Clearing the Dead: Apoptotic Cell Sensing, Recognition, Engulfment, and Digestion
TL;DR: This study focuses on the understanding of the steps involved in prompt cell clearance in vivo, including the sensing of corpses via "find me" signals, the recognition of corpses through "eat me" messages, the signaling pathways that regulate cytoskeletal rearrangement necessary for engulfment, and the responses of the phagocyte that keep cell clearance events "immunologically silent".
Abstract: Clearance of apoptotic cells is the final stage of programmed cell death. Uncleared corpses can become secondarily necrotic, promoting inflammation and autoimmunity. Remarkably, even in tissues with high cellular turnover, apoptotic cells are rarely seen because of efficient clearance mechanisms in healthy individuals. Recently, significant progress has been made in understanding the steps involved in prompt cell clearance in vivo. These include the sensing of corpses via “find me” signals, the recognition of corpses via “eat me” signals and their cognate receptors, the signaling pathways that regulate cytoskeletal rearrangement necessary for engulfment, and the responses of the phagocyte that keep cell clearance events “immunologically silent.” This study focuses on our understanding of these steps.
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TL;DR: A novel role for astrocytes in mediating synapse elimination in the developing and adult brain is revealed, MEGF10 and MERTK are identified as critical proteins in the synapse remodelling underlying neural circuit refinement, and have important implications for understanding learning and memory as well as neurological disease processes.
Abstract: To achieve its precise neural connectivity, the developing mammalian nervous system undergoes extensive activity-dependent synapse remodelling. Recently, microglial cells have been shown to be responsible for a portion of synaptic pruning, but the remaining mechanisms remain unknown. Here we report a new role for astrocytes in actively engulfing central nervous system synapses. This process helps to mediate synapse elimination, requires the MEGF10 and MERTK phagocytic pathways, and is strongly dependent on neuronal activity. Developing mice deficient in both astrocyte pathways fail to refine their retinogeniculate connections normally and retain excess functional synapses. Finally, we show that in the adult mouse brain, astrocytes continuously engulf both excitatory and inhibitory synapses. These studies reveal a novel role for astrocytes in mediating synapse elimination in the developing and adult brain, identify MEGF10 and MERTK as critical proteins in the synapse remodelling underlying neural circuit refinement, and have important implications for understanding learning and memory as well as neurological disease processes.
919 citations
Paris Descartes University1, Institut Gustave Roussy2, Mount Sinai Hospital3, University of Texas Southwestern Medical Center4, University of Kiel5, Thomas Jefferson University6, Seconda Università degli Studi di Napoli7, University of Toronto8, University of Massachusetts Medical School9, Louisiana State University10, Flanders Institute for Biotechnology11, Ghent University12, Queen Mary University of London13, Cancer Research UK14, Roswell Park Cancer Institute15, Karolinska Institutet16, University of Freiburg17, University of California, San Francisco18, Buck Institute for Research on Aging19, French Institute of Health and Medical Research20, Université Paris-Saclay21, University College London22, University of Rome Tor Vergata23, Northwestern University24, Memorial Sloan Kettering Cancer Center25, National Institutes of Health26, Technion – Israel Institute of Technology27, Johns Hopkins University28, University of Chieti-Pescara29, University of Ulm30, Genentech31, New York University32, Pennsylvania State University33, University of Salento34, Yale University35, Goethe University Frankfurt36, University of Burgundy37, Pasteur Institute38, University of Strasbourg39, University of Zurich40, University of Tokyo41, Technische Universität München42, University of Bern43, University of Michigan44, Medical Research Council45, University of Adelaide46, University of South Australia47, Medical University of South Carolina48, University of Texas at Dallas49, Howard Hughes Medical Institute50, St. John's University51, University of Oviedo52, University of Graz53, Istituto Superiore di Sanità54, Katholieke Universiteit Leuven55, Trinity College, Dublin56, University of Geneva57, University of Amsterdam58, Stony Brook University59, University of Washington60, University of Ferrara61, Royal College of Surgeons in Ireland62, La Trobe University63, University of Buenos Aires64, University of Virginia65, University of Padua66, University of Lisbon67, University of Cambridge68, University of Würzburg69, Soochow University (Suzhou)70, Columbia University71, University of Glasgow72, University of Crete73, Foundation for Research & Technology – Hellas74, Innsbruck Medical University75, Carlos III Health Institute76, Rutgers University77, University of Minnesota78, Harvard University79, City University of New York80, Moscow State University81
TL;DR: The Nomenclature Committee on Cell Death formulates a set of recommendations to help scientists and researchers to discriminate between essential and accessory aspects of cell death.
