Free
Editorial Views  |   March 2011
Toll-like Receptor Signaling during Myocardial Ischemia
Author Notes
  • Mucosal Inflammation Program, Department of Anesthesiology, University of Colorado Denver, Aurora, Colorado.
Article Information
Editorial Views / Cardiovascular Anesthesia
Editorial Views   |   March 2011
Toll-like Receptor Signaling during Myocardial Ischemia
Anesthesiology 3 2011, Vol.114, 490-492. doi:10.1097/ALN.0b013e31820a4d78
Anesthesiology 3 2011, Vol.114, 490-492. doi:10.1097/ALN.0b013e31820a4d78
ACUTE myocardial infarction and myocardial ischemia are leading causes of short- and long-term morbidity and mortality in the perioperative period. In part, this relates to the fact that treatment options for a myocardial infarction in surgical patients are extremely limited. For example, anticoagulation or treatment with highly effective inhibitors of platelet aggregation may not be an option due to the risk for bleeding from the operative site. Therefore, it is not surprising that the search for new pharmacologic approaches to treat myocardial ischemia or to render the myocardium more resistant to limited oxygen availability is an area of intense investigation for perioperative scientists.1–4 In fact, the introduction of intraoperative β-blockers for cardioprotection from ischemia was one of the most significant alterations in anesthesia practice during the past 15 y. In line with the search for novel therapeutic approaches to render the myocardium more resistant to ischemia, a very exciting research study by Wang et al.  , from the research laboratory of Dr. Wei Chao from the Anesthesia Center for Critical Care Research of the Massachusetts General Hospital in Boston, provides new insight into signaling pathways involving toll-like receptor (TLR)-elicited cardioprotection from ischemia.1 
TLRs are named after the German word “toll,” which carries the meaning of “great” or “amazing.” In fact, a team of scientists led by Dr. Christiane Nüsslein-Volhard, Ph.D. (Professor, Max Planck Institute, Tübingen, Germany) identified Toll as a gene critical in the embryogenesis of fruit flies by establishing the dorsal-ventral axis of Drosophila  .5,6 The name derives from Christiane Nüsslein-Volhard's 1985 exclamation, “Das war ja toll,” which translates as “that's amazing,” in reference to the underdeveloped ventral portion of a fruit fly larva. Together with Edward B. Lewis and Eric Wieschaus, she received the Nobel Prize in Physiology or Medicine in 1995 for her discoveries concerning the genetic control of early embryonic development. Ten years after the original discovery of the Toll gene, a novel role for Toll beyond its function in embryogenesis was discovered.7 In fact, mutant flies deficient in Toll were more susceptible to fungal or bacterial infections.7 In 1997, a group of scientists from Yale University led by Ruslan Medzhitov (Professor, Yale University School of Medicine, New Haven, CT) and Charles Janeway Jr. (Professor, Yale University School of Medicine, New Haven, CT) (1943–2003) described a human homolog of the Drosophila  Toll protein that had the resemblance of a cytoplasmatic receptor (“Toll-like”).8 In this landmark study, human Toll was identified as a type I transmembrane protein with an extracellular domain consisting of a leucine-rich repeat domain, and a cytoplasmic domain homologous to the cytoplasmic domain of the human interleukin (IL)-1 receptor. Moreover, constitutively active mutant of human Toll transfected into human cell lines could induce the activation of nuclear factor (NF)κB and the expression of NFκB-target genes.8 As such, today's view of “Toll-like receptors” has evolved toward a class of receptor proteins that play a key role in the innate immune system.9 They are single-membrane–spanning, noncatalytic receptors that recognize structurally conserved molecules derived from microbes (pattern recognition receptors). For example, studies of mice deficient in TLR4—the key TLR for the recognition of lipopolysaccharide—show hyporesponsiveness to lipopolysaccharide treatment.10 Currently, 10 TLR family members have been described in human.11 They differ in their specificity for different microbial compounds. For example TLR2 can be activated by lipoteichoic acid from Gram-positive bacteria, TLR4 by lipopolysaccharide from Gram-negative bacteria, or TLR5 by flagellin, the filament of bacterial flagella found on most motile bacteria.12,13 Activation of TLRs involves specific signal transduction pathways, including myeloid differentiation factor 88 (MyD88)- or TIR–domain-containing adaptor protein-inducing interferon-β–mediated transcription factor (Trif)-dependent (MyD88-independent) pathways that culminate in the activation of NFκB, thus promoting a pro-inflammatory host program.11,14 TLR signaling has been implicated in many forms of inflammatory disorders in human. For instance, a cosegregating missense mutation (Asp299Gly and Thr399Ile) affecting the extracellular domain of the TLR4 receptor is associated with a blunted response to inhaled lipopolysaccharide in humans.15 This common Asp299Gly polymorphism of TLR4 also predicts low concentrations of circulating inflammatory mediators and confers an increased risk of severe infections but carries a reduced risk of atherosclerosis in human.16 
