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Big eater macrophages dominate inflammation resolution following myocardial infarction
Journal of Molecular and Cellular Cardiology 87 (2015) 225–227
Contents lists available at ScienceDirect
Journal of Molecular and Cellular Cardiology journal homepage: www.elsevier.com/locate/yjmcc
Editorial
Big eater macrophages dominate inflammation resolution following myocardial infarction Keywords: Myocardial infarction Macrophages Efferocytosis Resolution of inflammation
1. Diverse roles for big eater macrophages in myocardial healing Cardiomyocyte cell homeostasis is challenged during cell death, cell senescence, and cell replacement which occurs in response to injuries, including myocardial infarction (MI) or infection. Injured, infected, or aged myocytes die according to regular processes i.e. apoptosis and necrosis and are cleared in a chronological manner by macrophages through a process termed phagocytosis. Phagocytosis is an inherent role of the macrophages and is why these cells were referred to as ‘big eater’ immune cells by Élie Metchnikoff, who first described the process to win the Nobel Prize in 1908 [1]. Post-MI, phagocytic macrophages arrive at the site of injury to clear first responder neutrophils and myocardial debris, stimulated by a diverse array of peptide and lipid-mediated ‘find me’ signals. Of note, all macrophages are not the same; for example, embryoderived macrophages expand to protect the left ventricle from injury. Post-MI, however, embryo-derived resident macrophages are replaced by monocyte-derived macrophages to promote unresolved inflammation [2]. Therefore, optimal and timely clearance of dead cells requires efficient coordination of the receptors including scavenger receptors, the phosphatidyl serine receptor, thrombospondin receptor, integrins, and complement receptor 3 [3]. Thus, proximate interactions of dead myocytes with macrophages provide the foundation for effective efferocytosis and resolution of inflammation. Efferocytosis refers to clearing of dead cells (e.g. apoptotic or necrotic) by macrophages, which possess receptors for engulfing them. Surface characteristics of receptors mediate the clearance of dead cells with chemotactic guidance; and insufficient clearance and defective efferocytosis have been shown to advance the disease pathology of atherosclerosis, as well as heart failure [4,5]. The temporal and spatial kinetics of phagocytes and their interaction with myocyte or fibroblast-sourced debris are essential to understanding post-MI pathophysiology. The macrophages are the "big eaters," which coordinate with both immune and non-immune cells to resolve unremitting inflammation. Due to their role in resolving the inflammation, a subset of these cells are also referred to as M2 or antiinflammatory macrophages [4]. Post-MI, hypoxic cardiomyocytes and fibroblasts drive the inflammatory response, releasing intracellular
http://dx.doi.org/10.1016/j.yjmcc.2015.08.019 0022-2828/© 2015 Elsevier Ltd. All rights reserved.
components at the site of injury. The macrophage synthesizes and releases both pro-inflammatory and pro-resolving signals in collaboration with neutrophil and platelets to clear the inflammation. The coordination of leukocytes for the systematic clearance of dead cells is essential for expediting the resolution process [4]. Several co-morbidities, such as obesity and aging, impair the resolution ability of the macrophage. Macrophages exposed to obesity superimposed on aging develop defects in this process to generate a pro-inflammatory environment and, therefore, experience delayed resolution [5]. Several studies have linked the defective resolution axis to the non-clearance of apoptotic bodies, which contributes to chronic inflammation. Genetic manipulation in various targets, such as tissue transglutaminase, milk-fat globule EGF factor-8 (MFG-E8), complement C1q, lysophosphatidylcholines, Fas, and myeloid-epithelial-reproductive tyrosine kinase (MERTK), leads to the development of non-resolving inflammation in atherosclerosis