National Plan for Eye and Vision Research
Lens and Cataract Program
In contrast to the cellular and molecular complexities present in most other tissues, the lens is a relatively simple system, composed of a single layer of metabolically active epithelial cells that differentiate into quiescent, but structurally highly differentiated, fiber cells. The ease of obtaining lens epithelial and fiber cells and the relative molecular simplicity of the fully differentiated fiber cells make the lens one of the best tissues to use in the study of events that control aging. Likewise, the lens provides an accessible system for studying the fundamental aspects of embryonic induction.
Nonetheless, it is the transparent properties of the lens and its ability to focus light that present some of the most clinically relevant challenges in eye research. Cataract is an opacity in the normally clear lens that interferes with vision and is by far the most serious problem associated with the lens. The World Health Organization cites cataracts as the leading cause of blindness worldwide. In the United States, cataracts affect an estimated 20.5 million or about one in six Americans older than age 40 years. By the age of 80 years, over one-half of all Americans have cataracts. In spite of readily available effective cataract surgery, cataracts account for a significant amount of vision impairment, particularly among older Americans with limited financial resources. The goals of early detection, prevention, and universal treatment of cataract are central to the Lens and Cataract Program, and advancing these goals requires increased understanding of the basic molecular processes occurring in the normal and cataractous lens.
Most people in midlife face another problem associated with the lens—presbyopia, the loss of the ability of the lens to focus on near objects (known as accommodation). By understanding changes in the physical properties of the normal lens and its surrounding support structures as a function of age, it may be possible to develop treatments that delay or prevent presbyopia.
Developmental defects in the lens are a major cause of blindness and visual impairment among children. Since many of the pathways required for formation of the lens are also important for lens maintenance, a detailed understanding of lens development will provide a rational basis for the treatment of childhood cataract and could shed light on the diseases of aging associated with the lens. Since lens formation is critical to eye development, these studies will help explain the etiology of common congenital eye malformations such as microphthalmia.
The Lens and Cataract Program objectives listed in this Strategic Plan have been selected with the understanding that basic lens physiology will provide the framework for learning more about the mechanisms involved in presbyopia and cataract, as well as developmental anomalies associated with the lens, thereby allowing researchers to develop more effective treatments.
One of the major highlights in cataract research in the past 5 years was the definitive establishment of an association between smoking and cataract formation. Recent studies also confirm that cataract development may be delayed by protection from ultraviolet ray exposure, for example, by wearing sunglasses and a hat with a brim.
In the area of molecular genetics, a number of new genes for hereditary cataracts have been mapped in humans and laboratory mice. These include some expected genes, such as those encoding lens crystallins, and unexpected genes, such as those encoding membrane proteins and cytoskeletal components. This accomplishment was significantly driven by expanded public genomic DNA and mouse resources, including sequence and mapping reagents and murine cataract alleles recovered from ethylnitrosourea mutagenesis screens.
This work has provided insight into underlying molecular mechanisms leading to opacification. Knowledge that the functional inactivation of genes—whose activity is mainly required during early stages of lens formation—also leads to cataract formation due to their downstream effects in structural lens fiber genes such as crystallins has brought researchers closer to a mechanistic understanding of certain types of cataract. In addition, new genes have been identified for hereditary malformations of the anterior chamber, including Rieger's anomaly, anterior segment mesodermal dysgenesis, and anophthalmia/microphthalmia.
Pioneering studies using molecular, cellular, and whole-animal approaches have resulted in significant progress in defining the contribution of the crystallins to the function of the lens and to the long-term maintenance of its optical properties. Consistent with molecular and cellular studies demonstrating a critical role for α-crystallin in maintaining lens transparency, mutants of human αA and αB crystallins have been genetically linked to autosomal dominant cataract. Laboratory mice in which the αA crystallin gene has been disrupted have smaller lenses and develop opacity shortly after birth. The nonlenticular role of the crystallins has been highlighted by the identification of mutant αB crystallin associated with desmin-related myopathy, the implication of the involvement of the β-crystallins in development, and the discovery of the expression of the crystallin in the retina.
