Abstract
The most unique biochemical characteristic of the eye lens fiber cell plasma membrane is its extremely high cholesterol content, the need for which is still unclear. It is evident, however, that the disturbance of Chol homeostasis may result in damages associated with cataracts. Electron paramagnetic resonance methods allow discrimination of two types of lipid domains in model membranes overloaded with Chol, namely, phospholipid-cholesterol domains and pure Chol bilayer domains. These domains are also detected in human lens lipid membranes prepared from the total lipids extracted from lens cortices and nuclei of donors from different age groups. Independent of the age-related changes in phospholipid composition, the physical properties of phospholipid-Chol domains remain the same for all age groups and are practically identical for cortical and nuclear membranes. The presence of Chol bilayer domains in these membranes provides a buffering capacity for cholesterol concentration in the surrounding phospholipid-Chol domains, keeping it at a constant saturating level and thus keeping the physical properties of the membrane consistent with and independent of changes in phospholipid composition. It seems that the presence of Chol bilayer domains plays an integral role in the regulation of cholesterol-dependent processes in fiber cell plasm membranes and in the maintenance of fiber cell membrane homeostasis.
Highlights
The lens has a unique structure consisting of densely packed fiber cells, most of which lose their organelles soon after they are formed [1, 2]
The effects of Chol and Chol bilayer domains (CBDs) on the organization and properties of lens lipid membranes are illustrated by the data obtained using the electron paramagnetic resonance (EPR) spin-labeling methods
Chol content in the human eye fiber cell plasma membranes increases with age and with the depth in the lens. These Chol contents are always high enough to saturate the PL bilayers of these membranes. This is evidenced from results that show the CBD is always present in human lens lipid membranes prepared from the total lipids extracted from clear lens cortices and the nuclei of donors aged 0–70 years
Summary
The lens has a unique structure consisting of densely packed fiber cells, most of which lose their organelles soon after they are formed [1, 2]. The most unique biochemical characteristic of the eye lens fiber cell plasma membrane is its extremely high cholesterol (Chol) content. Human studies are not clear; some of them have found an increased risk of cataracts in statin users [19, 20], while others show beneficial effects of statins that decrease the risk of cataracts [21, 22] or no effects at all [23, 24] All these studies indicate, that Chol plays an important physiological role in the eye lens. The main hypothesis of this review is that the high Chol content in the fiber cell plasma membranes of the eye lens, and the presence of the CBDs, play a significant function in maintaining lens transparency and, in protecting against cataract development. The effects of Chol and CBDs on the organization and properties of lens lipid membranes are illustrated by the data obtained using the electron paramagnetic resonance (EPR) spin-labeling methods.
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