This laboratory is appropriately titled Translational Research, as we use inherited retinal degenerations identified in the clinic as both a source of clues about retinal function and dysfunction and a target for research in therapeutic intervention. The broad direction for our laboratory involves the biology of photoreceptor rescue and repair and opportunities to initiate human clinical rescue trials for RP and allied diseases based on animal studies. We have studied a number of mouse and rat models of human retinal degeneration diseases to elucidate the mechanisms of retinal neural signaling deficiencies and degeneration leading to blindness. We use normal rodents and rodents that are genetically altered to mimic human retinal disease to study the characteristics (phenotype), molecular genetics, physiological mechanisms and possible treatments of these inherited retinal degenerations. Our laboratory applies the techniques of light and electron microscopy, immunohistochemistry, biochemistry, and molecular biology to human and animal retinal tissue, as well as the electroretinogram (ERG), ocular coherence tomography (OCT) and behavioral measurements in living animals to access retinal structure and function in ways similar to those used to evaluate human vision in the clinic. These studies address human conditions of retinal and macular degenerations and age-related macular degeneration. Mechanisms of Retinal Degeneration: A critical facet of retinal neurodegenerative disease involves the structural changes, particularly to the photoreceptor outer segments (OS), that precede photoreceptor death, causing loss of vision. As photoreceptor cells undergo primary degeneration through progressive outer segment (OS) shortening in many of these conditions, a critical question is whether the outer segment may exhibit sufficient structural plasticity to support elongation of OS that have been shortened by disease states and whether this would promote survival of the photoreceptor cell. The goal of the work is to investigate the molecules that are important in the regulation of OS length under light stress and genetic degenerative conditions. We are focusing on neurotrophic factors, such as CNTF, and on small molecules that regulate cytoskeletal growth, including Rac1. This year we continue a molecular approach to studying retinal disease mechanisms by investigating the role Rac1 in photoreceptor plasticity and homeostasis in normal and diseased retinas using Rac1 transgenic and conditional knockout mice. Rac1 is a protein that can function as an intracellular molecular switch, which is activated by various types of membrane receptors and produce a variety of downstream biological effects in many different cell types. We use a method call conditional gene targeting to modify the gene for Rac1 to learn about its role in photoreceptors. By this method only the gene in these cells is altered, leaving the Rac1 gene in other cell types unaffected. One of the photoreceptor specific functions of Rac1 in invertebrate photoreceptors is to regulate photoreceptor morphogenesis, and in particular the photoreceptive membrane analogous to outer segments in mammals. This was discovered using conditional gene targeting to produce depletion of Rac1 or constitutive activation of Rac1 in photoreceptors. We showed that conditional knockdown of Rac1 in mouse photoreceptors protected them from cell death resulting from overexposure to light, which indicates Rac1 is involved in one form of oxidative damage in photoreceptors. This may be useful in understanding the mechanisms of some types of inherited or environmental retinal degenerations and in designing treatments.To further explore the role of Rac1 in mammalian photoreceptors, we used conditional gene targeting to make a mouse which expresses a modified form of Rac1 in rod photoreceptors. This transgenic Rac1 was constructed so that its expression in photoreceptors coincided with the major outer segment protein rhodopsin, which begins about postnatal day 4. This allowed us to test its effect on postnatal development. Three lines of mice expressing different levels of this transgenic Rac1 are being studied. By 14 days of age, the amount of modified Rac1 protein in these lines is between 2 times and the level of normal protein. Results so far indicate that the modified Rac1 disturbs the development of the normal laminar structure of the photoreceptor layer and some cell nuclei were mislocalized to the layer on either side of the photoreceptor layer. In addition, the number of photoreceptors was reduced in the medium and high expressing lines by postnatal day 21, but all lines had folds and whorls in the photoreceptor layer with some cells oriented toward the inner retina rather than toward the outer margin formed by the retinal pigmented epithelial cells. The outer segment portion of the displaced cells was either absent or severely shortened. We are now investigating genetic and biochemical identity of the mislocalized cells to determine the pathways by which transgenic Rac1 altered their morphology. This will give us information about the role Rac1 in postnatal retinal layer formation and photoreceptor morphogenesis. Retinoschisnin Function in Photoreceptors: Mutations in the gene for retinoschisin found on the X chromosome cause X-linked retinoschisis (XLRS). XLRS is an inherited retinal disease and is a leading cause of juvenile macular degeneration in human males. The retinoschisin protein is found primarily on the outer membrane of photoreceptor inner segments. However, the role of retinoschisin in photoreceptor function is not known. This year we showed that young mice lacking retinoschisin have a specific defect in how their photoreceptors respond to light. While their electrical response to a light flash measured with the ERG is normal, the process of light activated protein translocation in photoreceptors (the movement of proteins from one compartment of the cell to another) in response to continuous illumination is ten times less sensitive in these mice at a young age than in litter mates who have the retinoschisin protein. This defect is limited to one specific protein, transducin, important for the signaling. When the mice are a few weeks older, however, the light sensitivity of translocation is near normal. Furthermore, during this period, the photoreceptor outer segments in the mice lacking retinoschisin grow from much shorter than normal to near normal. This suggests that the photoreceptors in these mice have a delay in their maturation. Since development and maturation is regulated by a set of proteins called transcription factors, we checked the levels of some of them in retinoschisin null mice. We found that two of them were decreased at young age but were in normal amounts at the older age and that transducin levels were lower and its inactivation higher at young age but not at older age. These results support the idea that photoreceptors lacking retinoschisin have reduced function at young age which is due to a delay in their maturity. This indicates that retinoschisin, a structural protein, may be linked to photoreceptor function involving light signaling by its interaction photoreceptor growth processes.
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