The cell membrane of Mycoplasma penetrans: lipid composition and phospholipase A1 activity

A tuneable minimal cell membrane reveals that two lipid species suffice for life

All cells are encapsulated by a lipid membrane that facilitates their interactions with the environment. How cells manage diverse mixtures of lipids, which dictate membrane property and function, is experimentally challenging to address. Here, we present an approach to tune and minimize membrane lipid composition in the bacterium Mycoplasma mycoides and its derived 'minimal cell' (JCVI-Syn3A), revealing that a two-component lipidome can support life. Systematic reintroduction of phospholipids with different features demonstrates that acyl chain diversity is more important for growth than head group diversity. By tuning lipid chirality, we explore the lipid divide between Archaea and the rest of life, showing that ancestral lipidomes could have been heterochiral. However, in these simple organisms, heterochirality leads to impaired cellular fitness. Thus, our approach offers a tunable minimal membrane system to explore the fundamental lipidomic requirements for life, thereby extending the concept of minimal life from the genome to the lipidome.

A tuneable minimal cell membrane reveals that two lipid species suffice for life

All cells are encapsulated by a lipid membrane which facilitates the interaction between life and its environment. How life exploits the diverse mixtures of lipids that dictate membrane property and function has been experimentally challenging to address. We introduce an approach to tune and minimize lipidomes in Mycoplasma mycoides and the Minimal Cell (JCVI-Syn3A) revealing that a 2-component lipidome can support life. Systematically reintroducing phospholipid features demonstrated that acyl chain diversity is more critical for growth than head group diversity. By tuning lipid chirality, we explored the lipid divide between Archaea and the rest of life, showing that ancestral lipidomes could have been heterochiral. Our approach offers a tunable minimal membrane system to explore the fundamental lipidomic requirements for life, thereby extending the concept of minimal life from the genome to the lipidome.

Infection strategies of mycoplasmas: Unraveling the panoply of virulence factors

Mycoplasmas, the smallest bacteria lacking a cell wall, can cause various diseases in both humans and animals. Mycoplasmas harbor a variety of virulence factors that enable them to overcome numerous barriers of entry into the host; using accessory proteins, mycoplasma adhesins can bind to the receptors or extracellular matrix of the host cell. Although the host immune system can eradicate the invading mycoplasma in most cases, a few sagacious mycoplasmas employ a series of invasion and immune escape strategies to ensure their continued survival within their hosts. For instance, capsular polysaccharides are crucial for anti-phagocytosis and immunomodulation. Invasive enzymes degrade reactive oxygen species, neutrophil extracellular traps, and immunoglobulins. Biofilm formation is important for establishing a persistent infection. During proliferation, successfully surviving mycoplasmas generate numerous metabolites, including hydrogen peroxide, ammonia and hydrogen sulfide; or secrete various exotoxins, such as community-acquired respiratory distress syndrome toxin, and hemolysins; and express various pathogenic enzymes, all of which have potent toxic effects on host cells. Furthermore, some inherent components of mycoplasmas, such as lipids, membrane lipoproteins, and even mycoplasma-generated superantigens, can exert a significant pathogenic impact on the host cells or the immune system. In this review, we describe the proposed virulence factors in the toolkit of notorious mycoplasmas to better understand the pathogenic features of these bacteria, along with their pathogenic mechanisms.

Model-driven design allows growth of Mycoplasma pneumoniae on serum-free media

Mycoplasma pneumoniae is a slow-growing, human pathogen that causes atypical pneumonia. Because it lacks a cell wall, many antibiotics are ineffective. Due to its reduced genome and dearth of many biosynthetic pathways, this fastidious bacterium depends on rich, undefined medium for growth, which makes large-scale cultivation challenging and expensive. To understand factors limiting growth, we developed a genome-scale, constraint-based model of M. pneumoniae called iEG158_mpn to describe the metabolic potential of this bacterium. We have put special emphasis on cell membrane formation to identify key lipid components to maximize bacterial growth. We have used this knowledge to predict essential components validated with in vitro serum-free media able to sustain growth. Our findings also show that glycolysis and lipid metabolism are much less efficient under hypoxia; these findings suggest that factors other than metabolism and membrane formation alone affect the growth of M. pneumoniae. Altogether, our modelling approach allowed us to optimize medium composition, enabled growth in defined media and streamlined operational requirements, thereby providing the basis for stable, reproducible and less expensive production.

