Muramyl dipeptide enhances the response to endotoxin to cause multiple organ injury in the anesthetized rat

MURAMYL DIPEPTIDE CAUSES MITOCHONDRIAL DYSFUNCTION AND INTESTINAL INFLAMMATORY CYTOKINE RESPONSES IN RATS

ABSTRACT

Introduction: Intestinal flora and the translocation of its products, such as muramyl dipeptide (MDP), are common causes of sepsis. MDP is a common activator of the intracellular pattern recognition receptor NOD2, and MDP translocation can cause inflammatory damage to the small intestine and systemic inflammatory responses in rats. Therefore, this study investigated the effects of MDP on the intestinal mucosa and distant organs during sepsis and the role of the NOD2/AMPK/LC3 pathway in MDP-induced mitochondrial dysfunction in the intestinal epithelium. Methods: Fifty male Sprague Dawley rats were randomly divided into five treatment groups: lipopolysaccharide (LPS) only, 1.5 and 15 mg/kg MDP+LPS, and 1.5 and 15 mg/kg MDP+short-peptide enteral nutrition (SPEN)+LPS. The total caloric intake was the same per group. The rats were euthanized 24 h after establishing the model, and peripheral blood and small intestinal mucosal and lung tissues were collected. Results: Compared to the LPS group, both MDP+LPS groups had aggravated inflammatory damage to the intestinal mucosal and lung tissues, increased IL-6 and MDP production, increased NOD2 expression, decreased AMPK and LC3 expression, increased mitochondrial reactive oxygen species production, and decreased mitochondrial membrane potential. Compared to the MDP+LPS groups, the MDP+SPEN+LPS groups had decreased IL-6 and MDP production, increased AMPK and LC3 protein expression, and protected mitochondrial and organ functions. Conclusions: MDP translocation reduced mitochondrial autophagy by regulating the NOD2/AMPK/LC3 pathway, causing mitochondrial dysfunction. SPEN protected against MDP-induced impairment of intestinal epithelial mitochondrial function during sepsis.

Inhibition of specific signaling pathways rather than epigenetic silencing of effector genes is the leading mechanism of innate tolerance

Introduction

Macrophages activated through a pattern-recognition receptor (PRR) enter a transient state of tolerance characterized by diminished responsiveness to restimulation of the same receptor. Signaling-based and epigenetic mechanisms are invoked to explain this innate tolerance. However, these two groups of mechanisms should result in different outcomes. The epigenetic scenario (silencing of effector genes) predicts that activation of a PRR should broadly cross-tolerize to agonists of unrelated PRRs, whereas in the signaling-based scenario (inhibition of signaling pathways downstream of specific PRRs), cross-tolerization should occur only between agonists utilizing the same PRR and/or signaling pathway. Also, the so-called non-tolerizeable genes have been described, which acquire distinct epigenetic marks and increased responsiveness to rechallenge with the same agonist. The existence of such genes is well explained by epigenetic mechanisms but difficult to explain solely by signaling mechanisms.

Methods

To evaluate contribution of signaling and epigenetic mechanisms to innate tolerance, we tolerized human macrophages with agonists of TLR4 or NOD1 receptors, which signal via distinct pathways, and assessed responses of tolerized cells to homologous restimulation and to cross-stimulation using different signaling, metabolic and transcriptomic read-outs. We developed a transcriptomics-based approach to distinguish responses to secondary stimulation from continuing responses to primary stimulation.

Results

We found that macrophages tolerized with a NOD1 agonist lack responses to homologous restimulation, whereas LPS-tolerized macrophages partially retain the ability to activate NF-κB pathway upon LPS rechallenge, which allows to sustain low-level expression of a subset of pro-inflammatory genes. Contributing to LPS tolerance is blockade of signaling pathways required for IFN-β production, resulting in 'pseudo-tolerization' of IFN-regulated genes. Many genes in NOD1- or TLR4-tolerized macrophages are upregulated as the result of primary stimulation (due to continuing transcription and/or high mRNA stability), but do not respond to homologous restimulation. Hyperresponsiveness of genes to homologous rechallenge is a rare and inconsistent phenomenon. However, most genes that have become unresponsive to homologous stimuli show unchanged or elevated responses to agonists of PRRs signaling via distinct pathways.

Discussion

Thus, inhibition of specific signaling pathways rather than epigenetic silencing is the dominant mechanism of innate tolerance.

