National Plan for Eye and Vision Research
The retina is a complex tissue in the back of the eye that contains specialized photoreceptor cells called rods and cones. The photoreceptors connect to a network of nerve cells for the local processing of visual information. This information is sent to the brain for decoding into a visual image. The adjacent retinal pigment epithelium (RPE) supports many of the retina's metabolic functions.
The retina is susceptible to a variety of diseases, including age-related macular degeneration (AMD), diabetic retinopathy (DR), retinitis pigmentosa (RP) and other inherited retinal degenerations, uveitis, retinal detachment, and eye cancers (ocular melanoma and retinoblastoma). Each of these can lead to visual loss or complete blindness.
The leading cause of visual loss among elderly persons is AMD, which has an increasingly important social and economic impact in the United States. As the size of the elderly population increases in this country, AMD will become a more prevalent cause of blindness than both DR and glaucoma combined. Although laser treatment has been shown to reduce the risk of extensive macular scarring from the "wet" or neovascular form of the disease, there are currently no effective treatments for the vast majority of patients with AMD.
DR is also a major cause of blindness. In the proliferative stage of the disease, newly formed, abnormal blood vessels can break through the retinal surface and hemorrhage into the normally transparent, gelatin-like vitreous in the middle of the eye. Scar tissue may subsequently form and pull the retina away from the back of the eye, causing a retinal detachment to occur. Laser treatment (laser photocoagulation) is a highly effective clinical tool for treating proliferative retinopathy.
The inherited retinal degenerations, typified by RP, result in the destruction of photoreceptor cells and the RPE. This group of debilitating conditions affects approximately 100,000 people in the United States.
One of the major achievements in biology has been defining the cellular events involved in the process of visual transduction—the process that captures light by photoreceptor cells and initiates the electrical signals utilized by the brain in processing visual information. This is now a classic model of how signal processing works in other systems. Advances in understanding visual biochemistry have yielded important new insights into the causes of retinal diseases.
The brain decodes and interprets the visual images that we perceive when electrical impulses generated within the retina are transmitted by ganglion cells via the optic nerve to the visual cortex of the brain. The tools of modern neurobiology offer the potential to understand both light adaptation (sensitivity to varying light levels) and inactivation (turning off the sensitivity to light). A central unanswered question in neurobiology is how the complex retinal network enables the formation of images and the discrimination of colors.
After a thorough evaluation of the entire Program, the Retinal Diseases Panel recommends the following goals for the next 5-year period:
- With regard to macular degeneration, understand the molecular and biochemical bases for its different forms, improve early diagnosis, characterize environmental effects on its etiology, and develop new treatments.
- Understand the pathogenesis of DR and other vascular diseases of the retina and develop strategies for primary prevention and improved treatment.
- Identify the genes involved in both inherited and retinal degenerative diseases (including RP), determine the pathophysiological mechanisms underlying these mutations, and determine new potential therapeutic strategies for treatment such as gene transfer, tissue and cell transplantation, growth factor therapy, and pharmacological intervention.
- Establish the causes and etiology of uveitis and improve methods for its diagnosis, treatment, and prevention.
- Use both molecular and physiological approaches to study light adaptation in photoreceptors, with particular emphasis on the visual cycle, and identify the post photoreceptor neural components of adaptation.
- Build on knowledge gained from retinal neuroscience to understand how retinal networks process visual images, a central unanswered question of modern neurobiology.
- Understand the genetic, cellular, and immunologic changes characterizing eye cancers and develop innovative methods of diagnosis and treatment.
The Age-Related Eye Disease Study (AREDS), in which nearly 5,000 Americans at high risk for AMD were prescribed high doses of zinc and three antioxidant vitamins (C, E, and beta-carotene), has revealed encouraging effects. The treatment lowered the risk of developing advanced AMD by 25 percent and reduced the risk of vision loss caused by advanced AMD by 19 percent. AREDS is a powerful example of the importance of large-scale clinical trials and a strong indicator of the possible role of oxidative stress as a contributory factor in AMD and other ischemic retinopathies.
The identification of receptors for the binding and ingestion of spent rod outer segments by the RPE has been a long-awaited finding of considerable importance. It has long been suspected that dysfunction in outer segment phagocytosis by the RPE causes retinal degeneration and blindness. The identified receptors are also utilized for the uptake of apoptotic cells by other phagocytes. Therefore, the phagocytic mechanism of the RPE belongs to a group of related clearance mechanisms that share common elements. This is a major conceptual advance.
The most important development of recent decades in the field of visual transduction is the production of a rhodopsin crystal structure. This important achievement should make it possible to understand the molecular basis of some visual dysfunctions at a level enabling the design of strategies for cures.
