Peptidoglycan fragments decrease food intake and body weight gain in rats

Peptidoglycan O-Acetylation as a Virulence Factor: Its Effect on Lysozyme in the Innate Immune System

The peptidoglycan sacculus of both Gram-positive and Gram-negative bacteria acts as a protective mesh and provides structural support around the entirety of the cell. The integrity of this structure is of utmost importance for cell viability and so naturally is the first target for attack by the host immune system during bacterial infection. Lysozyme, a muramidase and the first line of defense of the innate immune system, targets the peptidoglycan sacculus hydrolyzing the β-(1→4) linkage between repeating glycan units, causing lysis and the death of the invading bacterium. The O-acetylation of N-acetylmuramoyl residues within peptidoglycan precludes the productive binding of lysozyme, and in doing so renders it inactive. This modification has been shown to be an important virulence factor in pathogens such as Staphylococcus aureus and Neisseria gonorrhoeae and is currently being investigated as a novel target for anti-virulence therapies. This article reviews interactions made between peptidoglycan and the host immune system, specifically with respect to lysozyme, and how the O-acetylation of the peptidoglycan interrupts these interactions, leading to increased pathogenicity.

Synergistic effects of NOD1 or NOD2 and TLR4 activation on mouse sickness behavior in relation to immune and brain activity markers

Toll-like receptors (TLRs) and nuclear-binding domain (NOD)-like receptors (NLRs) are sensors of bacterial cell wall components to trigger an immune response. The TLR4 agonist lipopolysaccharide (LPS) is a strong immune activator leading to sickness and depressed mood. NOD agonists are less active but can prime immune cells to augment LPS-induced cytokine production. Since the impact of NOD and TLR co-activation in vivo has been little studied, the effects of the NOD1 agonist FK565 and the NOD2 agonist muramyl dipeptide (MDP), alone and in combination with LPS, on immune activation, brain function and sickness behavior were investigated in male C57BL/6N mice. Intraperitoneal injection of FK565 (0.001 or 0.003mg/kg) or MDP (1 or 3mg/kg) 4h before LPS (0.1 or 0.83mg/kg) significantly aggravated and prolonged the LPS-evoked sickness behavior as deduced from a decrease in locomotion, exploration, food intake and temperature. When given alone, FK565 and MDP had only minor effects. The exacerbation of sickness behavior induced by FK565 or MDP in combination with LPS was paralleled by enhanced plasma protein and cerebral mRNA levels of proinflammatory cytokines (IFN-γ, IL-1β, IL-6, TNF-α) as well as enhanced plasma levels of kynurenine. Immunohistochemical visualization of c-Fos in the brain revealed that NOD2 synergism with TLR4 resulted in increased activation of cerebral nuclei relevant to sickness. These data show that NOD1 or NOD2 synergizes with TLR4 in exacerbating the immune, sickness and brain responses to peripheral immune stimulation. Our findings demonstrate that the known interactions of NLRs and TLRs at the immune cell level extend to interactions affecting brain function and behavior.

Reconstitution of the peptidoglycan cytoplasmic precursor biosynthetic pathway in cell-free system and rapid screening of antisense oligonucleotides for Mur enzymes

Bacterial peptidoglycan is the cell wall component responsible for various biological activities. Its cytoplasmic precursor UDP-N-acetylmuramyl pentapeptide is biosynthesized by the first six enzymes of peptidoglycan synthetic pathways (Mur enzymes), which are all proved to be important targets for antibiotic screening. In our present work, the genes encoding Mur enzymes from Escherichia coli were co-expressed in the cell-free protein synthesis (CFPS) system, and the activities of Mur enzymes derived from CFPS system were validated by the synthesis of the final product UDP-N-acetylmuramyl pentapeptide. Then this in vitro reconstituted Mur biosynthetic pathway was used to screen a panel of specific antisense oligonucleotides for MurA and MurB. The selected oligonucleotides were proved to eliminate the expression of Mur enzymes, and thus inhibit the Mur biosynthetic pathway. The present work not only developed a rapid method to reconstruct and regulate a biosynthetic pathway in vitro, but also may provide insight into the development of novel antibiotics targeting on peptidoglycan biosynthetic pathway.