Innate control of adaptive immunity and adaptive instruction of innate immunity: bi-directional flow of information

The ability of the innate and adaptive immune systems to communicate with each other is central to protective immune responses and maintenance of host health. Myeloid cells of the innate immune system are able to sense microbial ligands, perturbations in cellular homeostasis, and virulence factors, thereby allowing them to relay distinct pathogen-specific information to naïve T cells in the form of pathogen-derived peptides and a unique cytokine milieu. Once primed, effector T helper cells produce lineage-defining cytokines to help combat the original pathogen, and a subset of these cells persist as memory or effector-memory populations. These memory T cells then play a dual role in host protection by not only responding rapidly to reinfection, but by also directly instructing myeloid cells to express licensing cytokines. This means there is a bi-directional flow of information first from the innate to the adaptive immune system, and then from the adaptive back to innate immune system. Here, we focus on how signals, first from pathogens and then from primed effector and memory T cells, are integrated by myeloid cells and its consequences for protective immunity or systemic inflammation.

Interplay between NOD1 and TLR4 Receptors in Macrophages: Nonsynergistic Activation of Signaling Pathways Results in Synergistic Induction of Proinflammatory Gene Expression

Interactions between pattern-recognition receptors shape innate immune responses to pathogens. NOD1 and TLR4 are synergistically interacting receptors playing a pivotal role in the recognition of Gram-negative bacteria. However, mechanisms of their cooperation are poorly understood. It is unclear whether synergy is produced at the level of signaling pathways downstream of NOD1 and TLR4 or at more distal levels such as gene transcription. We analyzed sequential stages of human macrophage activation by a combination of NOD1 and TLR4 agonists (N-acetyl-d-muramyl-l-alanyl-d-isoglutamyl-meso-diaminopimelic acid [M-triDAP] and LPS, respectively). We show that events preceding or not requiring activation of transcription, such as activation of signaling kinases, rapid boost of glycolysis, and most importantly, nuclear translocation of NF-κB, are regulated nonsynergistically. However, at the output of the nucleus, the combination of M-triDAP and LPS synergistically induces expression of a subset of M-triDAP- and LPS-inducible genes, particularly those encoding proinflammatory cytokines (TNF, IL1B, IL6, IL12B, and IL23A). This synergistic response develops between 1 and 4 h of agonist treatment and requires continuous signaling through NOD1. The synergistically regulated genes have a lower basal expression and higher inducibility at 4 h than those regulated nonsynergistically. Both gene subsets include NF-κB-inducible genes. Therefore, activation of the NF-κB pathway does not explain synergistic gene induction, implying involvement of other transcription factors. Inhibition of IKKβ or p38 MAPK lowers agonist-induced TNF mRNA expression but does not abolish synergy. Thus, nonsynergistic activation of NOD1- and TLR4-dependent signaling pathways results in the synergistic induction of a proinflammatory transcriptional program.

NOD1 and NOD2: Molecular targets in prevention and treatment of infectious diseases

Nucleotide-binding oligomerization domain (NOD) 1 and NOD2 are pattern-recognition receptors responsible for sensing fragments of bacterial peptidoglycan known as muropeptides. Stimulation of innate immunity by systemic or local administration of NOD1 and NOD2 agonists is an attractive means to prevent and treat infectious diseases. In this review, we discuss novel data concerning structural features of selective and non-selective (dual) NOD1 and NOD2 agonists, main signaling pathways and biological effects induced by NOD1 and NOD2 stimulation, including induction of pro-inflammatory cytokines, type I interferons and antimicrobial peptides, induction of autophagy, alterations of metabolism. We also discuss interactions between NOD1/NOD2 and Toll-like receptor agonists in terms of synergy and cross-tolerance. Finally, we review available animal data on the role of NOD1 and NOD2 in protection against infections, and discuss how these data could be applied in human infectious diseases.

Muramyl dipeptide enhances thermal injury-induced inflammatory cytokine production and organ function injury in rats

The bacterial infection following thermal injury is a very important factor of excessive inflammatory response and multiple organ damage. Muramyl dipeptide (MDP) is the key structure of gram-positive bacteria and gram-negative bacteria triggering the innate immune system. The aim of the present study was to determine the effect of MDP on thermal injury-induced inflammatory responses, organ function injury, and mortality in rats. Fifty male Sprague-Dawlay rats were randomly divided into three groups: normal control group, scald group, and MDP group. Scald group only suffered 20% total body surface area third-degree thermal injury. Muramyl dipeptide 5 mg·kg was administered through the femoral vein at 24 h after thermal injury in the MDP group. Plasma inflammatory cytokine levels were measured by enzyme-linked immunosorbent assay. An additional 90 male Sprague-Dawley rats were randomly divided into three groups to observe the survival rate in 72 h. Plasma levels of interleukin-6, interleukin-10, interferon-γ, and high-mobility group box 1; the white blood cell counts; the serum concentrations of alanine aminotransferase, aspartate aminotransferase, total bilirubin, creatine kinase isoenzyme-MB, blood urea nitrogen, and creatinine; and the activity of lung tissue myeloperoxidase significantly increased after thermal injury alone. Compared with the scald group, MDP led to more serious inflammatory responses and organ function damage and higher mortality (P < 0.05, respectively). These data indicate that MDP exacerbates thermal injury-induced inflammatory cytokine production, accompanied by multiple organ dysfunction syndrome and high mortality in rats.