Although TLR signaling has been extensively studied in the context of innate immunity, other studies have also implicated TLR signaling in noninfectious tissue injury, such as occurs in the context of myocardial infarction. As such, the studies by Wang et al.  1 build on the observation that previous administration of a small, nonlethal dose of the TLR4 agonist lipopolysaccharide results in robust cardioprotection from subsequent myocardial ischemia–reperfusion injury.17 To gain mechanistic insight on how TLR4 activation confers cardioprotection from ischemia, the team of scientists combined very elegant pharmacologic and genetic approaches in mice treated with systemic lipopolysaccharide. For this purpose, lipopolysaccharide- or vehicle-treated mice were subsequently (24 h later) exposed to myocardial ischemia using a Langendorff apparatus, and myocardial infarct sizes and cardiac function were examined. These studies demonstrated robust cardioprotection with TLR4-agonist treatment (approximately 50% reduction in infarct size) that was TLR4 specific. In fact, Tlr4  −/−mice were not protected by lipopolysaccharide pretreatment, whereas Tlr2  −/−mice demonstrated similar cardioprotection as wild-type animals. Studies in mice deficient for MyD88 or Trif revealed MyD88-dependent and Trif-independent signal transduction. Consistent with previous in vitro  studies from the laboratory of Dr. Chao,18 their current studies provide genetic in vivo  evidence that these signaling events finally converge on inducible nitric oxide synthase and soluble guanylate cyclase. The authors have to be congratulated on putting together such a comprehensive number of genetic models that allowed their team to convincingly delineate this pathway in vivo  . In contrast to studies on anesthetic preconditioning, where an in vivo  knockdown of the receptor mediating the specific effects of a “cardioprotective anesthetic” has been literally impossible (currently, an “isoflurane-receptor knockout mouse” does not exist), the authors were able to target specifically the receptor critical for TLR-dependent cardioprotection and provide an extremely convincing level of evidence in support of their hypothesis via  a combination of genetic and pharmacologic approaches.
In contrast to their findings, and somewhat paradoxical to the central hypothesis of their article, are studies in TLR-deficient mice exposed directly to myocardial ischemia–reperfusion injury. For example, an in situ  study of myocardial ischemia–reperfusion injury exposed gene-targeted mice for Tlr4  to 60 min of coronary artery ligation, followed by 24 h of reperfusion. The authors observed attenuated infarct sizes in Tlr4  -deficient mice in conjunction with attenuated myocardial inflammation, as assessed by neutrophil accumulation, lipid peroxidase, and complement deposition.19 Similar to these apparent discrepancies, previous studies have found a dual role of signaling pathways that result in activation of NFκB during ischemia-induced inflammation. In fact, a very elegant study from the laboratory of Michael Karin investigated the role of NFκB in acute inflammation caused by intestinal ischemia–reperfusion through selective ablation of IκB kinase (IKK)-β, the catalytic subunit of IKK that is essential for NFκB activation.20 Ablation of IKK-β in enterocytes prevented the systemic inflammatory response that culminates in multiple organ dysfunction syndrome and is normally triggered by gut ischemia–reperfusion. However, IKK-β removal from enterocytes also resulted in severe apoptotic damage to the reperfused intestinal mucosa. Thus, these studies demonstrate a dual function of the NFκB system, which is responsible for both tissue protection and systemic inflammation.20 Consistent with these findings, a pro-inflammatory imbalance after myocardial ischemia–reperfusion injury is attenuated in Tlr4  −/−mice,19 whereas TLR-agonist treatment is associated with cardioprotective responses via  dampening ischemia-driven cardiomyocyte apoptosis (fig. 1).17 
Fig. 1.  Dual role of Toll-like receptor signaling in myocardial ischemia–reperfusion injury. A study by Wang et al.  published in the current edition of Anesthesiology provides genetic and pharmacologic evidence for a cardioprotective role of Toll-like receptor signaling (TLR) during myocardial ischemia. Pretreatment with lipopolysaccharide (LPS) provides potent protection from subsequent myocardial ischemia, involving signal transduction through MyD88, and convergence on inducible nitric oxide synthase and guanylate cyclase. As such, TLR4-dependent activation of nuclear factor (NF)κB could provide cardioprotection from ischemia by dampening programmed myocardial cell death (apoptosis). However, previous studies in knockout mice for TLR4 or MyD88 demonstrate attenuated susceptibility to myocardial ischemia–reperfusion injury. These later findings could reflect increases in myocardial inflammation during ischemia–reperfusion injury.
