Purification and characterization of neutral sphingomyelinase from Helicobacter pylori

Role and interaction of bacterial sphingolipids in human health

Sphingolipids, present in both higher animals and prokaryotes, involving in cell differentiation, pathogenesis and apoptosis in human physiological health. With increasing attention on the gut microbiome and its impact on wellbeing, there is a renewed focus on exploring bacterial sphingolipids. This review aims to consolidate the current understanding of bacterial sphingolipids and their impact on host health. Compared to mammalian sphingolipids, bacterial sphingolipids are characterized by odd chain lengths due to the presence of branched alkyl chains. Additionally, intestinal microbial sphingolipids can migrate from the gut to various host organs, affecting the immune system and metabolism. Furthermore, the intricate interplay between dietary sphingolipids and the gut microbiota is explored, shedding light on their complex relationship. Despite limited knowledge in this area, this review aims to raise awareness about the importance of bacterial sphingolipids and further our understanding of more uncharacterized bacterial sphingolipids and their significant role in maintaining host health.

Metagenomic characterization of sphingomyelinase C in the microbiome of humans and environments

Bacterial sphingomyelinases (SMases) hydrolyze sphingomyelin and play an important role in membrane dynamics and the host immune system. While the number of sequenced genomes and metagenomes is increasing, a limited number of experimentally validated SMases have been reported, and the genomic diversity of SMases needs to be elucidated extensively. This study investigated the sequence and structural characteristics of SMases in bacterial genomes and metagenomes. Using previously identified SMases, such as the β-toxin of Staphylococcus aureus, we identified 276 putative SMases and 15 metagenomic SMases by a sequence homology search. Among the predicted metagenomic SMases, six non-redundant metagenomic SMases (M-SMase1-6) were selected for further analysis. The predicted SMases were confirmed to contain highly conserved residues in the central metal-binding site; however, the edge metal-binding site showed high diversity according to the taxon. In addition, protein structure modeling of metagenomic SMases confirmed structural conservation of the central metal-binding site and variance of the edge metal-binding site. From the activity assay on M-SMase2 and M-SMase5, we found that they displayed sphingomyelinase activity compared to Bacillus cereus SMase. This study elucidates a comprehensive genomic characterization of SMases and provides insight into the sequence-structure-activity relationship.

Differences in the sequence of PlcR transcriptional regulator binding site affect sphingomyelinase production in Bacillus cereus

Bacillus cereus is an opportunistic pathogen that often causes severe infections such as bacteremia, with sphingomyelinase (SMase) being a crucial virulence factor. Although many strains of B. cereus carry the SMase gene, they are classified as SMase-producing and non-producing strains. The reason for different SMase production among B. cereus strains remains unknown. In this study, we investigated the relationship between SMase and the PlcR transcriptional regulation system to clarify the mechanism leading to varied SMase production among B. cereus strains. We analyzed the sequence of the PlcR box, which is a transcriptional regulator binding site, located at the promoter region of SMase and phosphatidylcholine-specific phospholipase C. Based on differences in the PlcR box sequences, we classified the B. cereus strains into three groups (I, II, and III). SMase expression and activity were hardly detected in Group III strains. In Group I strains, SMase activity and its expression were maximal at the onset of the stationary phase and decreased during the stationary phase, whereas those were maintained during the stationary phase in Group II stains. On injection of B. cereus strains into mice or incubation with macrophages for phagocytosis assay, the SMase-producing Group I and II strains showed higher pathogenicity than Group III strains. These findings suggest that PlcR box sequence in B. cereus affects the production of SMase, which may provide important clinical information for the detection of highly pathogenic B. cereus strains. This article is protected by copyright. All rights reserved.

Sphingomyelinase C from Streptomyces sp. A9107: unusual primary structure for bacterial sphingomyelinase C

A sphingomyelinase C (SMase) was identified in the culture supernatant of Streptomyces sp. A9107 (S-SMase). Although S-SMase seems to be a typical bacterial SMase, the primary structure of S-SMase was unusual for known bacterial SMase. The gene was functionally overexpressed in the culture medium of recombinant Rhodococcus erythropolis.

