Evidence for Distinct Cholesterol Domains in Fiber Cell Membranes from Cataractous Human Lenses

Abstract

Previous studies in our laboratory have provided direct evidence for the existence of distinct cholesterol domains within the plasma membranes of human ocular lens fiber cells. The fiber cell plasma membrane is unique in that it contains unusually high concentrations of cholesterol, with cholesterol to phospholipid (C/P) mole ratios ranging from 1 to 4. Since membrane cholesterol content is disturbed in the development of cataracts, it was hypothesized that perturbation of cholesterol domain structure occurs in cataracts. In this study, fiber cell plasma membranes were isolated from both normal (control) and cataractous lenses and assayed for cholesterol and phospholipid. Control and cataractous whole lens membranes had C/P mole ratios of 3.1 and 1.7, respectively. Small angle x-ray diffraction approaches were used to directly examine the structural organization of the cataractous lens plasma membrane versus control. Both normal and cataractous oriented membranes yielded meridional diffraction peaks corresponding to a unit cell periodicity of 34.0 Å, consistent with the presence of immiscible cholesterol domains. However, comparison of diffraction patterns indicated that cataractous lens membranes contained more pronounced and better defined cholesterol domains than controls, over a broad range of temperature (5-40°C) and relative humidity (52-92%) levels. In addition, diffraction analyses of the sterol-poor regions of cataractous membranes indicated increased membrane rigidity as compared with control membranes. Modification of the membrane lipid environment, such as by oxidative insult, is believed to be one potential mechanism for the formation of highly resolved cholesterol domains despite significantly reduced cholesterol content. The results of this x-ray diffraction study provide evidence for fundamental changes in the lens fiber cell plasma membrane structure in cataracts, including the presence of more prominent and highly ordered, immiscible cholesterol domains.

Lipid Peroxidation Induces Cholesterol Domain Formation in Model Membranes

“…Direct analysis of membranes isolated from early-stage cataractous lenses revealed that cholesterol domains were more pronounced and more stable than those of normal lens membranes. This was true despite a significantly lower (ϳ54%) C/P mole ratio (82). It was speculated that these structural changes may be due to modification of the lens membrane by age-related insults, such as lipid peroxidation.…”

Lipids and the ocular lens

“…2 and 12 ). Phospholipid oxidation promotes cholesterol-domain formation ( 198 ), and these domains are distinct from those formed with nonoxidized membranes ( 199 ). The relative amount of cholesterol is lowest in the outer cortical region of the lens where the molar cholesterol-phospholipid ratio is 1:2 ( 197,200,201 ).…”

Proteomic Analysis of Lipid Raft-Like Detergent-Resistant Membranes of Lens Fiber Cells

“…Evidence for immiscible cholesterol-rich and -poor domains in normal lens fiber cell membrane was reported, 14 and cholesterol-rich domains were more pronounced and better defined in cataractous lens. 15 Previously, lipid raft domains were isolated and lipids 13 and caveolin-1 16 content was examined. Sorting of AQP0 and connexins to raft and nonraft domains was also reported.…”