Changes in the Properties and Organization of Human Lens Lipid Membranes Occurring with Age

Abstract

Purpose This research was undertaken to document the changes in the organization and properties of human lens lipid membranes that occur with age. Materials and Methods Human lens lipid membranes prepared from the total lipids extracted from clear lens cortices and nuclei of donors from age groups 0–20 and 21–40 years were investigated. An electron paramagnetic resonance technique and nitroxide spin labels (analogues of phospholipids and cholesterol) were used. Results Two distinct lipid domains, the phospholipid-cholesterol domain (PCD) and the pure cholesterol bilayer domain (CBD), were detected in all investigated membranes. Profiles of the acyl chain order, fluidity, hydrophobicity, and oxygen transport parameter across discriminated coexisting lipid domains were assessed. Independent of the age-related changes in phospholipid composition, the physical properties of the PCD remained the same for all age groups and were practically identical for cortical and nuclear membranes. However, the properties of pure CBDs changed significantly with the age of the donor and were related to the size of the CBD, which increased with the age of the donor and was greater in nuclear than in cortical membranes. A more-detailed analysis revealed that the size of the CBD was determined mainly by the cholesterol content in the membrane. Conclusions This paper presents data from four age groups: 0–20, 21–40, 41–60, and 61–70 years. Data from age groups 41–60 and 61–70 years were published previously. Combining the previously published data with those data obtained in the present work allowed us to show the changes in the organization of cortical and nuclear lens lipid membranes as functions of age and cholesterol. It seems that the balance between age-related changes in membrane phospholipid composition and cholesterol content plays an integral role in the regulation of cholesterol-dependent processes in fiber cell membranes and in the maintenance of fiber cell membrane homeostasis.

Whales, lifespan, phospholipids, and cataracts

“…Bowhead whale lenses have one of the highest levels of cholesterol measured for lenses. Above a molar cholesterol/phospholipid ratio of about two, cholesterol crystalline bilayer domains form (91,92). Because the phospholipid domain of the whale lens membranes is saturated with cholesterol, "the physical properties of the phospholipid bilayer portion of lens lipid membranes constant and independent of changes in the PL [phospholipid] composition that occur with age" (91, p. 722).…”

“…Because the phospholipid domain of the whale lens membranes is saturated with cholesterol, "the physical properties of the phospholipid bilayer portion of lens lipid membranes constant and independent of changes in the PL [phospholipid] composition that occur with age" (91, p. 722). While cholesterol probably plays a minor role in determining lens membrane phospholipid structure (71,91), it is likely to play a role in raft formation (93) and protein aggregation. One purpose of cholesterol in the lens may be to antagonize the binding of -crystallin to lens membranes (94).…”

Saturation Recovery EPR Spin-Labeling Method for Quantification of Lipids in Biological Membrane Domains

“…In the plasma membranes of fiber cells from human and porcine eye lenses, which are dense in integral membrane proteins and have a high Chol content, we were able to discriminate only three of these domains using CW and SR EPR [17–19]. The CBD was not discriminated; even so, this domain was clearly seen in lens lipid membranes prepared from the total lipid extracted from eye lenses [19, 20]. Problems with discriminating the CBD can be related to the fact that properties of Chol-analog spin labels in the CBD are similar to those of Chol-analog spin labels in the surrounding PL bilayer.…”

Factors Determining the Oxygen Permeability of Biological Membranes: Oxygen Transport Across Eye Lens Fiber-Cell Plasma Membranes

“…The phospholipid composition of fiber-cell membranes changes significantly with age and between different regions of the lens. Surprisingly, independent of these differences, profiles of the oxygen diffusion-concentration product across phospholipid domains are very similar in all of the investigated membranes [15]. They have a rectangular shape with an abrupt increase in the oxygen diffusion-concentration product at the C9–C10 position.…”