Abstract: Cells exposed to extreme physicochemical or mechanical stimuli die in an uncontrollable manner, as a result of their immediate structural breakdown. Such an unavoidable variant of cellular demise is generally referred to as ‘accidental cell death’ (ACD). In most settings, however, cell death is initiated by a genetically encoded apparatus, correlating with the fact that its course can be altered by pharmacologic or genetic interventions. ‘Regulated cell death’ (RCD) can occur as part of physiologic programs or can be activated once adaptive responses to perturbations of the extracellular or intracellular microenvironment fail. The biochemical phenomena that accompany RCD may be harnessed to classify it into a few subtypes, which often (but not always) exhibit stereotyped morphologic features. Nonetheless, efficiently inhibiting the processes that are commonly thought to cause RCD, such as the activation of executioner caspases in the course of apoptosis, does not exert true cytoprotective effects in the mammalian system, but simply alters the kinetics of cellular demise as it shifts its morphologic and biochemical correlates. Conversely, bona fide cytoprotection can be achieved by inhibiting the transduction of lethal signals in the early phases of the process, when adaptive responses are still operational. Thus, the mechanisms that truly execute RCD may be less understood, less inhibitable and perhaps more homogeneous than previously thought. Here, the Nomenclature Committee on Cell Death formulates a set of recommendations to help scientists and researchers to discriminate between essential and accessory aspects of cell death.
809 citations
TL;DR: Atherosclerosis occurs in the subendothelial space (intima) of medium-sized arteries at regions of disturbed blood flow and is triggered by an interplay between endothelial dysfunction and sub-endothel lipoprotein retention as mentioned in this paper.
Abstract: Atherosclerosis occurs in the subendothelial space (intima) of medium-sized arteries at regions of disturbed blood flow and is triggered by an interplay between endothelial dysfunction and subendothelial lipoprotein retention. Over time, this process stimulates a nonresolving inflammatory response that can cause intimal destruction, arterial thrombosis, and end-organ ischemia. Recent advances highlight important cell biological atherogenic processes, including mechanotransduction and inflammatory processes in endothelial cells, origins and contributions of lesional macrophages, and origins and phenotypic switching of lesional smooth muscle cells. These advances illustrate how in-depth mechanistic knowledge of the cellular pathobiology of atherosclerosis can lead to new ideas for therapy.
736 citations
TL;DR: Recent advances in the understanding of lesional macrophage phenotype and function in different stages of atherosclerosis are discussed.
Abstract: The remarkable plasticity and plethora of biological functions performed by macrophages have enticed scientists to study these cells in relation to atherosclerosis for >50 years, and major discoveries continue to be made today. It is now understood that macrophages play important roles in all stages of atherosclerosis, from initiation of lesions and lesion expansion, to necrosis leading to rupture and the clinical manifestations of atherosclerosis, to resolution and regression of atherosclerotic lesions. Lesional macrophages are derived primarily from blood monocytes, although recent research has shown that lesional macrophage-like cells can also be derived from smooth muscle cells. Lesional macrophages take on different phenotypes depending on their environment and which intracellular signaling pathways are activated. Rather than a few distinct populations of macrophages, the phenotype of the lesional macrophage is more complex and likely changes during the different phases of atherosclerosis and with the extent of lipid and cholesterol loading, activation by a plethora of receptors, and metabolic state of the cells. These different phenotypes allow the macrophage to engulf lipids, dead cells, and other substances perceived as danger signals; efflux cholesterol to high-density lipoprotein; proliferate and migrate; undergo apoptosis and death; and secrete a large number of inflammatory and proresolving molecules. This review article, part of the Compendium on Atherosclerosis, discusses recent advances in our understanding of lesional macrophage phenotype and function in different stages of atherosclerosis. With the increasing understanding of the roles of lesional macrophages, new research areas and treatment strategies are beginning to emerge.
669 citations
Cites background from "Clearing the Dead: Apoptotic Cell S..."
...Efferocytosis is mediated by the interaction of apoptotic cell recognition motifs, macrophage receptors, such as MerTK (MER proto-oncogene and tyrosine kinase), and molecules that bridge these 2 components.(117,118) It is possible that the impaired ability of macrophages to efferocytose apoptotic cells in advanced lesions is because of impaired function of these proteins....