Anorexia, undernutrition, weight loss, sarcopenia, and cachexia of aging

During the period between 2000 and 2030, the number of persons older than 65 years is projected to grow from 550 to 937 million worldwide. Globally, this growth represents an increase from 6.9 to 12 %. Studies have found that 74.6 % of women and 67.4 % of men aged 65 years or older suffer from many clinical and subclinical syndromes and problems, including anorexia, undernutrition, weight loss, sarcopenia, and cachexia, which are often overlooked or managed inadequately. These issues are very common in the elder. The key clinical question is whether these changes in the elder are distinct entities or an interdependent continuum. In this article, we reviewed these issues of related basic, clinical knowledge. The purpose of this review is to enhance the recognition of anorexia, undernutrition, weight loss, sarcopenia, and cachexia.

Effect of muramyl dipeptides on postsynaptic GABA, NMDA, and AMPA receptors and presynaptic NMDA receptors in rat brain

We studied the effect of muramyl dipeptides on postsynaptic GABA, NMDA, and AMPA receptors and presynaptic NMDA receptors. L,D-Dipeptide more potently than L,L-dipeptide inhibited postsynaptic NMDA receptors, potentiated GABA and AMPA receptors, and inhibited (45)Ca(2+) uptake in synaptosomes from of rat brain cortex. Our results indicate that muramyl dipeptides modulate function of glutamate and GABA receptors.

Intracameral Toxicity of Bacterial Components Muramyl Dipeptide and Staurosporine: Ciliary Cyst Formation, Epithelial Cell Apoptosis and Necrosis

The uveitogenic bacterial cell wall component muramyl dipeptide (MDP) is apoptogenic in rabbit kidney cells. The purpose of this investigation was to assess the cytotoxic activity of MDP and staurosporine (STSP; induces cultured corneal and lens cells apoptosis) in rabbit ciliary body tissue. Anterior uveitis was determined by clinical symptoms and increased aqueous humor (AH) protein. Ciliary body tissue was assessed for histological changes, caspase-3 activity, dye uptake, distribution of immunoreactive caspase-3 and DNA ladders at 4 and 6 hours postinjection. Increases in caspase-3 activity, APOPercentage dye uptake, and localization of immunoreactive caspase-3 in ciliary epithelial cells were associated with ciliary cysts of detached nonpigmented epithelial (NPE) cells, as well as apoptotic and necrotic DNA ladders in ciliary body tissues from eyes injected with MDP and/or STSP. The results suggest that intracameral injection of the bacterial components MDP and STSP can induce acute endophthalmic changes in uveal tissue including formation of ciliary body, NPE and pigmented epithelial (PE) cell apoptosis, and ciliary body tissue necrosis.

Peptidoglycan--an endotoxin in its own right?

Studies aimed at dissecting the complex pathophysiology of sepsis with multiple organ failure have traditionally focused on lipopolysaccharide of gram-negative bacteria, which is widely regarded as the classical endotoxin. However, gram-positive sepsis now accounts for up to 50% of all cases, calling for a shift of focus. Peptidoglycan (PepG) is the major cell wall component of gram-positive bacteria and has been increasingly recognized as an important proinflammatory molecule. During gram-positive infections, PepG reaches the circulation by bacterial breakdown or translocation from the intestine. Administration of PepG induces all the classical features of infectious illness and endotoxemia and may cause systemic inflammation with organ failure in animal models. Its potency, however, is crucially dependent on various features of its complex structure. PepG interacts with the innate immune system through receptors mainly expressed on monocytes/macrophages but may induce inflammatory changes in other cell types as well. Among the most extensively studied receptor systems are the nucleotide-binding oligomerization domains, the toll-like receptors, and the PepG recognition proteins. Based on the current available literature, we would like to propose that PepG must be regarded as an endotoxin in its own right and to encourage further work in the field of PepG signaling.

Conditioned taste aversion with lipopolysaccharide and peptidoglycan does not activate cytokine gene expression in the spleen and hypothalamus of mice

Several reports show that behavioural and physiological components of the acute phase reaction can be conditioned. However, the mechanisms responsible for these effects remain obscure. The underlying assumption that the changes observed in conditioned animals are dependent on a conditioned production of cytokines has never been demonstrated. In the present study, the possibility of conditioning the production of cytokines or molecules implicated in their signalling pathways was tested by submitting mice to conditioned taste aversion with a new saccharin taste paired with intraperitoneal (i.p.) injections of lipopolysaccharide (LPS, 0.83 microg/g) or peptidoglycan (PGN, 20 microg/g). After two conditioning sessions, conditioned mice developed a clear aversion to saccharine that was not associated with activation of genes of the cytokine network either at the periphery, or in the hypothalamus, as demonstrated by a macroarray approach and confirmed by real time RT-PCR. In contrast, there was an activation of the genes coding for nuclear factor kappa B (NFkappaB) and mitogen activated protein kinase (MAPK) signalling pathways in the spleen and to a lesser extent in the hypothalamus. This modulation of the NFkappaB and MAPK signalling pathways is interpreted in terms of a possible conditioned sensitisation of the immune system.

