Direct Evidence for Immiscible Cholesterol Domains in Human Ocular Lens Fiber Cell Plasma Membranes

Abstract

The molecular structure of human ocular lens fiber cell plasma membranes was examined directly using small angle x-ray diffraction approaches. A distinct biochemical feature of these membranes is their high relative levels of free cholesterol; the mole ratio of cholesterol to phospholipid (C/P) measured in these membranes ranges from 1 to 4. The organization of cholesterol in this membrane system is not well understood, however. In this study, the structure of plasma membrane samples isolated from nuclear (3.3 C/P) and cortical (2.4 C/P) regions of human lenses was evaluated with x-ray diffraction approaches. Meridional diffraction patterns obtained from the oriented membrane samples demonstrated the presence of an immiscible cholesterol domain with a unit cell periodicity of 34.0 Å, consistent with a cholesterol monohydrate bilayer. The dimensions of the sterol-rich domains remained constant over a broad range of temperatures (5-20°C) and relative humidity levels (31-97%). In contrast, dimensions of the surrounding sterol-poor phase were significantly affected by experimental conditions. Similar structural features were observed in membranes reconstituted from fiber cell plasma membrane lipid extracts. The results of this study indicate that the lens fiber cell plasma membrane is a complex structure consisting of separate sterol-rich and -poor domains. Maintenance of these separate domains may be required for the normal function of lens fiber cell plasma membrane and may interfere with the cataractogenic aggregation of soluble lens proteins at the membrane surface.

Lipids and the ocular lens

“…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 ). This is a very high ratio considering a typical membrane such as that of the sarcoplasmic reticulum of muscles contains 0.08 mol of cholesterol/mole phospholipid ( 100 ).…”

“…Such a high ratio leads to the formation of pure cholesterol domains ( Fig. 12 ) ( 195,197 ). These domains are actually cholesterol bilayers, different from the rafts that contain laterally segregated phospholipids and are enriched in cholesterol ( Figs.…”

Effects of HMG-CoA Reductase Inhibitors on Endothelial Function

“…23,24 Certain drugs that inhibit cholesterol biosynthesis have been shown to induce cataract development in animal models as a result of reducing essential levels of lens membrane cholesterol. [25][26][27] …”

“…The only mammalian cell type that contains cholesterol microdomains under physiological conditions is the fiber cell of the human ocular lens. 22,23 The plasma membrane of the fiber cell contains molar ratios of Figure 1. Schematic illustration of vessel wall and endothelial cell membrane changes during hyperlipidemia.…”

Membrane Microdomains and Vascular Biology

“…11 The stability of membrane cholesterol domains is dependent on numerous factors, including temperature, membrane cholesterol-to-phospholipid mole ratio, composition of the surrounding phospholipid acyl chains (eg, degree of saturation), and the extent of lipid peroxidation. 8,11,12 …”