plaques, indicating defective efferocytosis [6]. In addition to these targets, Wang et al. recently discovered that big eater macrophage receptor (MERTK) is essential for the clearance of dying adult cardiomyocytes and is important in resolving inflammation following MI. Post-MI MERTK is specifically induced on Ly6C low pro-resolving (M2) macrophages, which indicates its role in the resolution of inflammation. MERTK deficiency leads to an accumulation of necrotic cardiomyocytes, independent of the phagocytosis of non-cardiomyocytes, which leads to reduced efferocytosis [7]. The next question raised was about whether the ratio of cadiomyocyte to fibroblast phagocytosis has a similar effect. To answer this question, a recent article by Zhang et al. confirmed MERTK expression in failing human hearts as well as its novel mechanism of clearing hypoxic cardiomyocytes [8]. This result strengthens our understanding of the ordered, sequential steps and phagocytic efficiency of dying adult cardiomyocytes by macrophages (Fig. 1). Using transmission electron microscopy, the authors examined the formation of phagocytic cups on MERTK+/+ macrophages, which were interacting with target cells positive for myocyte markers desmin and actinin in the human ischemic heart. To provide evidence of a MERTK role in phagocytosis of cardiomyocytes, the authors used an experimental MI model in transgenic mice. This model had a cardiomyocyte-specific mCherry reporter gene which carried the MERTK-signal into the phagocytic cups, confirming its presence in ischemic mouse heart. Systemic ex-vivo efferocytosis methodology validated the rate limiting steps before and during cardiomyocyte engulfment by macrophages. The authors used MER-deficient mice (MERTK−/−) to confirm that MERTK deficiency leads to inefficient clearance of dead cardiomyocytes, therefore suppressing phagocytosis. Evaluation of human failing heart revealed upregulation of MERTK, providing clinical relevance.
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Editorial
Fig. 1. Inefficient efferocytosis of cardiomyocytes due to myeloid-epithelial-reproductive tyrosine kinase (MERTK) deficiency leads to defective resolution of inflammation post-myocardial infarction (MI). Post-MI injury induces macrophages MERTK expression and clears the apoptotic cardiomyocytes thereby effective resolution of inflammation. Cardiomyocyte induces shedding of MERTK receptor making macrophages MERTK-deficient delaying clearance of dying cardiomyocytes resulting in defective resolution of inflammation.
2. Big eater macrophages digest debris post-MI The clearance of myocyte and other myocardial debris in a timely manner post-MI was used to determine whether the resolution process in cardiac healing and repair was efficient [9]. Heterogeneous monocyte populations commit to the macrophage lineage, which are phagocytic in nature and play role in post-MI healing by clearing dead cells [10]. The primary function of the macrophage is to engulf and digest necrotic cells within an injured tissue, while facilitating wound healing to stimulate resolution of inflammation. Several receptors have been described for macrophages, including CD36, low-density lipoprotein receptorrelated protein 1 (LRP), and MERTK [11,12,13].
The distinct presence of MERTK on macrophages and its role in cardiac repair post-MI has been experimentally demonstrated by Wang et al. [7]. The authors highlighted that MERTK+/+ macrophages digest a large size variety of cardiomyocyte and size does not affect efferocytosis efficiency. Using these criteria, the authors showed that the inefficient phagocytosis due to the shedding of the MERTK ectodomain from the macrophages was induced by cardiomyocytes themselves. The detailed experiments using an ex-vivo approach showed that enhancement of cardiomyocyte efferocytosis by cleavage-resistant MERTK was significant and could be one of the potential contributing mechanisms that affects cardiomyocyte phagocytic efficiency. Future studies will help to identify potential cardiomyocyte-specific triggers of MERTK cleavage, the time