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TL;DR: Phagocytic monocyte-derived macrophages associate with the nodes of Ranvier and initiate demyelination while microglia clear debris and display a suppressed metabolic gene signature in EAE.
Abstract: In the human disorder multiple sclerosis (MS) and in the model experimental autoimmune encephalomyelitis (EAE), macrophages predominate in demyelinated areas and their numbers correlate to tissue damage. Macrophages may be derived from infiltrating monocytes or resident microglia, yet are indistinguishable by light microscopy and surface phenotype. It is axiomatic that T cell–mediated macrophage activation is critical for inflammatory demyelination in EAE, yet the precise details by which tissue injury takes place remain poorly understood. In the present study, we addressed the cellular basis of autoimmune demyelination by discriminating microglial versus monocyte origins of effector macrophages. Using serial block-face scanning electron microscopy (SBF-SEM), we show that monocyte-derived macrophages associate with nodes of Ranvier and initiate demyelination, whereas microglia appear to clear debris. Gene expression profiles confirm that monocyte-derived macrophages are highly phagocytic and inflammatory, whereas those arising from microglia demonstrate an unexpected signature of globally suppressed cellular metabolism at disease onset. Distinguishing tissue-resident macrophages from infiltrating monocytes will point toward new strategies to treat disease and promote repair in diverse inflammatory pathologies in varied organs.
667 citations
Cites background from "Clearing the Dead: Apoptotic Cell S..."
..., 2004); and stress-induced eat-me signals (Hochreiter-Hufford and Ravichandran, 2013), it may be feasible to identify a direct molecular pathway for initiating demyelination in this model....
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References
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Journal Article•
TL;DR: The data suggest that macrophages specifically recognize phosphatidylserine that is exposed on the surface of lymphocytes during the development of apoptosis, and suggest that apoptotic lymphocytes lose membrane phospholipid asymmetry and expose phosphorus on the outer leaflet of the plasma membrane.
Abstract: During normal tissue remodeling, macrophages remove unwanted cells, including those that have undergone programmed cell death, or apoptosis. This widespread process extends to the deletion of thymocytes (negative selection), in which cells expressing inappropriate Ag receptors undergo apoptosis, and are phagocytosed by thymic macrophages. Although phagocytosis of effete leukocytes by macrophages has been known since the time of Metchnikoff, only recently has it been recognized that apoptosis leads to surface changes that allow recognition and removal of these cells before they are lysed. Our data suggest that macrophages specifically recognize phosphatidylserine that is exposed on the surface of lymphocytes during the development of apoptosis. Macrophage phagocytosis of apoptotic lymphocytes was inhibited, in a dose-dependent manner, by liposomes containing phosphatidyl-L-serine, but not by liposomes containing other anionic phospholipids, including phosphatidyl-D-serine. Phagocytosis of apoptotic lymphocytes was also inhibited by the L isoforms of compounds structurally related to phosphatidylserine, including glycerophosphorylserine and phosphoserine. The membranes of apoptotic lymphocytes bound increased amounts of merocyanine 540 dye relative to those of normal cells, indicating that their membrane lipids were more loosely packed, consistent with a loss of membrane phospholipid asymmetry. Apoptotic lymphocytes were shown to express phosphatidylserine (PS) externally, because PS on their surfaces was accessible to derivatization by fluorescamine, and because apoptotic cells expressed procoagulant activity. These observations suggest that apoptotic lymphocytes lose membrane phospholipid asymmetry and expose phosphatidylserine on the outer leaflet of the plasma membrane. Macrophages then phagocytose apoptotic lymphocytes after specific recognition of the exposed PS.
3,344 citations
Additional excerpts
...3) (Fadok et al. 1992, 1998a, 2001)....
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TL;DR: The current understanding of myeloid lineage development is reviewed and the developmental pathways and cues that drive differentiation are described, which are central to the development of immunologic memory and tolerance in mice.
Abstract: Monocytes and macrophages are critical effectors and regulators of inflammation and the innate immune response, the immediate arm of the immune system. Dendritic cells initiate and regulate the highly pathogen-specific adaptive immune responses and are central to the development of immunologic memory and tolerance. Recent in vivo experimental approaches in the mouse have unveiled new aspects of the developmental and lineage relationships among these cell populations. Despite this, the origin and differentiation cues for many tissue macrophages, monocytes, and dendritic cell subsets in mice, and the corresponding cell populations in humans, remain to be elucidated.
