Physiological effects of light on mammals: role of the novel melanopsin-containing retinal ganglion photoreceptors .
To enjoy the beauty of a spring blossom or the fascinating fall colors, we depend upon the visual discrimination of colors, brightness, and contrast. This type of vision requires the classical photoreceptor cells, known as rods and cones, and their respective visual pigments. The light-absorbing pigments, composed of a protein moiety (opsin, which is a G-protein coupled receptor) and a vitamin-A-based chromophore (11-cis-retinal), transduce light first into a chemical signal, and eventually into an electrical signal in the rods and cones. However, the eyes have other light dependent physiological functions that do not depend on image formation in the retina. These functions include adjusting our internal body clocks to the outside solar day, constricting our pupils to control the light input and direct light effects on our behaviors that affect alertness and even mood. Previously, the cells within the retina responsible for transducing light for these collectively called non-image-forming (NIF) functions were not known.
Research from several laboratories uncovered a third type of photoreceptor cell in the mammalian retina. These cells express the photopigment melanopsin first identified by Ignacio Provencio and colleagues, and were shown to be intrinsically photosensitive by David Berson and colleagues. Robert Lucas and colleagues were the first to show conclusively that the melanopsin cells absorb light maximally at different wavelength than those of rods and cones. We in close, fruitful and continuing collaborations with Robert Lucas and David Berson have shown that these cells target specifically non-image-forming centers in the brain including the suprachiasmatic nucleus (center for the circadian pacemaker), and the olivary pretectal nucleus (the area responsible for pupil constriction) among many others. The main purpose of our research is to understand both the mode of action of these newly identified photoreceptors, and the individual contributions of the rods, cones and these novel photoreceptors in signaling light for non-image-forming visual functions.
To understand how these new photoreceptors function, we have genetically engineered mice that lack the protein that resembles the visual pigments found in these cells, a molecule called melanopsin. We found that melanopsin, an opsin-like protein, is absolutely necessary for the ability of these new photoreceptor cells to detect light. Concomitant with removing melanopsin, we added an enzyme, beta-galactosidase, to the mice. This enzyme stains the cells that would normally express melanopsin, with a blue color. These new photoreceptor cells are actually a subset of retinal ganglion cells, neurons that directly connect to the brain through their axons. To trace these axons to their brain targets, we attached a peptide moiety, tau, to the beta-galactosidase protein that allowed the fusion protein, tau-beta-galactosidase, to be transported down the length of the axon. This allowed us to not only stain the cell bodies of the melanopsin cells but also their axons and their eventual targets in the brain.
We have found that these new cells, with the rods and cones, are the only photoreceptor cells in the retina that signal light for an array of non-image-forming visual functions including pupillary constriction, and adjustment of the biological clock. These results were determined by using mutant mice that lack rod and cone functions, in addition to lacking melanopsin. Using various mouse lines that have one or two types of photoreceptors disabled but retain the remainder will enable us to assign the relative importance of each system in conveying light information to the brain for each specific non-image visual function. We also want to know whether rods and cones connect to brain centers for non-image-forming visual functions exclusively through the melanopsin-expressing retinal ganglion cells. To this end, we have used a genetic technique to deliver a Diphtheria toxin specifically to melanopsin expressing cells. With this tool, we can ask how the loss of these cells impacts the non-image-forming visual functions. Can we create an animal that can see images but is not able to detect light for non-image-forming visual functions? Finally, there is currently very little understood about the development of these cells and how they detect light, we are currently pursuing this line of research as well.