The disruption of a number of enzymes and binding proteins involved in the metabolism and transport of retinoids was shown to cause visual dysfunction. The gene RPE-65 was shown to play a critical role in retinoid metabolism and to be essential for the production of 11-cis-retinol, the precursor for the photopigment 11-cis-retinal. Mutations in this gene were discovered in humans and in dogs. This then culminated in a dramatic and successful, adeno-associated virus-vectored gene replacement therapy. The NEI has capitalized on this event by funding additional preclinical investigations intended to take this gene-based therapy into human clinical trials.
It had been assumed that the pathway for regeneration of the visual pigment 11-cis-retinal was the same in both rods and cones. This has been shown not to be the case; recent research indicates that rods and cones use different pathways. Three new enzymatic activities in cone-dominant retinas were discovered that represent a novel retinoid cycle that is apparently required, since visual pigment bleaching in sunlight greatly exceeds the calculated maximal rate of conversion of all-trans-retinal to 11-cis-retinal in the known retinoid cycle. The newly discovered retinoid cycle takes advantage of biochemical pathway-sharing between Müller cells and cones. This new pathway is 20 times faster than the rod cell pathway and selectively supplies cones with 11-cis-retinal, eliminating competition with rod cells for chromophore.
The field of protein kinesis (protein trafficking) has undergone dramatic advances during the past 5 years. This biological process is vital for all eukaryotic cells, but understanding the process has important implications in the retina. In the highly specialized and polarized photoreceptors, rhodopsin moves from its site of synthesis to the outer segment discs of the photoreceptors, where it is available to transduce light into a signal. Along with trafficking, the translocation of proteins involved in the process of phototransduction in light/dark adaptation and in a circadian rhythm is an important finding that will help unravel the complex regulation of photoreception.
Molecular genetic studies have shown that ABCR gene mutations are the cause of recessive Stargardt disease. A major breakthrough came with the development of an ABCR knockout mouse. This provided a convenient laboratory model for a systematic study of mechanisms underlying recessive Stargardt disease and resulted in fundamental new insights into the retinoid cycle. As a result, scientists are gaining insight into new players and potential new candidate disease genes.
There have been a number of notable accomplishments in retinal neurobiology in recent years. These include detailed biophysical characterization of the mechanisms of synaptic transmission at ribbon synapses, improved understanding of the processing of contrast signals in the retina, and important insights into circadian signaling.
A nonvisual imaging pathway in the central nervous system (CNS) has been identified on the basis of the initial discovery of a subtype of ganglion cell expressing a photopigment. These photoreceptive ganglion cells have been demonstrated to mediate entrainment of the circadian clock in the hypothalamus.
An elemental advance in the area of retinal development is the discovery that the order in which cell types are generated is determined in large part by molecular programs intrinsic to multipotent retinal stem cells. Significant progress has been achieved in understanding the interplay between these factors and the microenvironment in cellular determination.
Another important advance is the discovery that multipotent retinal stem cells persist in the pigmented ciliary margin. Similarly, Müller glia in postnatal chicken retinas, long thought to be incapable of regeneration, behave as retinal stem cells, generating new retinal neurons after retinal damage. The possibility that mammalian Müller glia may have a similar ability is raised by recent findings that mammalian retinal stem cells and Müller glia share the expression of many genes.
The development of angiostatic agents for the control of blood vessel growth in retinal vascular disease is important in DR and the wet form of AMD. The critical issue of the underlying vessel loss after angiostatic therapy has not received similar attention. A breakthrough came with the discovery of a subset of systemically administered bone marrow-derived hematopoietic stem cells (HSCs) from mice, which can function as blood vessel progenitors during retinal neovascularization. When HSCs were engineered to express an antiangiogenic, angiogenesis was inhibited; these cells also can rescue and stabilize a vasculature destined to degenerate. A positive trophic effect on photoreceptors resulted from HSC injection into mouse eyes, resulting in their increased survival.
More than 130 genes causing inherited retinopathies in humans have been identified. This makes it possible to identify the cause of RP in approximately 50 percent of patients and the cause of Usher syndrome in 75 percent of patients. Sophisticated analytical techniques such as serial analysis of gene expression (SAGE) have been used to identify over 80 genes that are retina specific or are enriched in the human retina. This genomic information will be useful in identifying candidate genes involved in retinal disease.
Genetically dominant eye diseases are gain-of-function mutations and cannot be corrected by adding a corrective gene. The defective gene first must be inactivated. Progress has been made in this arena by using a mutation-specific, ribozyme-based therapeutic strategy with longstanding rescue in two different lines of transgenic rats, each with its own rhodopsin mutation. Two classes, hammerhead and hairpin ribozymes, were found to be effective.
