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. These light-absorbing pigments, composed of a protein moiety (opsin) 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 functions that depend upon light but not upon image formation on the retina. These functions include the setting of our body clocks to the new time imposed by traveling across different time zones. The inability to adjust our body clock leads to prolonged symptoms of "jet-lag." Previously, the cells within the retina responsible for transducing light for these functions were not known. A new type of photodetecting cell in the retina was recently identified. The main purpose of my 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, I 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, I added an enzyme, b-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, I attached a peptide moiety, tau, to the b-galactosidase protein that allowed the fusion protein, tau-b-galactosidase, to be transported down the length of the axon. This allowed me not only to stain the cell bodies of the melanopsin cells but also their axons and their eventual targets in the brain. One of the proposed aims in this grant is to identify all the brain targets that receive innervation from these new photoreceptors. Finding the innervated brain regions could uncover the multitude of functions mediated by these photoreceptors. 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, I 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. I hope to pursue these questions as well.
|Organization:||Johns Hopkins University|