2,832 citations
"Clearing the Dead: Apoptotic Cell S..." refers background in this paper
...For instance, engulfment of uninfected apoptotic cells by tissue resident macrophages is usually immunologically silent; however, engulfment of apoptotic cells via infiltrating macrophages and/or dendritic cells in an inflamed environment has the potential to drive an antigen-specific immune response and break tolerance (Geissmann et al. 2010; Hochreiter-Hufford and Ravichandran 2012)....
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...…usually immunologically silent; however, engulfment of apoptotic cells via infiltrating macrophages and/or dendritic cells in an inflamed environment has the potential to drive an antigen-specific immune response and break tolerance (Geissmann et al. 2010; Hochreiter-Hufford and Ravichandran 2012)....
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TL;DR: Macrophages also play an important role in the recognition and clearance of apoptotic cells; a notable feature of this process is the absence of an inflammatory response.
Abstract: Phagocytosis of pathogens by macrophages initiates the innate immune response, which in turn orchestrates the adaptive response. In order to discriminate between infectious agents and self, macrophages have evolved a restricted number of phagocytic receptors, like the mannose receptor, that recognize conserved motifs on pathogens. Pathogens are also phagocytosed by complement receptors after relatively nonspecific opsonization with complement and by Fc receptors after specific opsonization with antibodies. All these receptors induce rearrangements in the actin cytoskeleton that lead to the internalization of the particle. However, important differences in the molecular mechanisms underlying phagocytosis by different receptors are now being appreciated. These include differences in the cytoskeletal elements that mediate ingestion, differences in vacuole maturation, and differences in inflammatory responses. Infectious agents, such as M. tuberculosis, Legionella pneumophila, and Salmonella typhimurium, enter macrophages via heterogeneous pathways and modify vacuolar maturation in a manner that favors their survival. Macrophages also play an important role in the recognition and clearance of apoptotic cells; a notable feature of this process is the absence of an inflammatory response.
2,774 citations
TL;DR: It is shown that anthracyclin-induced CRT translocation induces the rapid, preapoptotic translocation of calreticulin (CRT) to the cell surface and is identified as a key feature determining anticancer immune responses.
Abstract: Anthracyclin-treated tumor cells are particularly effective in eliciting an anticancer immune response, whereas other DNA-damaging agents such as etoposide and mitomycin C do not induce immunogenic cell death. Here we show that anthracyclins induce the rapid, preapoptotic translocation of calreticulin (CRT) to the cell surface. Blockade or knockdown of CRT suppressed the phagocytosis of anthracyclin-treated tumor cells by dendritic cells and abolished their immunogenicity in mice. The anthracyclin-induced CRT translocation was mimicked by inhibition of the protein phosphatase 1/GADD34 complex. Administration of recombinant CRT or inhibitors of protein phosphatase 1/GADD34 restored the immunogenicity of cell death elicited by etoposide and mitomycin C, and enhanced their antitumor effects in vivo. These data identify CRT as a key feature determining anticancer immune responses and delineate a possible strategy for immunogenic chemotherapy.
2,550 citations
"Clearing the Dead: Apoptotic Cell S..." refers background in this paper
...For example, treatment of tumor cells with antracyclins led to cell-surface exposure of calreticulin early during the death program, which was essential for phagocytosis of dying tumor cells by dendritic cells and the initiation of an immune response (Obeid et al. 2007b)....
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...Additionally, calreticulin exposure was linked to immunogenic cell death caused by irradiation and UVC-light treatment (Obeid et al. 2007a)....
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...…and oxidized low-density lipoprotein (LDL)-like moiety (Devitt et al. 1998; Fadok et al. 1998b; Aderem and Underhill 1999; Schlegel et al. 1999), and the exposure of certain intracellular proteins such as calreticulin and annexin I (Arur et al. 2003; Gardai et al. 2005; Obeid et al. 2007a,b)....
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TL;DR: Recent experiments have defined new functions of ROCKs in cells, including centrosome positioning and cell-size regulation, which might contribute to various physiological and pathological states.
Abstract: ROCKs, or Rho kinases, are serine/threonine kinases that are involved in many aspects of cell motility, from smooth-muscle contraction to cell migration and neurite outgrowth. Recent experiments have defined new functions of ROCKs in cells, including centrosome positioning and cell-size regulation, which might contribute to various physiological and pathological states.
1,878 citations