Fig. 1. 
	Dual role of Toll-like receptor signaling in myocardial ischemia–reperfusion injury. A study by Wang et al. 
	published in the current edition of Anesthesiology provides genetic and pharmacologic evidence for a cardioprotective role of Toll-like receptor signaling (TLR) during myocardial ischemia. Pretreatment with lipopolysaccharide (LPS) provides potent protection from subsequent myocardial ischemia, involving signal transduction through MyD88, and convergence on inducible nitric oxide synthase and guanylate cyclase. As such, TLR4-dependent activation of nuclear factor (NF)κB could provide cardioprotection from ischemia by dampening programmed myocardial cell death (apoptosis). However, previous studies in knockout mice for TLR4 or MyD88 demonstrate attenuated susceptibility to myocardial ischemia–reperfusion injury. These later findings could reflect increases in myocardial inflammation during ischemia–reperfusion injury.
Fig. 1.  Dual role of Toll-like receptor signaling in myocardial ischemia–reperfusion injury. A study by Wang et al.  published in the current edition of Anesthesiology provides genetic and pharmacologic evidence for a cardioprotective role of Toll-like receptor signaling (TLR) during myocardial ischemia. Pretreatment with lipopolysaccharide (LPS) provides potent protection from subsequent myocardial ischemia, involving signal transduction through MyD88, and convergence on inducible nitric oxide synthase and guanylate cyclase. As such, TLR4-dependent activation of nuclear factor (NF)κB could provide cardioprotection from ischemia by dampening programmed myocardial cell death (apoptosis). However, previous studies in knockout mice for TLR4 or MyD88 demonstrate attenuated susceptibility to myocardial ischemia–reperfusion injury. These later findings could reflect increases in myocardial inflammation during ischemia–reperfusion injury.
×
Ultimately the goal of the study by Wang et al.  is to identify novel therapeutic approaches for perioperative myocardial injury,1 but some future challenges have to be overcome. The current studies were carried out in a Langendorff apparatus in the absence of inflammatory cells. It is conceivable that activation of myocardial TLRs and subsequent activation of an NFκB-elicited pro-inflammatory program could enhance inflammatory cell trafficking into the myocardium. As such, it will be an important step to test TLR4 agonists in intact models of myocardial ischemia–reperfusion injury in the presence of inflammatory cells.21,22 Although other studies seek to identify ways to dampen hypoxia-elicited inflammation,23–26 the consequences of a single bolus treatment with lipopolysaccharide on myocardial inflammation after ischemia reperfusion injury needs to be clearly addressed. In addition, the therapeutic window for TLR agonist treatment has yet to be defined. The authors can produce robust cardioprotection with a 24-h pretreatment approach, but it would be highly desirable for the perioperative setting to use a TLR agonist at the onset of myocardial ischemia. As pointed out by the authors, additional limitations of current TLR4 agonists are their pyrogenic side effects. As such, the development of nonpyrogenic TLR agonists will be required to directly target this pathway for the perioperative treatment of myocardial ischemia reperfusion injury. However, none of these comments should distract from the enthusiasm about the studies provided by the laboratory of Dr. Chao. In our mind, the elegant experimental design, the approach of combining pharmacologic and genetic studies, the vision of characterizing molecular targets and pathways beyond the field of anesthetics, and the important therapeutic implications of their research work set the stage for future innovations in perioperative cardioprotection. No doubt, Dr. Chao and his colleagues have set the bar very high.