Effects of exogenous vasoactive intestinal peptide on mesenteric lymph pathway during early intestinal ischemia-reperfusion injury in rats

Mesenteric lymph pathway serves as the primary route by which gut injury leads to systemic inflammation and distant organ injury. The inflammation of the intestinal tract is partially mediated by vasoactive intestinal peptide (VIP). Therefore, the aim of this study was to test whether exogenous VIP affects mesenteric lymph pathway during early intestinal ischemia-reperfusion (IIR) injury. Rats were randomized into control, control+VIP, IIR and IIR+VIP groups. The observation of mesenteric lymph flow was carried out by cannulation of mesenteric lymphatics. The distribution of in vivo lymphocyte trafficking was performed by (51)Cr labeled lymphocytes and was measured by γ-counter. Endotoxin concentration was assayed using the limulus test kit and TNF-α level was detected by ELISA. When IIR injury treated with VIP, the volumes of lymph flow increased by 80%, which caused the number of lymphocytes exiting in mesenteric lymphatic increased by 50% while the proportion of (51)Cr-lymphocytes in Peyer's patches, intestinal effector tissues, mesenteric nodes, large intestine, stomach decreased by 58%, 51%, 58%, 63%, 64% respectively at the 6th h after reperfusion following intestinal ischemia. Meanwhile, endotoxin and TNF-α levels in intestinal lymph decreased by 51% and 83%. These results suggest that exogenous VIP ameliorates IIR induced splanchnic organ damage via inhibition of toxic mediators reaching systemic circulation and reinforcement of the effective immune responses in gut-associated lymphoid tissues (GALT).

Phenotyping of Nod1/2 double deficient mice and characterization of Nod1/2 in systemic inflammation and associated renal disease

It is indispensable to thoroughly characterize each animal model in order to distinguish between primary and secondary effects of genetic changes. The present study analyzed Nod1 and Nod2 double deficient (Nod1/2 DKO) mice under physiological and inflammatory conditions. Nod1 and Nod2 are members of the Nucleotide-binding domain and Leucine-rich repeat containing Receptor (NLR) family. Several inflammatory disorders, such as Crohn's disease and asthma, are linked to genetic changes in either Nod1 or Nod2. These associations suggest that Nod1 and Nod2 play important roles in regulating the immune system.

Three-month-old wildtype (Wt) and Nod1/2 DKO mice were sacrificed, body and organ weight were determined, and blood was drawn. Except for lower liver weight in Nod1/2 DKO mice, no differences were found in body/organ weight between both strains. Leukocyte count and composition was comparable. No significant changes in analyzed plasma biochemical markers were found. Additionally, intestinal and vascular permeability was determined. Nod1/2 DKO mice show increased susceptibility for intestinal permeability while vascular permeability was not affected. Next we induced septic shock and organ damage by administering LPS+PGN intraperitoneally to Wt and Nod1/2 DKO mice and sacrificed animals after 2 and 24 hours. The systemic inflammatory and metabolic response was comparable between both strains. However, renal response was different as indicated by partly preserved kidney function and tubular epithelial cell damage in Nod1/2 DKO at 24 hours. Remarkably, renal inflammatory mediators Tnfα, KC and Il-10 were significantly increased in Nod1/2 DKO compared with Wt mice at 2 hours.

Systematic analysis of Nod1/2 DKO mice revealed a possible role of Nod1/2 in the development of renal disease during systemic inflammation.