Editorial
course of MERTK in heart failure pathology, and the physiological relevance of MERTK cleavage post-MI. Unresolved inflammation is a major cause of heart failure pathology. Several efforts have been ongoing to resolve the inflammation axis in order to delay heart failure. Future studies with a major focus on MERTK agonists and antagonists or contribution of MERTK to activate adaptive response in post-MI setting will advance new knowledge in regards to chronic heart failure pathology. At present, it is unclear whether embryo-derived resident macrophages or monocyte-derived infiltrating macrophages dominates MERTK expression. The identification of MERTK on human and mice tissue certainly opens up a new avenue for pursuing therapeutic targets for resolving inflammation post-MI cardiac remodeling. It is important to consider that the phagocytosis of dead cardiomyocytes is one of the processes that is involved in cardiac healing post-MI. Other cell types, particularly the fibroblasts, neutrophils, dendritic cells, B cells, and T cells, play an important role during the cardiac repair process. Taking into consideration these encouraging findings and future studies will provide more information about ligands and receptors associated with the efferocytosis capacity of the macrophage in post-MI pathological remodeling. Disclosure statement None. Acknowledgements The authors acknowledge support from NIH-NCCAM K99/R00 AT006704 and UAB start-up funds, awarded to GVH. The authors used Servier Medical Arts for Fig. 1. References [1] S. Gordon, Elie Metchnikoff: father of natural immunity, Eur. J. Immunol. 38 (2008) 3257–3264. [2] K.J. Lavine, S. Epelman, K. Uchida, K.J. Weber, C.G. Nichols, J.D. Schilling, et al., Distinct macrophage lineages contribute to disparate patterns of cardiac recovery and
227
[3] [4]
[5]
[6] [7]
[8]
[9] [10] [11]
[12]
[13]
remodeling in the neonatal and adult heart, Proc. Natl. Acad. Sci. U. S. A. 111 (2014) 16029–16034. D.M. Mosser, J.P. Edwards, Exploring the full spectrum of macrophage activation, Nat. Rev. Immunol. 8 (2008) 958–969. V. Kain, S.D. Prabhu, G.V. Halade, Inflammation revisited: inflammation versus resolution of inflammation following myocardial infarction, Basic Res. Cardiol. 109 (2014) 444. E.F. Lopez, J.H. Kabarowski, K.A. Ingle, V. Kain, S. Barnes, D.K. Crossman, et al., Obesity superimposed on aging magnifies inflammation and delays the resolving response after myocardial infarction, Am. J. Physiol. Heart Circ. Physiol. 308 (2015) H269–H280. E. Thorp, I. Tabas, Mechanisms and consequences of efferocytosis in advanced atherosclerosis, J. Leukoc. Biol. 86 (2009) 1089–1095. E. Wan, X.Y. Yeap, S. Dehn, R. Terry, M. Novak, S. Zhang, et al., Enhanced efferocytosis of apoptotic cardiomyocytes through myeloid-epithelial-reproductive tyrosine kinase links acute inflammation resolution to cardiac repair after infarction, Circ. Res. 113 (2013) 1004–1012. X.-Y.Y. Shuang Zhang, L. Grigoryeva, S. Dehn, D.B.M.T. Matthew, E. Rostlund, D. Schrijvers, Z. Jenny, R.S. Zhang, G. Warren, D.L. Tourtellott, J. Lomasney, J.M. AEBT, Cardiomyocytes induce macrophage receptor shedding to suppress phagocytosis, J. Mol. Cell. Cardiol. (2015). N.G. Frangogiannis, Regulation of the inflammatory response in cardiac repair, Circ. Res. 110 (2012) 159–173. J.M. Lambert, E.F. Lopez, M.L. Lindsey, Macrophage roles following myocardial infarction, Int. J. Cardiol. 130 (2008) 147–158. K.J. Moore, J. El Khoury, L.A. Medeiros, K. Terada, C. Geula, A.D. Luster, et al., A CD36initiated signaling cascade mediates inflammatory effects of beta-amyloid, J. Biol. Chem. 277 (2002) 47373–47379. A. Rohlmann, M. Gotthardt, R.E. Hammer, J. Herz, Inducible inactivation of hepatic LRP gene by cre-mediated recombination confirms role of LRP in clearance of chylomicron remnants, J. Clin. Invest. 101 (1998) 689–695. T.D. Camenisch, B.H. Koller, H.S. Earp, G.K. Matsushima, A novel receptor tyrosine kinase, Mer, inhibits TNF-alpha production and lipopolysaccharide-induced endotoxic shock, J. Immunol. 162 (1999) 3498–3503.
