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The Role of Phosphatidylserine in Recognition of Apoptotic Cells by Phagocytes

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The Role of Phosphatidylserine in Recognition of Apoptotic Cells by Phagocytes Cell Death and Differentiation (1998) 5, 551 ± 562 1998 Stockton Press All rights reserved 13509047/98 $12.00 http://www.stockton-press.co.uk/cdd Review The role of phosphatidylserine in recognition of apoptotic cells by phagocytes Valerie A. Fadok1,2, Donna L. Bratton1, S. Courtney Frasch1, epithelial cells and vascular smooth muscle cells). To date, Mary L. Warner1 and Peter M. Henson1 there have been a number of receptors described for macrophages and other cells which bind to apoptotic cells 1 Department of Pediatrics, National Jewish Medical and Research Center, 1400 and mediate their uptake. These include lectin-like receptors Jackson Street, Denver, Colorado 80206 USA (Duvall et al, 1985, Dini et al, 1992; 1995; Hall et al, 1994; 2 corresponding author: tel: 1-303-398-1281 fax: 1-303-398-1381 Morris et al, 1994; Falasca et al, 1996), the vitronectin receptor email: [email protected] avb3 (Savill et al, 1990; Hall et al, 1994; Hughes et al, 1997), CD36 (Savill et al, 1992), an uncharacterized phosphatidyl- Received: 15.10.97; revised: 23.3.98; accepted: 2.4.98 Edited by M. Piacentini serine-recognizing receptor (Fadok et al, 1992a,b, 1993; Pradhan et al, 1997), CD14 (Flora and Gregory 1994; Devitt et al, 1998), and scavenger receptors (Sambrano and Steinberg, Abstract 1995; Fukasawa et al, 1996; Platt et al, 1996; Murao et al, 1997). The ABC1 transporter, also involved in uptake of Exposure of phosphatidylserine on the outer leaflet of the mammalian apoptotic cells, has recently been shown to plasma membrane is a surface change common to many mediate anion transport (Luciani and Chimini, 1996; Becq et apoptotic cells. Normally restricted to the inner leaflet, al, 1997). It is homologous to the ced-7 gene associated with phosphatidylserine appears as a result of decreased phagocytosis of apoptotic bodies in C. elegans (Ellis et al, aminophospholipid translocase activity and activation of a 1991). How any of these receptors mediate phagocytosis of calcium-dependent scramblase. Phosphatidylserine expo- apoptotic cells is not known. In addition, the ligands for these sure has several potential biological consequences, one of receptors are not well characterized; however, exposure of which is recognition and removal of the apoptotic cell by phosphatidylserine on the external leaflet of the plasma cell membrane appears to be common to many apoptotic cells phagocytes. It is still not clear which receptors mediate PS (Fadok et al, 1992a,b; Schlegel et al, 1993; Mower et al, 1994; recognition on apoptotic cells; however, several interesting Koopman et al, 1994; Bennet et al, 1995; Homburg et al, 1995; candidates have been proposed. These include the Class B Martin et al, 1995a; Shiratsuchi et al, 1997), and this scavenger and thrombospondin receptor CD36, an oxLDL phospholipid appears to be recognized in a stereospecific receptor (CD68), CD14, annexins, b2 glycoprotein I, gas-6 and fashion by subsets of macrophages (Fadok et al, 1992a,b, a novel activity expressed on macrophages stimulated with 1993; Pradhan et al, 1997), by melanoma cells (Fadok, 1995), digestible particles such as b-glucan. Whether PS is the sole by smooth muscle vascular cells (Bennett et al, 1995), and by ligand recognized by phagocytes or whether it associated with Sertoli cells (Shiratsuchi et al, 1997). other molecules to form a complex ligand remains to be Normally, the phospholipids of the plasma membrane determined. are distributed asymmetrically. Choline-containing lipids, such as phosphatidylcholine (PC) and sphingomyelin, are concentrated on the outer leaflet, whereas the aminopho- Keywords: phosphatidylserine; apoptosis; phagocytosis; recep- spholipids, including phosphatidylethanolamine (PE) and tors; membranes phosphatidylserine (PS), are present in highest abundance on the inner leaflet. Phosphatidylserine, in fact, is restricted Abbreviations: 2-GPI: b2-glycoprotein IBMDM: bone marrow- b exclusively to the inner leaflet in most cells. Loss of derived macrophages; CL: cardiolipin; GPS: glycerophosphoryl- phospholipid asymmetry and exposure of phosphatidylser- serine; HMDM: human monocyte-derived macrophages; PC: ine was first demonstrated for apoptotic lymphocytes phosphatidylcholine; PE: phosphatidylethanolamine; PI: phospha- (Fadok et al, 1992a; Schlegel et al, 1993; Mower et al, tidylinositol; PS: phosphatidylserine; SR: scavenger receptors; 1994); however, these findings have been confirmed for a TCR: T cell receptor; TSP: thrombospondin; UV: ultraviolet light; number of cell types, including neutrophils, tumor cell lines, VnR: vitronectin receptor. smooth muscle vascular cells, spermatogonia, and Jurkat T cells and their