They allow us to see the world in thousands of colours: red strawberries, green leaves, the blue sky. They also enable us to see all the objects around us clearly. And they allow us to perceive fast movements, such as the rush of a train or the flight of a dragonfly. We're talking about cone opsins – tiny, light-sensitive receptor-proteins in our retina. Often, however, these all-rounders of daylight vision are also involved in retinal diseases. Impairment of cone receptor function, caused by genetic mutations or other degenerative processes, can lead to disorders such as colour blindness and age-related macular degeneration (AMD), a disease affecting the central retina and causing progressive vision loss.
In a new study, Polina Isaikina and Sarah L. Schmidt, two researchers from the Center for Life Sciences at PSI, have succeeded for the first time in determining the three-dimensional structure of human cone opsins in their dark state and showing how their molecular architecture enables their rapid activation by light. This provides important new insights into human vision and its evolution and may offer new starting points for the study of eye diseases that currently lack effective treatment. The study was carried out in collaboration with colleagues at PSI, the Extreme Light Infrastructure in the Czech Republic and the University of Tokyo in Japan and has now been published in the journal Science.
Uneasy companions
Cone opsins are photoreceptor proteins found in the cone cells, which are densely packed in the fovea centralis. This area of the human retina is responsible for sharp vision. We humans have six to seven million cones in each eye. Their receptor proteins are activated by light, triggering a signalling cascade that ultimately produces electrical signals processed by the brain. Because this process is exceptionally fast, cone opsins enable us to track fast-moving objects with our eyes. However, they operate mainly during the day when the light levels are high. In low light, at dusk and at night, their evolutionarily younger relative, the rod opsin in rod cells, takes over this task.
Human colour vision is mediated by three types of cone opsins, each tuned to a different region of the visible spectrum. L cones are most sensitive to red light, M cones to green light, and S cones to blue light. Although there are only three cone types, we see the world in more than just three colours, as our colour perception arises from the interplay of their overlapping spectral sensitivities.
The 3D architecture of cone opsins before they are activated by light, and the reasons behind their exceptionally rapid responses, have been challenging to resolve. These receptors are highly dynamic and can undergo spontaneous activation even in darkness, which makes it extremely challenging to isolate them in a single, well-defined state.
To avoid accidental activation, the researchers worked exclusively under very dim red light in the lab, at wavelengths far outside the sensitivity of cone opsins. "In order to determine the three-dimensional structure of these receptors in their dark state and understand their rapid activation, we had to overcome major technical hurdles." says Polina Isaikina. "We had to combine several advanced approaches, including cryo-electron microscopy, ultrafast laser spectroscopy, biochemical and cellular assays, as well as computational tools that allowed us to design and optimise these receptors for detailed study."
The effort paid off: the research team now presents previously unresolved structures of human cone opsins, in particular the blue- and green-sensitive variants in their inactive states under dark conditions. Although the red cone opsin was not studied directly, its close genetic similarity to the green cone opsin suggests that similar molecular principles are likely to apply.
Manoeuvring room for a molecule
To understand why cone opsins are able to convert light pulses into electrical signals in a flash, it's worth taking a look at their structural organisation. "At the heart of every cone opsin is the so-called retinal, a light-sensitive molecule derived from vitamin A," explains Sarah L. Schmidt, a doctoral candidate and first author of the study.
When light hits the eye, it transfers energy to the retinal, causing it to change its shape. This in turn triggers the activation of the photoreceptor and the generation of an electrical signal to the brain, where visual information is processed. "Our new structural and functional data indicate that cone opsins are optimized for rapid signal transmission," says Schmidt. Their molecular structure includes a network of internal "microswitches" that allow them to connect with their intracellular signalling partner, the transducing G protein. Because this interaction already happens in the resting state, signal transmission can proceed extremely rapidly once the light is absorbed. This molecular readiness helps to explain how cone opsins fulfil the needs of daylight vision.
Another factor contributing to the speed of cone opsins lies in the architecture of the retinal binding site. In the green cone opsin, for example, this retinal binding pocket is relatively open at the entrance and exit. This allows the retinal to be quickly displaced after a light pulse, thus preparing for the next pulse. Such a rapid turnover supports fast updating of visual information in the brain.
The PSI researchers discovered something else: The retinal binding site of the blue-sensitive opsin is more confined, with "closed doors" that effectively restrict retinal movement. As a result, a higher-energy light stimulus is required to induce a shape change in the retinal ligand. Blue light naturally carries more energy than green or red light and is therefore well suited to trigger this transition. In contrast, the retinal in the green-sensitive opsin can move much more freely, allowing the receptor to respond to lower-energy green light and even to activate spontaneously in the absence of light.
Cone opsins as therapeutic targets
The findings of this study may provide a new molecular framework for understanding eye diseases associated with the loss or dysfunction of photoreceptors in the cone cells. Worldwide, hundreds of millions of people live with different types of vision impairments. Colour-vision deficiencies for example affect around 5% of the global population, predominantly males. More severe age-related macular degeneration (AMD) can lead to central vision loss and, in advanced cases, blindness. "Our new findings provide detailed molecular and structural insights into how cone opsins achieve their functions," says Polina Isaikina. "A detailed structural understanding of these mechanisms helps us identify where things go wrong in such diseases and where targeted therapies might be possible."
In the long term, the researchers hope that their results will advance the development of drugs that directly target cone opsins, with the aim of stabilizing their function and slowing vision loss. The new findings from the study also open up possibilities for the development of more precise optogenetic treatments, in which light-sensitive proteins are engineered to restore or modulate cellular signalling.
Text: Andreas Lorenz-Meyer
About PSI
The Paul Scherrer Institute PSI develops, builds and operates large, complex research facilities and makes them available to the national and international research community. The institute's own key research priorities are in the fields of future technologies, energy and climate, health innovation and fundamentals of nature. PSI is committed to the training of future generations. Therefore about one quarter of our staff are post-docs, post-graduates or apprentices. Altogether PSI employs 2300 people, thus being the largest research institute in Switzerland. The annual budget amounts to approximately CHF 450 million. PSI is part of the ETH Domain, with the other members being the two Swiss Federal Institutes of Technology, ETH Zurich and EPFL Lausanne, as well as Eawag (Swiss Federal Institute of Aquatic Science and Technology), Empa (Swiss Federal Laboratories for Materials Science and Technology) and WSL (Swiss Federal Institute for Forest, Snow and Landscape Research).
