Direct evidence for cholesterol crystalline domains in biological membranes: role in human pathobiology.

This review will discuss the use of small-angle X-ray diffraction approaches to study the organization of lipids in plasma membranes derived from two distinct mammalian cell types: arterial smooth muscle cells and ocular lens fiber cells. These studies indicate that cholesterol at an elevated concentration can self-associate and form immiscible domains in the plasma membrane, a phenomenon that contributes to both physiologic and pathologic cellular processes, depending on tissue source. In plasma membrane samples isolated from atherosclerotic smooth muscle cells, the formation of sterol-rich domains is associated with loss of normal cell function, including ion transport activity and control of cell replication. Analysis of meridional diffraction patterns from intact and reconstituted plasma membrane samples indicates the presence of an immiscible cholesterol domain with a unit cell periodicity of 34 A, consistent with a cholesterol monohydrate tail-to-tail bilayer, under disease conditions. These cholesterol domains were observed in smooth muscle cells enriched with cholesterol in vitro as well as from cells obtained ex vivo from an animal model of atherosclerosis. By contrast, well-defined cholesterol domains appear to be essential to the normal physiology of fiber cell plasma membranes of the human ocular lens. The organization of cholesterol into separate domains underlies the role of lens fiber cell plasma membranes in maintaining lens transparency. These domains may also interfere with cataractogenic aggregation of soluble lens proteins at the membrane surface. Taken together, these analyses provide examples of both physiologic and pathologic roles that sterol-rich domains may have in mammalian plasma membranes. These findings support a model of the membrane in which cholesterol aggregates into structurally distinct regions that regulate the function of the cell membrane.