A method for the enzymatic synthesis and HPLC purification of the peptidoglycan precursor UDP-N-acetylmuramic acid

UDP-N-acetylmuramic acid (UDP-MurNAc) is a precursor for peptidoglycan biosynthesis in bacteria. A major difficulty in the study of this pathway is that UDP-MurNAc is not commercially available. We have developed an enzymatic synthesis scheme for UDP-MurNAc using two easily purified Escherichia coli polyhistidine tagged peptidoglycan biosynthesis enzymes, MurZ and MurB, followed by a single-step purification of UDP-MurNAc by high-performance liquid chromatography. The identity of the UDP-MurNAc synthesized by our method was confirmed by electrospray ionization mass spectrometry. Furthermore, we show that the UDP-MurNAc can support a UDP-MurNAc-L-alanine ligase reaction.

Pharmacokinetics and feeding responses to muramyl dipeptide in rats

N-acetyl-muramyl-L-alanine-D-isoglutamine or muramyl dipeptide (MDP) is the minimally active subunit of bacterial peptidoglycan. During a systemic infection, the involvement of MDP has been demonstrated in food intake depression by the macrophage hydrolysis of Gram-positive bacteria. Under normal conditions, mammals are constantly exposed to the release of endogenous MDP from degraded gut flora and that of exogenous MDP from the diet. However, MDP digestion and absorption in the gastrointestinal tract are not fully understood, and their physiological significance needs to be clarified. After gavage (1.5 mg/kg), very low levels of MDP were found in the systemic circulation of rats and feeding patterns were not altered. In contrast, after the intraperitoneal injection of a similar dose, a depression in food intake was observed. The rats reduced their meal frequency and constant feeding rate, showing signs of satiety. The behavioral satiety sequence (BSS) was modified by behavioral changes, similar to those which appear during sickness, such as an increase in resting and a reduction in grooming. Our data suggest that the hypophagic effect of MDP may result from satiety and sickness behavior.

Stereo-isomer specific induction of renal cell apoptosis by synthetic muramyl dipeptide (N-acetylmuramyl-L-alanyl-D-isoglutamine)

The cytotoxicity of bacterial cell wall components, muramyl dipeptide (synthetic N-acetylmuramyl-L-alanyl-D-isoglutamine; L,D-MDP) and lipopolysaccharide (LPS), was investigated in several kidney cell lines. MDP and LPS were toxic to rabbit and monkey kidney cells, MDP was toxic to canine kidney cells, but not to human or porcine kidney cells. Notably, L,D-MDP was >100-fold more cytotoxic/microg than the D,D-MDP and L,L-MDP, as well as LPS. L,D-MDP and analogs containing L,D-MDP were the most widely cytotoxic of the MDP tested. The MDP-induced cytotoxicity was characterized as apoptosis by DAPI staining and DNA laddering. The acute rabbit kidney (RK13) cell apoptosis (cell death in < 5 h) induced by apical or basal application of MDP was associated with glutamate (Glu) release, decreased gamma-glutamyltranspeptidase (GGT) and acidosis and was suppressed by Indomethacin, Naproxen and Curcumin. The cytotoxic activity of L,D-MDP was decreased significantly by 24 h incubation in human sera. Aged (> 2 year-old) rabbits that apparently failed to quickly clear and excrete a uveitogenic dose of MDP within 24 h died in I week. The results indicate that minute amounts (5 ng/ml) of MDP containing L-alanyl-D-isoglutamine can induce renal cell apoptosis in vitro and support MDP-induced kidney cytotoxicity in rabbits. Also, the results indicate that MDP in sera can be detected utilizing the RK13 cell bioassay and that failure to rapidly clear and excrete L,D-MDP is associated with uveitis and death in aged rabbits.

Elevation of plasma peptidoglycan and peripheral blood neutrophil activation during hemorrhagic shock: plasma peptidoglycan reflects bacterial translocation and may affect neutrophil activation

OBJECTIVE

To investigate the relations among bacterial transloation, plasma peptidoglycan elevation, and peripheral blood neutrophil activation during hemorrhagic shock.

DESIGN

Prospective, randomized, unblinded animal study.

SETTING

Surgical research laboratories of Shiga University of Medical Science.

SUBJECTS

Male, specific pathogen-free Sprague-Dawley rats.