Physiological functions and clinical implications of sphingolipids in the gut

Studies of sphingolipids have become one of the most rapidly advancing fields in the last two decades. These highly diverse lipids have been known to have multiple physiological functions and clinical implications in several diseases, including tumorigenesis, inflammation, atherosclerosis and neural degenerative diseases. Unlike other organs, sphingolipids in the intestinal tract are present not only as lipid constituents in the cells but also as dietary compositions for digestion in the lumen. The present review focuses on the presence of sphingolipids and their catalytic enzymes in the gut; the metabolism and the signaling effects of the metabolites and their impacts on barrier functions, cholesterol absorption, inflammatory diseases and tumor development in the gut.

[Structure and function of sphingomyelinase]

Bacillus cereus is one that causes of opportunistic human infections. Sphingomyelinase produced by B. cereus is assumed a virulence factor for the infection. Sphingomyelinase from Bacillus cereus (Bc-SMase) is Mg(2+)-containing metalloenzyme. Bc-SMase is a family of neutral SMase (nSMase) and mimics the actions of the endogenous mammalian nSMase in causing differentiation, development, and apoptosis. Bc-SMase may be a good model for the poorly characterized mammalian nSMase. Activation of Bc-SMase by divalent metal ions was in the order Co(2+)>Mn(2+)>Mg(2+)>>Ca(2+)>Sr(2+). Crystal structure analysis of Bc-SMase bound to Co(2+), Mg(2+), or Ca(2+) revealed that the water-bridged double divalent metal ions at the center of the cleft in both the Co(2+)- and Mg(2+)-bound forms is the catalytic architecture required for sphingomyelinase activity. In contrast, the architecture of Ca(2+) binding at the site showed only one binding site. A further single metal-binding site existed at one side edge of the cleft. Based on the highly conserved nature of amino acid residues of the binding sites, the crystal structure of Bc-SMase with Mg(2+) or Co(2+) provided a common structural framework applicable to phosphohydrolases belonging to the DNase I-like folding superfamily. In addition, our analysis provided evidence that beta-hairpin containing the aromatic amino acid residues and the metal ion of the side-edge participate in binding to sphinogmyelin and membranes containing sphingomyelin. This article summarized current knowledge of characteristics and mode of action of Bc-SMase.

Sphingomyelin functions as a novel receptor for Helicobacter pylori VacA

The vacuolating cytotoxin (VacA) of the gastric pathogen Helicobacter pylori binds and enters epithelial cells, ultimately resulting in cellular vacuolation. Several host factors have been reported to be important for VacA function, but none of these have been demonstrated to be essential for toxin binding to the plasma membrane. Thus, the identity of cell surface receptors critical for both toxin binding and function has remained elusive. Here, we identify VacA as the first bacterial virulence factor that exploits the important plasma membrane sphingolipid, sphingomyelin (SM), as a cellular receptor. Depletion of plasma membrane SM with sphingomyelinase inhibited VacA-mediated vacuolation and significantly reduced the sensitivity of HeLa cells, as well as several other cell lines, to VacA. Further analysis revealed that SM is critical for VacA interactions with the plasma membrane. Restoring plasma membrane SM in cells previously depleted of SM was sufficient to rescue both toxin vacuolation activity and plasma membrane binding. VacA association with detergent-resistant membranes was inhibited in cells pretreated with SMase C, indicating the importance of SM for VacA association with lipid raft microdomains. Finally, VacA bound to SM in an in vitro ELISA assay in a manner competitively inhibited by lysenin, a known SM-binding protein. Our results suggest a model where VacA may exploit the capacity of SM to preferentially partition into lipid rafts in order to access the raft-associated cellular machinery previously shown to be required for toxin entry into host cells.

