Phosphatidylserine plasma membrane

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). 

One of the early events of the apoptotic process involves the translocation of phosphatidylserine on the surface of cell membranes annexin V binding and propidium iodide uptake reveals various cellular states. After treatment with organotin(IV) compounds the cells could be categorized into populations vital cells (annexin V /P ), early apoptotic cells (annexin V /P ), late apoptotic cells (annexin V /P ), and necrotic cells (annexin V /P" ). Cells are observed with a fluorescence microscope and it is possible to observe translocation of phosphatidylserine (PS) from the inner side of the plasma membrane to the outer one and to see a green stain for annexin V FLUOS bound to PS, and a red stain for propidium iodide. 

It can be seen from Figure 1 that the choline-containing phospholipids, phosphatidylcholine and sphingomyelin are localized predominantly in the outer monolayer of the plasma membrane. The aminophospholipids, conprising phosphatidylethanolamine and phosphatidylserine, by contrast, are enriched in the cytoplasmic leaflet of the membrane (Bretcher, 1972b Rothman and Lenard, 1977 Op den Kamp, 1979). The transmembrane distribution of the minor membrane lipid components has been determined by reaction with lipid-specific antibodies (Gascard et al, 1991) and lipid hydrolases (Biitikofer et al, 1990). Such studies have shown that phosphatidic acid, phosphatidylinositol and phosphatidylinositol-4,5-fc -phosphate all resemble phosphatidylethanolamine in that about 80% of the phospholipids are localized in the cytoplasmic leaflet of the membrane. 

Farge, E., 1995, Increased vesicle endocytosis due to an increase in the plasma membrane phosphatidylserine concentration. Biophys. J., 69 2501-2506. 

Lecoeur, H., Prevost, M.C. and Gougeon, M.L., 2001, Oncosis is associated with exposure of phosphatidylserine residues on the outside layer of the plasma membrane A reconsideration ofthe specificity of the annexin V/propidium iodide assay. Cytometry, 44 65-72. 

Martin, O.C. and Pagano, R.E., 1987, Transbilayer movement of florescent analogs of phosphatidylserine and phosphatidylethanolamine at the plasma membrane of cultured cehs. /. Biol. Chem., 2lSl 5890-5898. 

Fadeel, B., Gleiss, B., Hogstrand, K., Chandra, J., Wiedmer, T., Sims, P., Henter, J.-I, Orrenius, S., and Samah, A., 1999, Phosphatidylserine exposure during apoptosis is a cell type-specific event and does not correlate with plasma membrane phospholipid scramblase expression, Biochem. Biophys. Res. Commun. 266 504-511. 

Kawai, K., Tyurina, Y.Y, Tyurin, V.A., Kagan, V.E., and Fabisiak, J., 2000, Peroxidation and externalization of phosphatidylserine in plasma membrane of HL-60 ceUs during tert-butyl hydroperoxide-induced apoptosis Role of cytochrome c, The Toxicologist 54 SuppL 776. 

Changes occur at the cell surface and plasma membrane in the early stages of apoptosis. One of the major plasma membrane alterations is the translocation of phosphatidylserine (PS) from the inner side of the plasma membrane to the outer layer for external exposure (FI). This change of exposure requires the activation of caspase-3, a Ca flux over the plasma membrane, and a change in Bcl-2 family (B8,B13, M6). 

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. 

Translocation of the phosphatidylserine from the inner to the outer leaflet of the plasma membrane is an initial event related to the apoptotic process and possibly serves as a signal for the removal of apoptotic bodies by phagocytic cells (Martin et al., 1995). The exposure of this phospholipid has been largely used as a specific apoptosis marker. 

Unfortunately, annexin V also binds to phosphatidylserine residues in the inner leaflet of the plasma membrane of necrotic cells due to the loss of integrity of the membrane. However, the use of propidium iodide as a counter-marker allows viable, apoptotic, and necrotic cells to be distinguished (Plasier et al., 1999). 

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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