INTERVENTIONS

The rats were randomly divided into three groups: a conventional group with normal intestinal flora (NF), an antibiotic (streptomycin and penicillin G) decontaminated group (AD), and a sham shock group with normal intestinal flora. The NF and AD groups were subjected to hemorrhagic shock (mean arterial pressure 30 mm Hg, for 30 to 90 mins). Rats were killed at 30, 60, and 90 mins after shock induction. Systemic blood and mesenteric lymph nodes (MLNs) were cultured for the determination of bacterial translocation (BT). Systemic plasma peptidoglycan and endotoxin concentrations were measured. To evaluate peripheral blood neutrophil activation, phagocytosis and hydrogen peroxide generation were assayed by flow cytometry.

MEASUREMENTS AND MAIN RESULTS

In the NF group, BT to MLNs was significantly increased from 30 mins after shock induction. Blood culture and plasma endotoxin were positive at 90 mins but there were no significant differences. Assayed plasma peptidoglycan was significantly increased at 90 mins. Phagocytosis and hydrogen peroxide generation were significantly increased. Assayed plasma peptidoglycan concentrations showed significant positive correlations with the magnitude of BT to MLNs (r2 = .54) and hydrogen peroxide generation (r2 = .22) in individual animals. Furthermore, BT and these parameters were significantly suppressed in the AD group.

CONCLUSIONS

First, we concluded that assayed plasma peptidoglycan reflects BT induced by hemorrhage because the increase in assayed plasma peptidoglycan was suppressed, as was BT, by antibiotic decontamination. Second, peripheral blood neutrophil activation was also suppressed when BT was prevented. We concluded BT to be involved in neutrophil activation. Our findings suggest hydrogen peroxide generation by neutrophils to be involved in plasma peptidoglycan elevation.

Detection of peptidoglycan in human plasma using the silkworm larvae plasma test

Silkworm larvae plasma (SLP) reagent, which is prepared from the body fluid of the silkworm, reacts with peptidoglycan (PG), a fragment of both the Gram-positive and Gram-negative bacterial cell wall, as well as with beta-glucan, a component of fungi. We developed a quantitative method for the detection of PG in human plasma from cases with bacterial infection using the SLP reagent. Tested in this way, the SLP method showed 86.2% sensitivity, 90.6% specificity, 89.3% positive predictive value, and 88.5% efficiency. The SLP method provides a valuable tool for the diagnosis of systemic infection using patients' blood.

Probiotic properties of lactic-acid bacteria: plenty of scope for fundamental R & D

Probiotic products are marketed widely throughout the world. This is especially true of yogurts that contain strains of lactic-acid bacteria of intestinal origin. Consumption of these products is aimed at promoting the wellbeing of the consumer by impacting on the collection of microorganisms that normally inhabit the intestinal tract. The development of scientifically valid probiotics requires more detailed knowledge of this intestinal microflora than is currently available.

Tolerance to appetite suppression induced by peptidoglycan

Physiologically realistic peptidoglycan (PG) fragments, derived from Neisseria gonorrhoeae, were shown previously to dose-dependently suppress food consumption and body weight gain in rats following single intraperitoneal injections. The present study, examining the effects of repeated daily injection of PG, provides additional support to our underlying hypothesis, i.e., that soluble PG fragments contribute to the loss of appetite commonly associated with bacterial infections. An initial intraperitoneal injection of purified, soluble, macromolecular, extensively O-acetylated PG fragments (S-O-PG) (240 micrograms/kg of body weight) decreased overnight food consumption in male Lewis rats (150 g) by approximately 35% relative to animals receiving diluent alone (P < 0.05). However, subsequent daily injections of S-O-PG resulted in progressively smaller effects on food consumption until, by the fourth day, rats were completely nonresponsive (tolerant) to S-O-PG-induced hypophagia. Rats that developed tolerance to the effects of S-O-PG on appetite were also tolerant to three other known hypophagic agents, lipopolysaccharide (LPS), muramyl dipeptide, and interleukin-1, when challenged one day after establishment of S-O-PG tolerance. Similarly, rats developed tolerance to repeated injections of muramyl dipeptide or LPS and were cross-tolerant to S-O-PG when challenged 1 day later. However, 30 days after establishment of S-O-PG tolerance, rats remained nonresponsive to S-O-PG but regained full responsiveness to LPS-mediated hypophagia. Thus, at least two mechanisms of tolerance to S-O-PG hypophagia exist: an early tolerance which is nonspecific and a late tolerance which is specific for S-O-PG. Late, but not early, tolerance to S-O-PG-mediated suppression of appetite was associated with an increase in specific anti-PG antibody activity as measured in an enzyme-linked immunosorbent assay.