Novel synthetic lipopeptides derived from Mycoplasma hyorhinis upregulate calpastatin in SH-SY5Y neuroblastoma cells and induce a neuroprotective effect against amyloid-β-peptide toxicity

Previously, we showed that contamination of SH-SY5Y neuroblastoma cells by Mycoplasma hyorhinis strains NDMh and MCLD leads to increased levels of calpastatin (the endogenous, specific inhibitor of the Ca2+-dependent protease calpain), resulting in inhibition of calpain activation. We have found that the increased calpastatin level is promoted by the lipoprotein fraction (MhLpp) of the mycoplasmal membrane. Here, we present MhLpp-based novel synthetic lipopeptides that induce upregulation of calpastatin in SH-SY5Y neuroblastoma cells, leading to protection of the treated cells against Ca2+/amyloid-β-peptide toxicity. These lipopeptides present a new class of promising agents against calpain-induced cell toxicity.

Spiroplasma eriocheiris Adhesin-Like Protein (ALP) Interacts with Epidermal Growth Factor (EGF) Domain Proteins to Facilitate Infection

Spiroplasma eriocheiris is a novel pathogen found in recent years, causing the tremor disease (TD) of Chinese mitten crab Eriocheir sinensis. Like Spiroplasma mirum, S. eriocheiris infects the newborn mouse (adult mice are not infected) and can cause cataract. Adhesion-related protein is an important protein involved in the interaction between pathogen and host. In this study, the Adhesin-like Protein (ALP) of S. eriocheiris was detected on its outer membrane by using immune electron microscopy, and was found to be involved in the bacterium's infection of mouse embryo fibroblasts (3T6-Swiss albino). Yeast two-hybrid analysis demonstrated that ALP interacts with a diverse group of mouse proteins. The interactions between recombinant partial fibulin7 (FBLN7; including two epidermal growth factor [EGF] domains) and ALP were confirmed by Far-western blotting and colocalization. We synthetized the domains of FBLN7 [EGF domain: amino acids 136–172 and complement control protein (CCP) domain: 81–134 amino acids], and demonstrated that only EGF domain of FBLN7 can interact with ALP. Because the EGF domain has high degree of similarity to EGF, it can activate the downstream EGFR signaling pathway, in key site amino acids. The EGFR pathway in 3T6 cells was restrained after rALP stimulation resulting from competitive binding of ALP to EGF. The unborn mouse, newborn mouse, and the adult mouse with cataract have a small amount of expressed FBLN7; however, none was detected in the brain and very little expression was seen in the eye of normal adult mice. In short, ALP as a S. eriocheiris surface protein, is critical for infection and further supports the role of ALP in S. eriocheiris infection by competitive effection of the EGF/EGFR axis of the target cells.

Mycoplasma agalactiae Induces Cytopathic Effects in Infected Cells Cultured In Vitro

Mycoplasma agalactiae is the etiological agent of the contagious agalactia syndrome in sheep and goats and causes significant economic losses worldwide. Yet the mechanism of pathogenesis is largely unknown. Even whole-genome sequence analysis of its pathogenic type strain did not lead to any conclusions regarding its virulence or pathogenicity factors. Although inflammation and tissue destruction at the local site of M. agalactiae infection are largely considered as effects of the host immune response, the direct effect of the agent on host cells is not completely understood. The aim of this study was to investigate the effect of M. agalactiae infection on the quality and viability of host cells in vitro. Changes in cell morphology including cell elongation, cytoplasm shrinkage and membrane blebbing were observed in infected HeLa cells. Chromatin condensation and increased caspase-3 cleavage in infected HeLa cells 48 h after infection suggests an apoptosis-like phenomenon in M. agalactiae-infected cells. In compliance with these results, decreased viability and cell lysis of M. agalactiae-infected HeLa cells was also observed. Measurement of the amount of LDH released after M. agalactiae infection revealed a time- and dose-dependent increase in HeLa cell lysis. A significant decrease in LDH released after gentamicin treatment of infected cells confirmed the major role of cytadherent M. agalactiae in inducing host cell lysis. This is the first study illustrating M. agalactiae's induction of cytopathic effects in infected HeLa cells. Further detailed investigation of infected host tissue for apoptotic markers might demonstrate the association between M. agalactiae-induced host cell lysis and the tissue destruction observed during M. agalactiae natural infection.

