Abstract
The unusually high levels of saturation and thus order contribute to the uniqueness of human lens membranes. In addition, and unlike in most biomembranes, most of the lens lipids are associated with proteins, thus reducing their mobility. The major phospholipid of the human lens is dihydrosphingomyelin. Found in significant quantities only in primate lenses, particularly human ones, this lipid is so extremely stable that it was reported to be the only lipid remaining in a frozen mammoth 40,000 years after its death. Unusually high levels of cholesterol add peculiarity to the composition of lens membranes. Beyond the lateral segregation of lipids into dynamic domains known as rafts, the high abundance of cholesterol in the human lens leads to the formation of patches of pure cholesterol. Changes in human lens lipid composition with age and disease as well as differences among species are greater than those observed for any other biomembrane. The relationships among lens membrane composition, structure, and lipid conformation reviewed in this article are unique to the mammalian lens and offer exciting insights into lens membrane function. This review focuses on findings reported over the last two decades that demonstrate the uniqueness of mammalian lens membranes regarding their morphology and composition. Because the membranes of human lenses do undergo the most dramatic changes with age and cataractogenesis, the final sections of this review address our current knowledge of the unusual composition and organization of adult human lens membranes with and without opacification. Finally, the questions that still remain to be answered are presented.
Highlights
The unusually high levels of saturation and order contribute to the uniqueness of human lens membranes
These membranes serve as impermeable barriers to cations and as a matrix for the major lens membrane proteins, aquaporin (AQPO), plasma membrane Ca2+-ATPase (PMCA), and Na, K-ATPase that are necessary for the control of lens water, calcium, sodium, and potassium homeostasis, respectively, all of which are required for maintenance of lens clarity
Membranes of adult human lenses are some of the most saturated, ordered membranes in the human body, and their high level of cholesterol leads to the formation of patches of pure cholesterol bilayers (Fig. 2 and “Cholesterol” section)
Summary
The purpose of the lens is to focus light onto the retina in the back of the eye. The lens is avascular, avoiding light scattering, and is in a hypoxic environment, containing less. Light passing through a human lens traverses through approximately 2,800 cellular membranes. The most abundant phospholipid is dihydrosphingomyelin, found in significant quantities only in primate lenses, human ones (Fig. 1 and “Lipid compositional changes in human lens membranes with age and cataract”). Epithelial cells in the equatorial region differentiate into fiber cells (Fig. 3). Packing diminishes, and intracellular organelles are lost (Fig. 4, blue dots) for cells in the remodeling zone and the adjacent transition region. At these zones, compounds such as red-dextran cannot permeate. As a consequence of the loss of organelles, turnover of cell membranes and proteins does not take place in the deeper fibers (nucleus). The lack of cell turnover contributes to the increase in size and weight of the lens with age
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