Sensitivity to toxins produced by pathogenic bacteria is largely dictated by the presence or absence of toxin receptors on the plasma membrane of host cells. VacA is an important toxin produced by the pathogenic bacterium Helicobacter pylori, which infects the human stomach and causes gastric ulcer disease and stomach cancer. VacA binds and enters human cells, and induces several changes resulting ultimately in the death of the intoxicated cells. However, the identity of the VacA receptor responsible for toxin binding and function has remained a topic of debate. In this paper, we demonstrate that sphingomyelin, a lipid on the surface of cells with important membrane structural and signaling properties, functions as a VacA receptor. We demonstrate that VacA binds to sphingomyelin, and that presence or absence of sphingomyelin on the plasma membrane dictates how much VacA binds to the cell surface, and therefore, how sensitive cells are to the toxin. The identification of sphingomyelin also provides a conceptual framework for how VacA may enter cells through specialized functional domains on the surface of cells. This is the first example of a bacterial toxin that exploits sphingomyelin as a receptor, and future work will focus on developing strategies to block VacA interactions with sphingomyelin, thereby protecting cells from the downstream consequences of toxin action.

Sphingomyelinase of Helicobacter pylori-induced cytotoxicity in AGS gastric epithelial cells via activation of JNK kinase

The aim of this study was to determine whether the Helicobacter pylori-derived sphigomyelinase (SMase) affects the sphingomyelin pathway and growth in AGS epithelial cells. We showed that the exogenous SMase increased the intracellular level of ceramide in AGS cells and led to rapid stimulation of extracellular signal-regulated kinase (ERK) and c-Jun kinase (JNK) activities. Incubation of AGS cells with H. pylori-derived SMase also resulted in suppression of cell growth and a concomitant induction of apoptosis. Data showed that PD98059 (up to 50 microM), an ERK inhibitor, did not affect the cell viability, whereas the cytotoxicity of exogenous SMase was completely blocked by SP600125, a JNK inhibitor at a concentration of 210 nM. We conclude that the activation of the mitogen-activated protein (MAP) kinases in AGS cells by exogenous H. pylori SMase is a major pathway to mediate the cytotoxicity.

Phosphatidylcholine-specific phospholipase C and sphingomyelinase activities in bacteria of the Bacillus cereus group

Bacillus anthracis is nonhemolytic, even though it is closely related to the highly hemolytic Bacillus cereus. Hemolysis by B. cereus results largely from the action of phosphatidylcholine-specific phospholipase C (PC-PLC) and sphingomyelinase (SPH), encoded by the plc and sph genes, respectively. In B. cereus, these genes are organized in an operon regulated by the global regulator PlcR. B. anthracis contains a highly similar cereolysin operon, but it is transcriptionally silent because the B. anthracis PlcR is truncated at the C terminus. Here we report the cloning, expression, purification, and enzymatic characterization of PC-PLC and SPH from B. cereus and B. anthracis. We also investigated the effects of expressing PlcR on the expression of plc and sph. In B. cereus, PlcR was found to be a positive regulator of plc but a negative regulator of sph. Replacement of the B. cereus plcR gene by its truncated orthologue from B. anthracis eliminated the activities of both PC-PLC and SPH, whereas introduction into B. anthracis of the B. cereus plcR gene with its own promoter did not activate cereolysin expression. Hemolytic activity was detected in B. anthracis strains containing the B. cereus plcR gene on a multicopy plasmid under control of the strong B. anthracis protective antigen gene promoter or in a strain carrying a multicopy plasmid containing the entire B. cereus plc-sph operon. Slight hemolysis and PC-PLC activation were found when PlcR-producing B. anthracis strains were grown under anaerobic-plus-CO(2) or especially under aerobic-plus-CO(2) conditions. Unmodified parental B. anthracis strains did not demonstrate obvious hemolysis under the same conditions.