The authors acknowledge Shelley A. Eltzschig, B.S.B.A., artist, Mucosal Inflammation Program, University of Colorado Denver, for the artwork included in this article.
Mucosal Inflammation Program, Department of Anesthesiology, University of Colorado Denver, Aurora, Colorado.
References
Wang E, Feng Y, Zhang M, Zou L, Li Y, Buys ES, Huang P, Brouckaert P, Chao W: Toll-like receptor 4 signaling confers cardiac protection against ischemic injury via  inducible nitric oxide synthase- and soluble guanylate cyclase-dependent mechanisms. Anesthesiology 2011; 114:603–13Wang, E Feng, Y Zhang, M Zou, L Li, Y Buys, ES Huang, P Brouckaert, P Chao, W
Eckle T, Köhler D, Lehmann R, El Kasmi K, Eltzschig HK: Hypoxia-inducible factor-1 is central to cardioprotection: A new paradigm for ischemic preconditioning. Circulation 2008; 118:166–75Eckle, T Köhler, D Lehmann, R El Kasmi, K Eltzschig, HK
Eckle T, Krahn T, Grenz A, Köhler D, Mittelbronn M, Ledent C, Jacobson MA, Osswald H, Thompson LF, Unertl K, Eltzschig HK: Cardioprotection by ecto-5′-nucleotidase (CD73) and A2B adenosine receptors. Circulation 2007; 115:1581–90Eckle, T Krahn, T Grenz, A Köhler, D Mittelbronn, M Ledent, C Jacobson, MA Osswald, H Thompson, LF Unertl, K Eltzschig, HK
Köhler D, Eckle T, Faigle M, Grenz A, Mittelbronn M, Laucher S, Hart ML, Robson SC, Müller CE, Eltzschig HK: CD39/ectonucleoside triphosphate diphosphohydrolase 1 provides myocardial protection during cardiac ischemia/reperfusion injury. Circulation 2007; 116:1784–94Köhler, D Eckle, T Faigle, M Grenz, A Mittelbronn, M Laucher, S Hart, ML Robson, SC Müller, CE Eltzschig, HK
Anderson KV, Bokla L, Nüsslein-Volhard C: Establishment of dorsal-ventral polarity in the Drosophila  embryo: The induction of polarity by the Toll gene product. Cell 1985; 42:791–8Anderson, KV Bokla, L Nüsslein-Volhard, C
Anderson KV, Jürgens G, Nüsslein-Volhard C: Establishment of dorsal-ventral polarity in the Drosophila  embryo: Genetic studies on the role of the Toll gene product. Cell 1985; 42:779–89Anderson, KV Jürgens, G Nüsslein-Volhard, C
Lemaitre B, Nicolas E, Michaut L, Reichhart JM, Hoffmann JA: The dorsoventral regulatory gene cassette spätzle/Toll/cactus controls the potent antifungal response in Drosophila  adults. Cell 1996; 86:973–83Lemaitre, B Nicolas, E Michaut, L Reichhart, JM Hoffmann, JA
Medzhitov R, Preston-Hurlburt P, Janeway CA Jr.: A human homologue of the Drosophila  Toll protein signals activation of adaptive immunity. Nature 1997; 388:394–7Medzhitov, R Preston-Hurlburt, P Janeway, CA
Cook DN, Pisetsky DS, Schwartz DA: Toll-like receptors in the pathogenesis of human disease. Nat Immunol 2004; 5:975–9Cook, DN Pisetsky, DS Schwartz, DA
Hoshino K, Takeuchi O, Kawai T, Sanjo H, Ogawa T, Takeda Y, Takeda K, Akira S: Cutting edge: Toll-like receptor 4 (TLR4)-deficient mice are hyporesponsive to lipopolysaccharide: Evidence for TLR4 as the Lps gene product. J Immunol 1999; 162:3749–52Hoshino, K Takeuchi, O Kawai, T Sanjo, H Ogawa, T Takeda, Y Takeda, K Akira, S
Chao W: Toll-like receptor signaling: A critical modulator of cell survival and ischemic injury in the heart. Am J Physiol Heart Circ Physiol 2009; 296:H1–12Chao, W
Akira S, Uematsu S, Takeuchi O: Pathogen recognition and innate immunity. Cell 2006; 124:783–801Akira, S Uematsu, S Takeuchi, O