NOD2 signaling contributes to host defense in the lungs against Escherichia coli infection

Bacterial pneumonia remains a significant cause of mortality in the United States. The innate immune response is the first line of defense against invading bacteria. Neutrophil recruitment to the lungs is the first step in a multistep sequence leading to bacterial clearance. Ligand interaction with pattern-recognizing receptors (PRRs) leads to chemokine production, which drives neutrophils to the site of infection. Although we demonstrated that RIP2 is important for host defense in the lungs against Escherichia coli, the individual roles of NOD1 and NOD2 in pulmonary defense have not been addressed. Here, we explored the role of NOD2 in neutrophil-mediated host defense against an extracellular pathogen, E. coli. We found enhanced bacterial burden and reduced neutrophil and cytokine/chemokine levels in the lungs of NOD2⁻/⁻ mice following E. coli infection. Furthermore, we observed reduced activation of NF-κB and mitogen-activated protein kinases (MAPKs) in the lungs of NOD2⁻/⁻ mice upon E. coli challenge. Moreover, NOD2⁻/⁻ neutrophils show impaired intracellular bacterial killing. Using NOD2/RIP2⁻/⁻ mice, we observed bacterial burden and neutrophil accumulation in the lungs similar to those seen with NOD2⁻/⁻ mice. In addition, bone marrow-derived macrophages obtained from NOD2/RIP2⁻/⁻ mice demonstrate a reduction in activation of NF-κB and MAPKs similar to that seen with NOD2⁻/⁻ mice in response to E. coli. These findings unveil a previously unrecognized role of the NOD2-RIP2 axis for host defense against extracellular Gram-negative bacteria. This pathway may represent a novel target for the treatment of lung infection/inflammation.

Pharmacological postconditioning effect of muramyl dipeptide is mediated through RIP2 and TAK1

AIMS

Despite their ability to cause septic shock and myocardial dysfunction, components of Gram-negative bacterial cell walls, like lipopolysaccharide, have been shown in numerous studies to induce myocardial protection during ischaemia-reperfusion injury. Muramyl dipeptide (MDP) is another such component recognized by an intracellular receptor, nucleotide-binding oligomerization domain 2. Receptor activation leads to intracellular signals through receptor interacting protein-2 (RIP2) and tumour growth factor-beta-activated kinase-1 (TAK1). However, little is known about the RIP2/TAK1 pathway in the heart. The aim of this study was to determine whether the RIP2/TAK1 pathway has a cardioprotective role in a mouse model of myocardial infarction.

METHODS AND RESULTS

We isolated and subjected wild-type (WT) and RIP2(-/-) mouse hearts to 30 min of global ischaemia and 120 min of reperfusion with or without perfusion of MDP (10 microg/mL) before or after the ischaemic period and determined the infarct size. We examined activation of the TAK1/nuclear factor kappaB (NFkappaB) signalling pathway. The effect of TAK1 inhibition on MDP-induced cardioprotection was also evaluated. Exposure to MDP during reperfusion significantly reduced infarct size in WT hearts (from 51.7 +/- 5.6% in control to 38.1 +/- 6.7%, P < 0.05), but not in RIP2(-/-) hearts or in WT hearts with coincident pharmacological inhibition of TAK1. MDP treatment significantly increased the levels of p-TAK1 and p-JNK (Jun N-terminal kinase) and led to NFkappaB activation via phosphorylation and degradation of IkappaB in the WT, but not in the RIP2(-/-), myocardium.

CONCLUSION

These results indicate that MDP at reperfusion induced cardioprotection through an RIP2/TAK1-dependent mechanism.

The role of RIP2 in p38 MAPK activation in the stressed heart

The activation of p38 MAPK by dual phosphorylation aggravates myocardial ischemic injury and depresses cardiac contractile function. SB203580, an ATP-competitive inhibitor of p38 MAPK and other kinases, prevents this dual phosphorylation during ischemia. Studies in non-cardiac tissue have shown receptor-interacting protein 2 (RIP2) lies upstream of p38 MAPK, is SB203580-sensitive and ischemia-responsive, and aggravates ischemic injury. We therefore examined the RIP2-p38 MAPK signaling axis in the heart. Adenovirus-driven expression of wild-type RIP2 in adult rat ventricular myocytes caused robust, SB203580-sensitive dual phosphorylation of p38 MAPK associated with activation of p38 MAPK kinases MKK3, MKK4, and MKK6. The effect of SB203580 was recapitulated by unrelated inhibitors of RIP2 or the downstream MAPK kinase kinase, TAK1. However, overexpression of wild-type, kinase-dead, caspase recruitment domain-deleted, or kinase-dead and caspase recruitment domain-deleted forms of RIP2 had no effect on the activating dual phosphorylation of p38 MAPK during simulated ischemia. Similarly, p38 MAPK activation and myocardial infarction size in response to true ischemia did not differ between hearts from wild-type and RIP2 null mice. However, both p38 MAPK activation and the contractile depression caused by the endotoxin component muramyl dipeptide were attenuated by SB203580 and in RIP2 null hearts. Although RIP2 can cause myocardial p38 MAPK dual phosphorylation in the heart under some circumstances, it is not responsible for the SB203580-sensitive pattern of activation during ischemia.