Investigations into the molecular, structural, and functional properties of α-crystallin have confirmed its role as a molecular chaperone, or "sensor," of protein stability that recognizes early events in protein unfolding, such as those brought about by age-related damage. Many types of cataract have been traced to protein aggregation and failures of the chaperone machinery. In this regard, they share molecular characteristics with leading aging pathologies, such as Parkinson's disease, Alzheimer's disease, and amyotrophic lateral sclerosis. Advances in the understanding of cataracts and the development of therapeutic strategies will likely have a far-reaching impact that transcends the lens.
Key to understanding lens function is an understanding of the controls of lens epithelial cell proliferation and differentiation into fiber cells, a process that begins during development and continues throughout life. Advances in the past 5 years demonstrate that control of lens epithelial cell proliferation and differentiation into fiber cells is complex and requires the coordination of multiple growth factor signaling pathways, including those for fibroblast growth factor (FGF), bone morphogenetic protein, and transforming growth factor-beta (TGFβ). These growth factor signaling pathways regulate the activity of cell cycle control proteins (including Rb, E2F, and the cyclin-dependent kinases and their inhibitors), which in turn controls epithelial cell proliferation and fiber cell differentiation. Their importance in understanding lens defects is emphasized by the involvement of FGF and TGFβ signaling in various types of cataracts.
The availability of genetically altered mouse models has contributed greatly to understanding lens cell cycle regulation and the differentiation of epithelial cells to fiber cells. The foundation of these studies comes from advances in identifying the promoter elements within genes expressed in the lens. Studies of the α and γ crystallin promoters, as well as those from developmentally expressed genes such as Pax-6, have provided critical insights into the structure of promoter elements, which in turn has led to the construction of promoters that allow for targeted tissue and cellular expression of genes that regulate critical functions in the lens.
Optical clarity not only must be achieved by the unique differentiation of lens cells but also must be maintained for decades after the differentiation process is complete. Identification of mutations in human lens fiber cell cytoskeletal genes, combined with information from studies on genetically engineered mice, establishes that some elements of the lens cytoskeleton are not required to achieve optical clarity but are required to maintain it. These studies suggest that the lens has evolved specific mechanisms with which to resist cataractogenic pressures associated with aging in these postmitotic cells.
Because the lens is avascular, cell-to-cell communication via gap junctions is essential for the maintenance of homeostasis. This is revealed by mutations in human lens fiber connexins, the protein components of gap junctions, resulting in congenital cataracts. Supporting these findings in humans is the demonstration that disruption of connexin genes in mice also results in cataracts, providing experimental models for study. Surprisingly, deletion of one of the fiber connexins, Cx50, results in smaller lenses in the mouse and an accompanying microphthalmia due to a slowing of the cell division rate. Deletion of the other fiber connexin, Cx46, does not have this effect on lens cell growth. Replacement of Cx50 with Cx46 results in rescue of the cataract phenotype, but not slower growth, demonstrating the unique functions of these different gap junction structural proteins. This finding is unique in the area of gap junction study and demonstrates a selectivity provided by the diversity of connexin intercellular channels.
Maintenance of transparency requires that, at the time of differentiation, fiber cells lose their nucleus along with other organelles needed to carry out metabolic processes. Over the past 5 years, the process by which organelles are lost—denucleation—has become better understood because of the recognition that degradation is synchronized and biochemically overlaps with the cell death (apoptotic) cascade.
Defects in development of the lens are a major cause of blindness and visual impairment. Since many of the pathways required for formation of a lens are also important for lens maintenance, a detailed understanding of lens development will provide a sound basis for the treatment of cataracts in both children and adults. In the past 5 years, exceptional progress has been made in defining a gene network critical for early lens development (lens induction) and subsequent lens function. Important findings have confirmed that a single evolutionarily conserved gene, Pax-6, can initiate all of the events required for lens and eye development and demonstrate that the lens is critical for normal development and maintenance of the retina and cornea. The roles of other critical genes in this network were significantly defined, including Prox-1, Sox-2, Maf, Pitx-2, and Pitx-3. These findings were possible because of major breakthroughs in developing new transgenic and conditional knockout lines in both mammalian and nonmammalian model systems, such as those of mouse, zebrafish, and Xenopus. In the past 5 years, this basic developmental framework has converged to a remarkable extent with the clinical genetic discoveries noted above, creating a critical synergy between human genetic analysis and developmental analysis in defining the gene networks required for lens development.