Clearance of Apoptotic Cells Kirsten Lauber, Sibylle G. Blumenthal, Michaela Waibel, Sebastian Wesselborg  Molecular Cell  Volume 14, Issue 3, Pages 277-287 (May 2004) DOI: 10.1016/S1097-2765(04)00237-0

Figure 1 The Engulfment Synapse The apoptotic cell displays various eat-me signals that are recognized directly or indirectly via different bridging molecules by diverse phagocyte receptors. ABC1, ATP binding-cassette-transporter 1; AnxI, Annexin I; β2-GPI, β2-glycoprotein-I; C1q, complement protein C1q; C3b/bi, complement protein C3b/bi; CD14, lipopolysaccharide receptor CD14; CD91, calreticulin/heat shock protein receptor; CR3, complement receptor 3; CR4, complement receptor 4; CRT, calreticulin; Gas6, growth arrest-specific 6; ICAM-3, intercellular adhesion molecule 3; LOX1, lectin-like oxidized low-density lipoprotein particle receptor 1; MBL, mannose binding lectin; Mer, receptor-tyrosin-kinase (expressed in monocytes and tissues of epithelial and reproductive origin); MFG-E8, milk-fat-globule-EGF-factor 8; OxLDL, oxidized low-density lipoprotein particle; Prot S, protein S; PS-receptor, phosphatidylserine receptor; SP-A/D, lung surfactant protein A or D; SR-A, class A macrophage scavenger receptor; TSP-1, thrombospondin-1. Molecular Cell 2004 14, 277-287DOI: (10.1016/S1097-2765(04)00237-0)

Figure 2 Lack of Don't-Eat-Me Signals on the Surface of Apoptotic Cells In vertebrates, professional phagocytes often meet potential target cells and interact with them through their respective CD31 proteins. If the target cell is viable, inside-out signaling is stimulated resulting in the target cell's active repulsion from the phagocyte. If the target cell is apoptotic or dying, inside-out signaling through CD31 is disabled, and the target cell does not actively reject the phagocyte anymore. This supports recognition of the dying cell by specialized engulfment receptors on the phagocyte. Molecular Cell 2004 14, 277-287DOI: (10.1016/S1097-2765(04)00237-0)

Figure 3 Hypothetical, Simplified Model for the Engulfment Synapse The puzzling variety of eat-me signals might be reduced to externalized phosphatidylserine, lysophosphatidylcholine, and modified sugar residues on the surface of the apoptotic cell, with lysophosphatidylcholine functioning both as a soluble attraction signal and as a membrane-bound eat-me signal. During apoptosis, caspase-activated calcium-independent phospholipase A2 hydrolizes phosphatidylcholine to arachidonic acid and lysophospatidylcholine, which in turn is externalized. In its secreted form, LPC might function as find-me signal attracting the phagocyte to its prey, while in its membrane-bound form it could be causative for the oxLDL-like, C1q, or C3b/bi binding nature of different sites on the apoptotic cell surface. The collectin and TSP-1 binding property of the apoptotic cell membrane might be attributed to well-known externalized phosphatidylserine and modified sugar residues. AnxI, Annexin I; β2-GPI, β2-glycoprotein-I; C1q, complement protein C1q; C3b/bi, complement protein C3b/bi; CD14, lipopolysaccharide receptor CD14; CD91, calreticulin/heat shock protein receptor CD91; CR3, complement receptor 3; CR4, complement receptor 4; CRT, calreticulin; Gas6, growth arrest-specific 6; ICAM-3, intercellular adhesion molecule 3; IgM, immunoglobulin M; iPLA2, calcium-independent phospholipase A2; LOX1, lectin-like oxidized low-density lipoprotein particle receptor 1; LPC, lysophosphatidylcholine; MBL, mannose binding lectin; Mer, receptor-tyrosin-kinase (expressed in monocytes and tissues of epithelial and reproductive origin); MFG-E8, milk-fat-globule-EGF-factor 8; OxLDL, oxidized low-density lipoprotein particle; Prot S, protein S; PS receptor, phsophatidylserine receptor; SP-A/D, lung surfactant protein A or D; SR-A, class A macrophage scavenger receptor; TSP-1, thrombospondin-1. Molecular Cell 2004 14, 277-287DOI: (10.1016/S1097-2765(04)00237-0)

Figure 4 Putative Model of Caspase- and iPLA2-Mediated Attraction of Phagocytes During apoptosis, iPLA2 is cleaved and activated by caspase-3, conceivably by releasing it from a putative regulator protein and facilitating access to its endogenous substrate phosphatidylcholine in the plasma membrane. Activated iPLA2 subsequently starts hydrolizing phosphatidylcholine, yielding arachidonic acid and lysophoshphatidylcholine. Lysophosphatidylcholine in turn is externalized and secreted by an as-yet unknown mechanism. Subsequent binding of lysophoshphatidylcholine to its respective receptor (e.g., the G protein-coupled receptors G2A or GPR4) would then trigger the phagocyte's migration toward the apoptotic cell. Casp-3, caspase-3; iPLA2, calcium-independent phospholipase A2; LPC, lysophoshatidylcholine. Molecular Cell 2004 14, 277-287DOI: (10.1016/S1097-2765(04)00237-0)

Figure 5 Known Gene Products Involved in Engulfment in the Nematode C. elegans and Their Mammalian Homologs In C. elegans, seven gene products have been described to be involved in apoptotic cell removal. They are arranged in two partially redundant signaling cassettes. CED-1/CD91, CED-6/GULP, and CED-7/ABC1 function in one pathway, whereas the other signaling module is comprised of CED-2/CrkII, CED-5/DOCK180, CED-10/Rac1, and CED-12/ELMO. Members of the second pathway are known to regulate rearrangements of the actin cytoskeleton and also play a role in migration processes. The upstream activating receptor of this pathway remains to be identified. Potential candidates are integrins, the phosphatidylserine receptor, or a migration signal receptor. Although little is known about the mutual interactions of the different engulfment gene products, there is evidence for a connection between the two signaling cassettes, since the CED-1, CED-6, CED-7 pathway has recently been demonstrated to be linked to CED-10 downstream. Molecular Cell 2004 14, 277-287DOI: (10.1016/S1097-2765(04)00237-0)

Figure 6 The Three Steps of Apoptotic Cell Removal (A) The dying cell releases soluble find-me signals in order to attract professional phagocytes. (B) Once at its site of action, having close contact to the apoptotic cell, the phagocyte is able to recognize its prey by the display of eat-me and the lack of don't-eat-me signals on the apoptotic cell surface, thus leading to the internalization of the apoptotic cell corpse. (C) Having ingested the apoptotic remains, the phagocyte starts producing anti-inflammatory cytokines, like IL-10 and TGF-β, thus creating an anti-inflammatory milieu around the site of the apoptotic cell death. Molecular Cell 2004 14, 277-287DOI: (10.1016/S1097-2765(04)00237-0)