Evidence implicating an immune component in the etiology of AMD has been added to genetic evidence as the underlying cause of this disease. Research shows that the RPE is replete with the ability to synthesize molecules involved in the immune response. Drusen, pathological deposits that form between the RPE and Bruch's membrane, are a significant risk factor for AMD development. Immune components include dendritic cells and antigen-presenting cells, and local inflammatory responses have been shown to be closely associated with drusen development. Proteomics has been used to confirm the notion that oxidative mechanisms also contribute to drusen formation. Emerging and evolving lines of evidence are shored up by powerful new tools such as deoxyribonucleic acid (DNA) microarray analysis, microscopic imaging, and proteomics.
Progress in a number of new technologies, methodologies, analyses, and resources have facilitated the advancement of research, including the following:
- High-throughput analysis of retinal expression, including microarrays and SAGE, which are powerful methods to generate complete profiles of specific cell types in the adult retina. These techniques show great promise for revealing new candidate genes that underlie retinal stem cell programs, cell type specification, and retinal disease states.
- High-throughput methodologies for proteomics.
- Genome-wide mutagenesis and phenotype-based screening.
- Large-scale generation, at national and international levels, of resources for genetic analyses.
- Animal models, including mouse, Xenopus, and zebrafish for visual system development, visual behavior, and retinal degeneration.
- Transgenic fish and mice expressing green fluorescent protein (GFP) in specific retinal cell types. Transgenic animals that express GFP specifically in retinal ganglion cells, bipolar cells, rods, or cones have been made in the past few years.
- Electroporation methods to introduce plasmid expression vectors encoding genes of interest or small inhibitory ribonucleic acids (siRNAs) into visual pathway cells in vivo, particularly retinal ganglion cells, recently have been developed.
- New methods for achieving loss and gain of function as part of experimental design include the use of morpholinos, conditional knockouts, inducible transgenic animals, and siRNAs.
After carefully considering the research advances that have been made in this Program and on the basis of a careful analysis of current research needs and opportunities, the Retinal Diseases Panel recommends the following laboratory and clinical research objectives:
- Understand the process and control of circadian shedding of photoreceptor outer segments and their phagocytosis by the RPE.
- Analyze the mechanisms underlying light adaptation and recovery following phototransduction and understand the changes in neural coding in light/dark adaptation.
- Continue to reveal the presence and effects of a circadian clock in photoreceptors. Several metabolic functions and the trafficking of proteins may be regulated or influenced by the circadian clock. These influences may present risk factors for AMD and other retinal disorders.
- Understand the cell biology of cones, including outer segment renewal and shedding, the phototransduction cascade, retinoid metabolism, opsin trafficking, and the regulation of gene expression for cone pigments.
- Understand the basic biology of synaptogenesis. The cellular, molecular, and biophysical determinants of synapse formation have important and far-reaching implications for all aspects of vision, including synaptic remodeling, transplantation, and the development of receptive fields.
- Explore the topographical and regional differences in the organization of the retina and the relationship of this topography to disease progression.
- Continue to develop and apply noninvasive technologies such as functional magnetic resonance imaging (fMRI), ocular coherence tomography, adaptive optics, and confocal imaging to better understand retinal function and changes in disease states.
- Develop strategies to enhance retinal ganglion cell regeneration.
- Understand the role of synchronous activity in sensory coding and the anatomic, functional, and genomic components that regulate neural coding (e.g., finding of synchronicity of firing of retinal ganglion cells).
- Understand the pathogenesis of inherited retinal diseases, DR and other vascular diseases, uveitis, and eye cancers and explore new therapeutic strategies for their treatment.
- Explore the role of glia in the maintenance of retinal neuron function, including photoreceptor rescue and neural remodeling.
- Understand the causes and etiology of uveitis and immune modulation in retinal disorders. Identify the factors that dictate the unique properties of intraocular immunity and inflammation and that alter systemic immunity to intraocular antigens. Investigate possible therapeutic approaches, including gene therapy.
- Study the interacting roles of the environment and genetics in risk factors for retinal disease.
- Examine the genetic component of proliferative vitreoretinopathy and retinal detachment, determine the immune mechanisms involved, and develop antiproliferative drugs.
- Explore the pathophysiological heterogeneity of AMD to hasten development of the tools needed for improved diagnosis, prevention, and therapy.
- Further develop and critically evaluate therapies involving gene delivery, growth factors, and transplantation for the treatment of retinal disease.
The Retinal Diseases Panel recognizes the following additional opportunities/resources that will advance retinal research:
- Standardization of the definitions and characteristics of retinal phenotypes in macular disease. This will allow more precise disease definitions based on genotype-phenotype-environment correlations for the study of disease progression and response to therapy.
- Continued funding of large-scale clinical studies and the comparison of diverse groups and the development of standards for reading centers to compare across studies.
- Expansion of the research resources made available through the NEIBank.
- Continued development of animal models and a coordinated system to share animal model data and resources in the vision community.
- Development of diagnostic methods and therapeutic approaches to distinguish among infectious, immunopathogenic, and autoimmune posterior segment intraocular inflammation.