Modification of red cell membrane structure by cholesterol-rich lipid dispersions. A model for the primary spur cell defect

Abstract

Cholesterol-rich membranes are the hallmark of "spur" red cells. Spur cells accumulate cholesterol from cholesterol-rich serum lipoproteins. Previous studies suggested that this added cholesterol is responsible for both the altered morphology and the destruction of spur cells. To examine this process in the absence of other serum factors, cholesterol-lecithin dispersions with varying amounts of unesterified cholesterol (C) relative to phospholipid (P) were prepared, and their influence on normal human red cells was studied. Cholesterol-rich lipid dispersions (C/P mole ration greater 1.0) transferred cholesterol to both red cell membranes and serum lipoproteins, and cholesterol-poor dispersions (C/P mole ration less 1.0) depleted red cells of cholesterol. Changes in membrane cholesterol paralleled changes in membrane surface area, as calculated from osmotic fragility, with a 0.22 percent variation in surface area per 1.0 percent variation in cholesterol content. Cold-induced compression of membrane surface area was increased in cholesterol-poor red cells (C/P equals 0.4), whereas the surface area of cholesterol-rich membranes (C/P equals 1.80) underwent no compression. Although the Na and K permeability of red cells severely depleted of cholesterol was increased, lesser degrees of depletion had no effect, and the permeability of cholesterol-rich cells was normal. However, increasing membrane cholesterol caused a progressive decrease in red cell deformability, as measured by filtration. Cholesterol-poor red cells were spherocytic in appearance and cholesterol-rich cells were broad and flat, indicative of their surface areas. In addition, cholesterol-rich cells had an irregular contour due to folding of the periphery of the cell. This shape abnormality was identical to that of both spur cells after splenectomy and normal red cells incubated in spur serum. Normalization of the C/P of spur serum by added phospholipid prevented the increase in membrane cholesterol and surface area and the transformation of cell shape. These studies establish that the cholesterol content of red cells is dependent on the C/P of their milieu, either lipoproteins or cholesterol-lecithin dispersions. Moreover, the surface area, deformability, and contour of cholesterol-rich red cells are a direct function of their increased membrane C/P. Although cholesterol-rich spur cells are further modified in the circulation of patients with spleens, this abnormality of the membrane lipid bilayer, induced by cholesterol-rich cholesterol-lecithin dispersions, represents the primary spur cell defect.