The discovery and characterisation of a third class of photoreceptor in the vertebrate eye.
Little more than a decade ago discussion that the eye might contain a non-rod, non-cone photoreceptor generated either polite amusement or hostile rebuttal by most vision scientists. Since the eye had been the subject of intense research for two centuries it seemed inconceivable that such a system could have been overlooked. The dogma was that all photoreception took place in the rods and cones of the outer retina whilst the cells of the inner retina provide the initial stages of signal processing prior to complex visual processing in the brain. However, two separate lines of study from my group showed that the inner retina also contains photosensory neurones.
Studies in Mammals
In the early 1990’s we used retinally degenerate and transgenic animal models to understand how the circadian system in rodents is regulated by light. The approach used mice which carry gene defects resulting in blindness (greatly diminished or undetectable visual responses) and monitor the impact of this loss on the ability of these rodents to adjust (entrain) their circadian rhythm system to a light/dark cycle. We showed that despite massive rod and cone photoreceptor loss these mice were not only able to entrain their circadian rhythms, but could do so with the same sensitivity as fully sighted animals. These, and a host of subsequent experiments, showed that the processing of light information by the circadian and visual systems was different and suggested that there might be another class of photoreceptor within the eye. But because we could not preclude the possibility that only a very small number of rods and/or cones are required for photoentrainment, we engineered mice (rd/rd cl) in which all the rods and cones were functionally ablated. Circadian entrainment, the regulation of pineal melatonin, and a variety of other responses to environmental irradiance (e.g. pupil constriction), were preserved in rd/rd cl mice and these data provided unambiguous evidence for a third class of ocular photoreceptor, quite different from the rods and cones, within the mammalian retina. To localise these photoreceptors Mark Hankins, Sum Sakaran, Rob Lucas and I utilised the isolated rd/rd cl mouse retina in combination with Ca2+ - imaging techniques. This approach showed that the retina contains a plexus of electrically coupled photosensitive ganglion cells (pRGCs) and that Ca2+ is likely to play an important role in the phototransduction cascade. Most recently this approach has shown that these pRGCs are photosensitive at birth and convey light information to the brain. Parallel studies on therd/rd cl mouse employed action spectroscopy to characterise a novel opsin/vitamin A photopigment (OP) with a maximum sensitivity in the “blue” part of the spectrum (λmax 479nm/ OP479). Although we had deduced the biochemistry of the photopigment, the molecular identity of OP479 remained a mystery. In collaboration with collegues in the USA, notably King-Wai Yau and Samar Hatter we then showed that melanopsin is likely to be this photopigment. Melanopsin is expressed in the pRGCs, and its genetic ablation in mice lacking all functional rods and cones abolishes circadian and pupillary responses to light. The functional properties of melanopsin have been assessed very recently by members of our group (and independently by two other laboratories in the USA) by combining the expression of melanopsin protein with physiological assays of cellular photosensitivity. Remarkably, Mark Hankins has shown that melanopsin can confer photosensitivity to a variety of non-photosensitive cell types (see Melyan et. al. 2005). Our recent and unpublished work has shown that pRGCs regulate multiple areas of physiology including sleep propensity and cardiac function (work in progress). Finally, novel microarray approaches developed by Stuart Peirson and studies in gene knock-out models by Henrik Oster in the group have identified new and unexpected proteins in the melanopsin phototransduction cascade.
Studies in Teleost Fish
In parallel with our studies in mammals, we discovered non-rod, non-cone ocular photoreception in fish by using a very different set of approaches. In 1997 we isolated a novel opsin gene family from teleost fish that we termed the VA (vertebrate ancient)-opsins. This gene encodes a functional VA opsin photopigment and is expressed in a sub-set of retinal horizontal and amacrine/ganglion cells. Since this discovery pre-dated our findings in mammals it represents the first demonstration of a non-rod, non-cone ocular photopigment in any vertebrate. Subsequent collaborative work with Mark Hankins has combined electrophysiological, molecular and anatomical approaches to study the cell biology of these novel retinal photoreceptors. They respond to environmental irradiance, integrate light information from rods and cones, and contain a photopigment with a maximum sensitivity in the “blue” part of the spectrum (λmax 477nm).
Significance of these discoveries
The eye has been considered the best characterised part of the central nervous system. The fundamental questions about the eye were considered answered, with only the details left to resolve. Our discovery of novel ocular photoreceptors in mammals and fish has forced a major reassessment of how the eye processes light information to regulate a variety of different photosensory tasks, and it is likely that much of this work will have important clinical implications. Not least on the classification of human blindness. Ophthalmologists are now beginning to appreciate the full consequences of eye loss, a state that deprives an individual of both their sense of space and time.