing ways of either stimulating the natural factors that promote cone survival or blocking the rod signals that initiate cone death. For patients with advanced retinal disease, RPB scientists are developing stem cell therapies to replace cones.
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June 21, 2013