Membrane lipids in plant-associated bacteria: their biosyntheses and possible functions

Membrane lipids in most bacteria generally consist of the glycerophospholipids phosphatidylglycerol, cardiolipin, and phosphatidylethanolamine (PE). A subset of bacteria also possesses the methylated derivatives of PE, monomethylphosphatidylethanolamine, dimethylphosphatidylethanolamine, and phosphatidylcholine (PC). In Sinorhizobium meliloti, which can form a nitrogen-fixing root nodule symbiosis with Medicago spp., PC can be formed by two entirely different biosynthetic pathways, either the PE methylation pathway or the recently discovered PC synthase pathway. In the latter pathway, one of the building blocks for PC formation, choline, is obtained from the eukaryotic host. Under phosphorus-limiting conditions of growth, S. meliloti replaces its membrane phospholipids by membrane-forming lipids that do not contain phosphorus; namely, the sulfolipid sulfoquinovosyl diacylglycerol, ornithine-derived lipids, and diacylglyceryl-N,N,N-trimethylhomoserine. Although none of these phosphorus-free lipids is essential for growth in culture media rich in phosphorus or for the symbiotic interaction with the legume host, they are expected to have major roles under free-living conditions in environments poor in accessible phosphorus. In contrast, sinorhizobial mutants deficient in PC show severe growth defects and are completely unable to form nodules on their host plants. Even bradyrhizobial mutants with reduced PC biosynthesis can form only root nodules displaying reduced rates of nitrogen fixation. Therefore, in the cases of these microsymbionts, the ability to form sufficient bacterial PC is crucial for a successful interplay with their host plants.

Biosynthesis of phosphatidylcholine in bacteria

Phosphatidylcholine (PC) is the major membrane-forming phospholipid in eukaryotes and can be synthesized by either of two pathways, the methylation pathway or the CDP-choline pathway. Many prokaryotes lack PC, but it can be found in significant amounts in membranes of rather diverse bacteria and based on genomic data, we estimate that more than 10% of all bacteria possess PC. Enzymatic methylation of phosphatidylethanolamine via the methylation pathway was thought to be the only biosynthetic pathway to yield PC in bacteria. However, a choline-dependent pathway for PC biosynthesis has been discovered in Sinorhizobium meliloti. In this pathway, PC synthase, condenses choline directly with CDP-diacylglyceride to form PC in one step. A number of symbiotic (Rhizobium leguminosarum, Mesorhizobium loti) and pathogenic (Agrobacterium tumefaciens, Brucella melitensis, Pseudomonas aeruginosa, Borrelia burgdorferi and Legionella pneumophila) bacteria seem to possess the PC synthase pathway and we suggest that the respective eukaryotic host functions as the provider of choline for this pathway. Pathogens entering their hosts through epithelia (Streptococcus pneumoniae, Haemophilus influenzae) require phosphocholine substitutions on their cell surface components that are biosynthetically also derived from choline supplied by the host. However, the incorporation of choline in these latter cases proceeds via choline phosphate and CDP-choline as intermediates. The occurrence of two intermediates in prokaryotes usually found as intermediates in the eukaryotic CDP-choline pathway for PC biosynthesis raises the question whether some bacteria might form PC via a CDP-choline pathway.

Sphingomyelinases: enzymology and membrane activity

This paper reviews our present knowledge of sphingomyelinases as enzymes, and as enzymes acting on a membrane constituent lipid, sphingomyelin. Six types of sphingomyelinases are considered, namely acidic, secretory, Mg(2+)-dependent neutral, Mg(2+)-independent neutral, alkaline, and bacterial enzymes with both phospholipase C and sphingomyelinase activity. Sphingomyelinase assay methods and specific inhibitors are reviewed. Kinetic and mechanistic studies are summarized, a kinetic model and a general-base catalytic mechanism are proposed. Sphingomyelinase-membrane interactions are considered from the point of view of the influence of lipids on the enzyme activity. Moreover, effects of sphingomyelinase activity on membrane architecture (increased membrane permeability, membrane aggregation and fusion) are described. Finally, a number of open questions on the above topics are enunciated.