The Ocular Lens: Normal Biology and Cataractogenesis

National Eye Institute

Investigators

Linked publications & trials

Abstract

The Lens and Cataract Biology Section investigates aspects of the normal biology of the ocular lens and the molecular mechanisms underlying cataractogenesis, with the ultimate goal being the development of therapeutic modalities to prevent or delay cataract development. One model system we use to study the induction and prevention of cataract is the organ cultured lens. Dr. Madhumita Ghosh is studying the effects of extended culture on the rat lens and has found that young rat lenses, actively growing in vivo, cease to grow when placed in organ culture, although they remain transparent and metabolically active for 1-2 weeks. Using bromodeoxyuridine (BrdU) labeling she determined that proliferation of epithelial cells was preceding normally in the cultured lenses, but that differentiation of lens fibers was apparently arrested. This conclusion was consistent with the lack of growth, and supported by abnormal histological findings and a specific decrease in the synthesis of lens crystallins in the cultured lenses. We are currently investigating whether lens organ culture can be used as a model system for probing the factors and signaling pathways which are important in the process of differentiation of lens epithelial cells into fibers. This complex process is important not only because it is a critical and poorly understood aspect of the normal biology of the lens, but also because mutations affecting function of the relevant factors and pathways are known to lead to cataracts. The amount of water, its localization, and its extent of association with proteins is critical to the transparency of the lens. Syneresis is the process whereby water is released from the hydration layer of macromolecules and becomes free water; entry of free water into the hydration layer is inverse syneresis. Dr. Fred Bettelheim has utilized NMR relaxation studies to measure changes in free and bound water in lenses exposed to different hydrostatic pressures. He found that human lenses, as well as lenses from other species, can respond to changes in hydrostatic pressure via a reversible syneretic process. This is potentially important because the lens is repeatedly exposed to changes in hydrostatic pressure during accommodation. Since the hydrostatic pressure on the lens surface has an equilibrium counterpart in osmotic pressure it produces a tendency for water fluxes between the lens and its surroundings. For the lens, having the capacity to counteract and prevent such water movement by modulating the amounts of free and bound water could be critical to the maintenance of transparency. Dr. Bettelheim has recently demonstrated that lenses do respond to pressure change in the physiological range; i.e. pressure changes of the magnitude reported to occur during accommodation. We also have evidence that the capacity to respond to pressure changes decreases with age in the human lens and that this loss is associated with cataract formation. Studies will soon begin to compare responses to pressure in clear and opaque areas of human lenses with early cataracts.