Kuhlicke J, Frick JS, Morote-Garcia JC, Rosenberger P, Eltzschig HK: Hypoxia inducible factor (HIF)-1 coordinates induction of Toll-like receptors TLR2 and TLR6 during hypoxia. PLoS ONE 2007; 2:e1364Kuhlicke, J Frick, JS Morote-Garcia, JC Rosenberger, P Eltzschig, HK
Feng Y, Zou L, Si R, Nagasaka Y, Chao W: Bone marrow MyD88 signaling modulates neutrophil function and ischemic myocardial injury. Am J Physiol Cell Physiol 2010; 299:C760–9Feng, Y Zou, L Si, R Nagasaka, Y Chao, W
Arbour NC, Lorenz E, Schutte BC, Zabner J, Kline JN, Jones M, Frees K, Watt JL, Schwartz DA: TLR4 mutations are associated with endotoxin hyporesponsiveness in humans. Nat Genet 2000; 25:187–91Arbour, NC Lorenz, E Schutte, BC Zabner, J Kline, JN Jones, M Frees, K Watt, JL Schwartz, DA
Kiechl S, Lorenz E, Reindl M, Wiedermann CJ, Oberhollenzer F, Bonora E, Willeit J, Schwartz DA: Toll-like receptor 4 polymorphisms and atherogenesis. N Engl J Med 2002; 347:185–92Kiechl, S Lorenz, E Reindl, M Wiedermann, CJ Oberhollenzer, F Bonora, E Willeit, J Schwartz, DA
Chao W, Shen Y, Zhu X, Zhao H, Novikov M, Schmidt U, Rosenzweig A: Lipopolysaccharide improves cardiomyocyte survival and function after serum deprivation. J Biol Chem 2005; 280:21997–2005Chao, W Shen, Y Zhu, X Zhao, H Novikov, M Schmidt, U Rosenzweig, A
Zhu X, Zhao H, Graveline AR, Buys ES, Schmidt U, Bloch KD, Rosenzweig A, Chao W: MyD88 and NOS2 are essential for toll-like receptor 4-mediated survival effect in cardiomyocytes. Am J Physiol Heart Circ Physiol 2006; 291:H1900–9Zhu, X Zhao, H Graveline, AR Buys, ES Schmidt, U Bloch, KD Rosenzweig, A Chao, W
Oyama J, Blais C, Jr., Liu X, Pu M, Kobzik L, Kelly RA, Bourcier T: Reduced myocardial ischemia-reperfusion injury in toll-like receptor 4-deficient mice. Circulation 2004; 109:784–9Oyama, J Blais, C Liu, X Pu, M Kobzik, L Kelly, RA Bourcier, T
Chen LW, Egan L, Li ZW, Greten FR, Kagnoff MF, Karin M: The two faces of IKK and NF-kappaB inhibition: Prevention of systemic inflammation but increased local injury following intestinal ischemia-reperfusion. Nat Med 2003; 9:575–81Chen, LW Egan, L Li, ZW Greten, FR Kagnoff, MF Karin, M
Eckle T, Grenz A, Köhler D, Redel A, Falk M, Rolauffs B, Osswald H, Kehl F, Eltzschig HK: Systematic evaluation of a novel model for cardiac ischemic preconditioning in mice. Am J Physiol Heart Circ Physiol 2006; 291:H2533–40Eckle, T Grenz, A Köhler, D Redel, A Falk, M Rolauffs, B Osswald, H Kehl, F Eltzschig, HK
Eltzschig HK, Köhler D, Eckle T, Kong T, Robson SC, Colgan SP: Central role of Sp1-regulated CD39 in hypoxia/ischemia protection. Blood 2009; 113:224–32Eltzschig, HK Köhler, D Eckle, T Kong, T Robson, SC Colgan, SP
Eckle T, Grenz A, Laucher S, Eltzschig HK: A2B adenosine receptor signaling attenuates acute lung injury by enhancing alveolar fluid clearance in mice. J Clin Invest 2008; 118:3301–15Eckle, T Grenz, A Laucher, S Eltzschig, HK
Morote-Garcia JC, Rosenberger P, Kuhlicke J, Eltzschig HK: HIF-1-dependent repression of adenosine kinase attenuates hypoxia-induced vascular leak. Blood 2008; 111: 5571–80Morote-Garcia, JC Rosenberger, P Kuhlicke, J Eltzschig, HK
Morote-Garcia JC, Rosenberger P, Nivillac NM, Coe IR, Eltzschig HK: Hypoxia-inducible factor-dependent repression of equilibrative nucleoside transporter 2 attenuates mucosal inflammation during intestinal hypoxia. Gastroenterology 2009; 136:607–18Morote-Garcia, JC Rosenberger, P Nivillac, NM Coe, IR Eltzschig, HK