Vasundhara Kain Ganesh V. Halade⁎ Division of Cardiovascular Disease, Department of Medicine, The University of Alabama at Birmingham, Alabama, United States ⁎Corresponding author. E-mail address: [email protected] (G.V. Halade). 24 August 2015
Contents lists available at ScienceDirect
Journal of Molecular and Cellular Cardiology journal homepage: www.elsevier.com/locate/yjmcc
Editorial
Big eater macrophages dominate inflammation resolution following myocardial infarction Keywords: Myocardial infarction Macrophages Efferocytosis Resolution of inflammation
1. Diverse roles for big eater macrophages in myocardial healing Cardiomyocyte cell homeostasis is challenged during cell death, cell senescence, and cell replacement which occurs in response to injuries, including myocardial infarction (MI) or infection. Injured, infected, or aged myocytes die according to regular processes i.e. apoptosis and necrosis and are cleared in a chronological manner by macrophages through a process termed phagocytosis. Phagocytosis is an inherent role of the macrophages and is why these cells were referred to as ‘big eater’ immune cells by Élie Metchnikoff, who first described the process to win the Nobel Prize in 1908 [1]. Post-MI, phagocytic macrophages arrive at the site of injury to clear first responder neutrophils and myocardial debris, stimulated by a diverse array of peptide and lipid-mediated ‘find me’ signals. Of note, all macrophages are not the same; for example, embryoderived macrophages expand to protect the left ventricle from injury. Post-MI, however, embryo-derived resident macrophages are replaced by monocyte-derived macrophages to promote unresolved inflammation [2]. Therefore, optimal and timely clearance of dead cells requires efficient coordination of the receptors including scavenger receptors, the phosphatidyl serine receptor, thrombospondin receptor, integrins, and complement receptor 3 [3]. Thus, proximate interactions of dead myocytes with macrophages provide the foundation for effective efferocytosis and resolution of inflammation. Efferocytosis refers to clearing of dead cells (e.g. apoptotic or necrotic) by macrophages, which possess receptors for engulfing them. Surface characteristics of receptors mediate the clearance of dead cells with chemotactic guidance; and insufficient clearance and defective efferocytosis have been shown to advance the disease pathology of atherosclerosis, as well as heart failure [4,5]. The temporal and spatial kinetics of phagocytes and their interaction with myocyte or fibroblast-sourced debris are essential to understanding post-MI pathophysiology. The macrophages are the "big eaters," which coordinate with both immune and non-immune cells to resolve unremitting inflammation. Due to their role in resolving the inflammation, a subset of these cells are also referred to as M2 or antiinflammatory macrophages [4]. Post-MI, hypoxic cardiomyocytes and fibroblasts drive the inflammatory response, releasing intracellular
http://dx.doi.org/10.1016/j.yjmcc.2015.08.019 0022-2828/© 2015 Elsevier Ltd. All rights reserved.
components at the site of injury. The macrophage synthesizes and releases both pro-inflammatory and pro-resolving signals in collaboration with neutrophil and platelets to clear the inflammation. The coordination of leukocytes for the systematic clearance of dead cells is essential for expediting the resolution process [4]. Several co-morbidities, such as obesity and aging, impair the resolution ability of the macrophage. Macrophages exposed to obesity superimposed on aging develop defects in this process to generate a pro-inflammatory environment and, therefore, experience delayed resolution [5]. Several studies have linked the defective resolution axis to the non-clearance of apoptotic bodies, which contributes to chronic inflammation. Genetic manipulation in various targets, such as tissue transglutaminase, milk-fat globule EGF factor-8 (MFG-E8), complement C1q, lysophosphatidylcholines, Fas, and myeloid-epithelial-reproductive tyrosine kinase (MERTK), leads to the development of non-resolving inflammation in atherosclerosis plaques, indicating defective