Phosphatidylserine asymmetry

There is also inside-outside (transverse) asymmetry of the phospholipids. The choline-containing phospholipids (phosphatidylcholine and sphingomyelin) are located mainly in the outer molecular layer the aminophospholipids (phosphatidylserine and phos-phatidylethanolamine) are preferentially located in the inner leaflet. Obviously, if this asymmetry is to exist at all, there must be limited transverse mobility (flip-flop) of the membrane phospholipids. In fact, phospholipids in synthetic bilayers exhibit an extraordinarily slow rate of flip-flop the half-life of the asymmetry can be measured in several weeks. However, when certain membrane proteins such as the erythrocyte protein gly-cophorin are inserted artificially into synthetic bilayers, the frequency of phospholipid flip-flop may increase as much as 100-fold. 

Methods used to demonstrate the existence of membrane phospholipid asymmetry, such as chemical labelling and susceptibility to hydrolysis or modification by phospholipases and other enzymes, are rmsuitable for dynamic studies because the rates of chemical and biochemical reactions are of a different order compared to the transmembrane translocahon of the phospholipids. Indirect methods have therefore been developed to measure the translocation rate which are consequent on the loss of membrane phospholipid asymmetry. Thus time scales appropriate to rates of lipid scrambling under resting conditions or when the forces preserving the asymmetric phospholipid distribution are disturbed can be monitored. Generally the methods rely on detecting the appearance of phosphatidylserine on the surface of cells. Methods of demonstrating Upid translocation in mammalian cells has been the subject of a recent review (Bevers etal., 1999). 

Reviews of the role of aminophospholipid translocase and scramblase (Schlegel et al, 2000) and the consequences of the appearance of phosphatidylserine on the cell surface (Williamson et al, 2001) in apoptosis of thymocytes have been published. The precise relationship between membrane phospholipid asymmetry and apoptosis is currently a topic of considerable interest. 

Fadok, V.A., de Cathelineau, A., Daleke, D.L., Henson, P.M. and Bratton, D.L., 2001, Loss of phosphohpid asymmetry and surface exposure of phosphatidylserine is required for phagocytosis of apoptotic cells by macrophages and fibroblasts. J. Biol Chem., 276 1071-1077. 

T5. Thornberry, N. A., andLazebnik, Y, Caspases Enemies within. SciencelSl, 1312—1316(1998). T6. Tsujimoto, Y., and Shimizu, S., Bcl-2 family Life-or-death switch. FEBSLett. 466,6-10 (2000). V1. Van den Eijnde, S. M., Boshart, L., Reutelingsperger, C. R, De Zeeuw, C. I., and Vermeij-Keers, C., Phosphatidylserine plasma membrane asymmetry in vivo A pancellular phenomenon which alters during apoptosis. Cell Death Differ. 4, 311-316 (1997). 

Plasma membrane lipids are asymmetrically distributed between the two monolayers of the bilayer, although the asymmetry, unlike that of membrane proteins, is not absolute. In the plasma membrane of the erythrocyte, for example, choline-containing lipids (phosphatidylcholine and sphingomyelin) are typically found in the outer (extracellular or exoplasmic) leaflet (Fig. 11-5), whereas phosphatidylserine, phosphatidyl-ethanolamine, and the phosphatidylinositols are much more common in the inner (cytoplasmic) leaflet. Changes in the distribution of lipids between plasma membrane leaflets have biological consequences. For example, only when the phosphatidylserine in the plasma membrane moves into the outer leaflet is a platelet able to play its role in formation of a blood clot. For many other cells types, phosphatidylserine exposure on the outer surface marks a cell for destruction by programmed cell death. 

Lipids also show asymmetrical distributions between the inner and outer leaflets of the bilayer. In the erythrocyte plasma membrane, most of the phosphatidylethanolamine and phosphatidylserine are in the inner leaflet, whereas the phosphatidylcholine and sphingomyelin are located mainly in the outer leaflet. A similar asymmetry is seen even in artificial liposomes prepared from mixtures of phospholipids. In liposomes containing a mixture of phosphatidylethanolamine and phosphatidylcholine, phosphatidylethanolamine localizes preferentially in the inner leaflet, and phosphatidylcholine in the outer. For the most part, the asymmetrical distributions of lipids probably reflect packing forces determined by the different curvatures of the inner and outer surfaces of the bilayer. By contrast, the disposition of membrane proteins reflects the mechanism of protein synthesis and insertion into the membrane. We return to this topic in chapter 29. 

Membrane asymmetry changes can be detected by flow cytometry using a fluorescent marker (e.g. fluorescein, FITC) conjugated to annexin V, a protein that has high affinity for phosphatidylserine. When using a fluorescence microscope, this technique can be quantitative if a hemocyt-ometer is used. 

In the case of red blood cells, it is assumed that the progressive loss of lipid asymmetry, possibly associated with the entry of calcium, is a signal that the cell is aging. This signal, in turn, is recognized by macrophages and leads to cell destruction. Dmgs which, for example, compete for calcium bound to phosphatidylserine could interfere with these processes and many other Ca2+-dependent processes such as protein kinase C activation. The influence of asymmetry in membranes of different phospholipid composition on the fusion of liposomes has been studied and reported [22]. 

Figure 9.26 Asymmetry of phospholipids in the human erythrocyte and B. megaterium plasma membranes. "Total lipid" indicates 50% of lipid on each of the two sides of the bilayer. SM, PC, PE, PS, and PG are sphingomyelin, phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidylglycerol, respectively. (Reproduced by permission from Vance DE, Vance JE. Biochemistry of Lipids and Membranes. Menlo Park Benjamin/Cummings, 1985, p. 477.)...

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