Rosenberger P, Schwab JM, Mirakaj V, Masekowsky E, Mager A, Morote-Garcia JC, Unertl K, Eltzschig HK: Hypoxia-inducible factor-dependent induction of netrin-1 dampens inflammation caused by hypoxia. Nat Immunol 2009; 10:195– 202Rosenberger, P Schwab, JM Mirakaj, V Masekowsky, E Mager, A Morote-Garcia, JC Unertl, K Eltzschig, HK
Fig. 1.  Dual role of Toll-like receptor signaling in myocardial ischemia–reperfusion injury. A study by Wang et al.  published in the current edition of Anesthesiology provides genetic and pharmacologic evidence for a cardioprotective role of Toll-like receptor signaling (TLR) during myocardial ischemia. Pretreatment with lipopolysaccharide (LPS) provides potent protection from subsequent myocardial ischemia, involving signal transduction through MyD88, and convergence on inducible nitric oxide synthase and guanylate cyclase. As such, TLR4-dependent activation of nuclear factor (NF)κB could provide cardioprotection from ischemia by dampening programmed myocardial cell death (apoptosis). However, previous studies in knockout mice for TLR4 or MyD88 demonstrate attenuated susceptibility to myocardial ischemia–reperfusion injury. These later findings could reflect increases in myocardial inflammation during ischemia–reperfusion injury.
Fig. 1. 
	Dual role of Toll-like receptor signaling in myocardial ischemia–reperfusion injury. A study by Wang et al. 
	published in the current edition of Anesthesiology provides genetic and pharmacologic evidence for a cardioprotective role of Toll-like receptor signaling (TLR) during myocardial ischemia. Pretreatment with lipopolysaccharide (LPS) provides potent protection from subsequent myocardial ischemia, involving signal transduction through MyD88, and convergence on inducible nitric oxide synthase and guanylate cyclase. As such, TLR4-dependent activation of nuclear factor (NF)κB could provide cardioprotection from ischemia by dampening programmed myocardial cell death (apoptosis). However, previous studies in knockout mice for TLR4 or MyD88 demonstrate attenuated susceptibility to myocardial ischemia–reperfusion injury. These later findings could reflect increases in myocardial inflammation during ischemia–reperfusion injury.
Fig. 1.  Dual role of Toll-like receptor signaling in myocardial ischemia–reperfusion injury. A study by Wang et al.  published in the current edition of Anesthesiology provides genetic and pharmacologic evidence for a cardioprotective role of Toll-like receptor signaling (TLR) during myocardial ischemia. Pretreatment with lipopolysaccharide (LPS) provides potent protection from subsequent myocardial ischemia, involving signal transduction through MyD88, and convergence on inducible nitric oxide synthase and guanylate cyclase. As such, TLR4-dependent activation of nuclear factor (NF)κB could provide cardioprotection from ischemia by dampening programmed myocardial cell death (apoptosis). However, previous studies in knockout mice for TLR4 or MyD88 demonstrate attenuated susceptibility to myocardial ischemia–reperfusion injury. These later findings could reflect increases in myocardial inflammation during ischemia–reperfusion injury.
×