efferocytosis [6]. In addition to these targets, Wang et al. recently discovered that big eater macrophage receptor (MERTK) is essential for the clearance of dying adult cardiomyocytes and is important in resolving inflammation following MI. Post-MI MERTK is specifically induced on Ly6C low pro-resolving (M2) macrophages, which indicates its role in the resolution of inflammation. MERTK deficiency leads to an accumulation of necrotic cardiomyocytes, independent of the phagocytosis of non-cardiomyocytes, which leads to reduced efferocytosis [7]. The next question raised was about whether the ratio of cadiomyocyte to fibroblast phagocytosis has a similar effect. To answer this question, a recent article by Zhang et al. confirmed MERTK expression in failing human hearts as well as its novel mechanism of clearing hypoxic cardiomyocytes [8]. This result strengthens our understanding of the ordered, sequential steps and phagocytic efficiency of dying adult cardiomyocytes by macrophages (Fig. 1). Using transmission electron microscopy, the authors examined the formation of phagocytic cups on MERTK+/+ macrophages, which were interacting with target cells positive for myocyte markers desmin and actinin in the human ischemic heart. To provide evidence of a MERTK role in phagocytosis of cardiomyocytes, the authors used an experimental MI model in transgenic mice. This model had a cardiomyocyte-specific mCherry reporter gene which carried the MERTK-signal into the phagocytic cups, confirming its presence in ischemic mouse heart. Systemic ex-vivo efferocytosis methodology validated the rate limiting steps before and during cardiomyocyte engulfment by macrophages. The authors used MER-deficient mice (MERTK−/−) to confirm that MERTK deficiency leads to inefficient clearance of dead cardiomyocytes, therefore suppressing phagocytosis. Evaluation of human failing heart revealed upregulation of MERTK, providing clinical relevance.
226
Editorial
Fig. 1. Inefficient efferocytosis of cardiomyocytes due to myeloid-epithelial-reproductive tyrosine kinase (MERTK) deficiency leads to defective resolution of inflammation post-myocardial infarction (MI). Post-MI injury induces macrophages MERTK expression and clears the apoptotic cardiomyocytes thereby effective resolution of inflammation. Cardiomyocyte induces shedding of MERTK receptor making macrophages MERTK-deficient delaying clearance of dying cardiomyocytes resulting in defective resolution of inflammation.
2. Big eater macrophages digest debris post-MI The clearance of myocyte and other myocardial debris in a timely manner post-MI was used to determine whether the resolution process in cardiac healing and repair was efficient [9]. Heterogeneous monocyte populations commit to the macrophage lineage, which are phagocytic in nature and play role in post-MI healing by clearing dead cells [10]. The primary function of the macrophage is to engulf and digest necrotic cells within an injured tissue, while facilitating wound healing to stimulate resolution of inflammation. Several receptors have been described for macrophages, including CD36, low-density lipoprotein receptorrelated protein 1 (LRP), and MERTK [11,12,13].
The distinct presence of MERTK on macrophages and its role in cardiac repair post-MI has been experimentally demonstrated by Wang et al. [7]. The authors highlighted that MERTK+/+ macrophages digest a large size variety of cardiomyocyte and size does not affect efferocytosis efficiency. Using these criteria, the authors showed that the inefficient phagocytosis due to the shedding of the MERTK ectodomain from the macrophages was induced by cardiomyocytes themselves. The detailed experiments using an ex-vivo approach showed that enhancement of cardiomyocyte efferocytosis by cleavage-resistant MERTK was significant and could be one of the potential contributing mechanisms that affects cardiomyocyte phagocytic efficiency. Future studies will help to identify potential cardiomyocyte-specific triggers of MERTK cleavage, the time
Editorial