cytoplasts (Fadok et al, 1992b; Bennet et al, 1995; Homburg et al, 1995; Martin et al, 1995a, 1996; Recognition and removal by phagocytes is the final common Shiratsuchi et al, 1997). Because exposure of phosphati- event in the lives of many apoptotic cells. Apoptotic cells are dylserine appears to be a universal feature of apoptotic removed prior to their lysis, suggesting that they express cells, it is now used as a marker in flow cytometry assays specific surface changes that signal phagocytes to bind and utilizing fluorochrome-labelled annexin V, which binds to PS engulf them. These phagocytes can be professional (the in a calcium-dependent manner (Vermes et al, 1995; Zhang macrophages) or amateur (including such cells as fibroblasts, et al, 1997). PS recognition on apoptotic cells FA Fadok et al 552 study PS and PC transport during development of Mechanisms for phosphatidylserine apoptosis. They found that prior to the development of exposure in apoptotic cells nuclear condensation, PS translocation to the inner leaflet The mechanisms for PS exposure on the surface of apoptotic was impaired. At the same time, the transport of PC was cells appear complex and will remain an active area of increased. These data showed for the first time that during investigation for some time. To determine these, an under- apoptosis, aminophospholipid translocase activity was standing of the maintenance of normal membrane phospho- impaired as the scramblase was activated. The net result lipid asymmetry is required. Two mechanisms have been was PS exposure on the surface of the cell. proposed. A membrane protein termed the aminophospholi- We have recently compared the roles of the aminophos- pid translocase transfers PS and to some extent, PE, from the pholipid translocase and scramblase in three additional cell outer leaflet to the inner leaflet (Devaux, 1991; Zachowski, types known to express PS when undergoing apoptosis: 1993; Williamson and Schlegel, 1994). This protein appears ultraviolet light (UV)-treated HL-60 cells, anti-Fas treated to be a Mg2+-ATPase which is inhibited by Ca2+ at Jurkat T cells, and UV-treated human peripheral blood concentrations achieved in many apoptotic cells (Daleke neutrophils (Bratton et al, 1997). Utilizing NBD-PS uptake and Huestis, 1985; Zachowski et al, 1986; Bitbol et al, 1987; as a measure of aminophospholipid translocase activity in Bevers et al, 1989; Morrot et al, 1990; Comfurius et al, 1990; both bulk uptake assays and flow cytometric assays, we Auland et al, 1994). Tang and coworkers recently cloned a found that translocase activity was impaired in all three cell novel P-type ATPase with aminophospholipid translocase types. The increase in NBD-PC uptake by all three cell activity (Tang et al, 1996). The second mechanism involves types supported the notion that scramblase was activated. the tethering of PS in the inner leaflet by its association with Changes in these activities preceded the expression of PS, membrane skeletal proteins such as fodrin (non-erythrocyte as determined by annexin V binding. Because both the spectrin), based on the known ability of PS to bind to spectrin aminophospholipid translocase activity and the scramblase (Mombers et al, 1980; Williamson et al, 1987; Maksymiw et al, are reported to be affected by alterations in calcium levels, 1987; Middlekoop et al, 1988). The tethering mechanism fell we studied the effects of Ca2+ depletion on PS exposure. out of favor for a period of time (see Calvez et al, 1988; Chelation of intracellular Ca2+ with BAPTA had no effect on Devaux, 1991; Williamson and Schlegel, 1994); for example, PS expression, loss of aminophospholipid translocase Pradhan and coworkers labelled cytoskeletal proteins with a activity, or activation of nonspecific phospholipid flip-flop. photoactivable analogue of phosphatidylethanolamine (2-(2- In contrast, depletion of extracellular Ca2+ was associated azido-4-nitrobenzoyl)-1-acyl-sn-glycero-3-phospho with a marked decrease in annexin V binding (the annexin [14C]ethanolamine) and found no differences in spectrin assay however was done in the presence of Ca2+, as labelling between normal red cells and PS-expressing red annexin binding is calcium-dependent) and loss of cells from patients with sickle cell anemia (Pradhan et al, nonspecific phospholipid transport by apoptotic cells. 1991). However, the tethering hypothesis appears to be There was no alteration in the percentage of apoptotic enjoying a resurgence of interest, given the observation that cells nor was there restoration of aminophospholipid fodrin is cleaved during apoptosis. Other potential intracellular translocase activity, suggesting that loss of aminophos- tethering molecules for phosphatidylserine could include pholipid translocase activity was not sufficient for PS annexins or polyamines (Chung et al, 1985; Meers et al, expression on apoptotic
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