course of MERTK in heart failure pathology, and the physiological relevance of MERTK cleavage post-MI. Unresolved inflammation is a major cause of heart failure pathology. Several efforts have been ongoing to resolve the inflammation axis in order to delay heart failure. Future studies with a major focus on MERTK agonists and antagonists or contribution of MERTK to activate adaptive response in post-MI setting will advance new knowledge in regards to chronic heart failure pathology. At present, it is unclear whether embryo-derived resident macrophages or monocyte-derived infiltrating macrophages dominates MERTK expression. The identification of MERTK on human and mice tissue certainly opens up a new avenue for pursuing therapeutic targets for resolving inflammation post-MI cardiac remodeling. It is important to consider that the phagocytosis of dead cardiomyocytes is one of the processes that is involved in cardiac healing post-MI. Other cell types, particularly the fibroblasts, neutrophils, dendritic cells, B cells, and T cells, play an important role during the cardiac repair process. Taking into consideration these encouraging findings and future studies will provide more information about ligands and receptors associated with the efferocytosis capacity of the macrophage in post-MI pathological remodeling. Disclosure statement None. Acknowledgements The authors acknowledge support from NIH-NCCAM K99/R00 AT006704 and UAB start-up funds, awarded to GVH. The authors used Servier Medical Arts for Fig. 1. References [1] S. Gordon, Elie Metchnikoff: father of natural immunity, Eur. J. Immunol. 38 (2008) 3257–3264. [2] K.J. Lavine, S. Epelman, K. Uchida, K.J. Weber, C.G. Nichols, J.D. Schilling, et al., Distinct macrophage lineages contribute to disparate patterns of cardiac recovery and
227
[3] [4]
[5]
[6] [7]
[8]
[9] [10] [11]
[12]
[13]
remodeling in the neonatal and adult heart, Proc. Natl. Acad. Sci. U. S. A. 111 (2014) 16029–16034. D.M. Mosser, J.P. Edwards, Exploring the full spectrum of macrophage activation, Nat. Rev. Immunol. 8 (2008) 958–969. V. Kain, S.D. Prabhu, G.V. Halade, Inflammation revisited: inflammation versus resolution of inflammation following myocardial infarction, Basic Res. Cardiol. 109 (2014) 444. E.F. Lopez, J.H. Kabarowski, K.A. Ingle, V. Kain, S. Barnes, D.K. Crossman, et al., Obesity superimposed on aging magnifies inflammation and delays the resolving response after myocardial infarction, Am. J. Physiol. Heart Circ. Physiol. 308 (2015) H269–H280. E. Thorp, I. Tabas, Mechanisms and consequences of efferocytosis in advanced atherosclerosis, J. Leukoc. Biol. 86 (2009) 1089–1095. E. Wan, X.Y. Yeap, S. Dehn, R. Terry, M. Novak, S. Zhang, et al., Enhanced efferocytosis of apoptotic cardiomyocytes through myeloid-epithelial-reproductive tyrosine kinase links acute inflammation resolution to cardiac repair after infarction, Circ. Res. 113 (2013) 1004–1012. X.-Y.Y. Shuang Zhang, L. Grigoryeva, S. Dehn, D.B.M.T. Matthew, E. Rostlund, D. Schrijvers, Z. Jenny, R.S. Zhang, G. Warren, D.L. Tourtellott, J. Lomasney, J.M. AEBT, Cardiomyocytes induce macrophage receptor shedding to suppress phagocytosis, J. Mol. Cell. Cardiol. (2015). N.G. Frangogiannis, Regulation of the inflammatory response in cardiac repair, Circ. Res. 110 (2012) 159–173. J.M. Lambert, E.F. Lopez, M.L. Lindsey, Macrophage roles following myocardial infarction, Int. J. Cardiol. 130 (2008) 147–158. K.J. Moore, J. El Khoury, L.A. Medeiros, K. Terada, C. Geula, A.D. Luster, et al., A CD36initiated signaling cascade mediates inflammatory effects of beta-amyloid, J. Biol. Chem. 277 (2002) 47373–47379. A. Rohlmann, M. Gotthardt, R.E. Hammer, J. Herz, Inducible inactivation of hepatic LRP gene by cre-mediated recombination confirms role of LRP in clearance of chylomicron remnants, J. Clin. Invest. 101 (1998) 689–695. T.D. Camenisch, B.H. Koller, H.S. Earp, G.K. Matsushima, A novel receptor tyrosine kinase, Mer, inhibits TNF-alpha production and lipopolysaccharide-induced endotoxic shock, J. Immunol. 162 (1999) 3498–3503.
Vasundhara Kain Ganesh V. Halade⁎ Division of Cardiovascular Disease, Department of Medicine, The University of Alabama at Birmingham, Alabama, United States ⁎Corresponding author. E-mail address: [email protected] (G.V. Halade). 24 August 2015