Mycoplasma gallisepticum inactivated by targeting the hydrophobic domain of the membrane preserves surface lipoproteins and induces a strong immune response

An innovative approach for inactivation of Mycoplasma gallisepticum using the hydrophobic photoinduced alkylating probe 1, 5-iodonaphthylazide (INA) is described. Treatment of washed M. gallisepticum mid-exponential culture (0.2 mg cell protein /mL) with INA followed by irradiation with far-ultraviolet light (310–380 nm) completely abolished viability. Transmission electron microscopy showed that the majority of the inactivated M. gallisepticum were comparable in size to intact cells, but that part of the INA-treated M. gallisepticum preparation also contained low density cells and membrane vesicles. Confocal microscopy revealed that untreated M. gallisepticum cells were internalized by chicken red blood cells (c-RBCs), whereas the INA-inactivated cells remained attached to the outer surface of the c-RBCs. INA treatment of M. gallisepticum resulted in a complete inactivation of F0F1 –ATPase and of the L-arginine uptake system, but the cytoplasmatic soluble NADH2 dehydrogenase was only partially affected. Western blot analysis of the lipoprotein fraction showed that the INA-treated M. gallisepticum retained their lipoproteins. Following subcutaneous injection of M. gallisepticum INA-bacterin, 100% and 68.8% of chickens were positive by the rapid serum agglutination test and enzyme-linked immunosorbent assay respectively, 2 weeks post-injection. These data suggest that the photoinducible alkylating agent INA inactivates M. gallisepticum but preserves its surface lipoproteins and thus has the potential to be used as a general approach for the inactivation of mycoplasmas for vaccine development.

Exploitation of host lipids by bacteria

Bacteria that interact with eukaryotic cells have developed a variety of strategies to divert host lipids, or cellular processes driven by lipids, to their benefit. Host lipids serve as building blocks for bacterial membrane formation and as energy source. They promote the formation of specific microdomains, facilitating interactions with the host. Host lipids are also critical players in the entry of bacteria or toxins into cells, and, for bacteria growing inside parasitophorous vacuoles, in building a secure shelter. Bacterial dissemination is often dependent on enzymatic activities targeting host lipids. Finally, on a larger scale, long lasting parasitic association can disturb host lipid metabolism so deeply as to 'reprogram' it, as proposed in the case of Mycobacterium infection.

The phospholipid profile of mycoplasmas

The de novo synthesized polar lipids of Mycoplasma species are rather simple, comprising primarily of the acidic glycerophospholipids PG and CL. In addition, when grown in a medium containing serum, significant amounts of PC and SPM are incorporated into the mycoplasma cell membrane although these lipids are very uncommon in wall-covered bacteria. The exogenous lipids are either incorporated unchanged or the PC incorporated is modified by a deacylation-acylation enzymatic cycle to form disaturated PC. Although their small genome, in some Mycoplasma species, other genes involved in lipid biosynthesis were detected, resulting in the synthesis of a variety of glycolipis, phosphoglycolipids and ether lipids. We suggest that analyses and comparisons of mycoplasma polar lipids may serve as a novel and useful tool for classification. Nonetheless, to evaluate the importance of polar lipids in mycoplasma, further systematic and extensive studies on more Mycoplasma species are needed. While studies are needed to elucidate the role of lipids in the mechanisms governing the interaction of mycoplasmas with host eukaryotic cells, the finding that a terminal phosphocholine containing glycolipids of M. fermentans serves both as a major immune determinants and as a trigger of the inflammatory responses, and the findings that the fusogenicity of M. fermentans with host cells is markedly stimulated by lyso-ether lipids, are important steps toward understanding the molecular mechanisms of M. fermentans pathogenicity.

Phospholipase A and glycerophosphodiesterase activities in the cell membrane of Mycoplasma hyorhinis

Mycoplasma hyorhinis, the major contaminant of tissue cultures, has been implicated in a variety of diseases in swine. Most human and animal mycoplasmas remain attached to the surface of epithelial cells. Nonetheless, we have recently shown that M. hyorhinis is able to invade and survive within nonphagocytic melanoma cells. The invasion process may require the damaging of the host cell membrane by either chemical, physical or enzymatic means. In this study, we show that M. hyorhinis membranes possess a nonspecific phospholipase A (PLA) activity capable of hydrolyzing both position 1 and position 2 of 1-acyl-2-(12-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)] aminododecanoyl) phosphatidylcholine. In silico analysis of the M. hyorhinis genome shows that the PLA of M. hyorhinis shares no homology to described phospholipases. The PLA activity of M. hyorhinis was neither stimulated by Ca (2+) nor inhibited by EGTA and had a broad pH spectrum. Mycoplasma hyorhinis also possess a potent glycerophosphodiesterase (GPD), which apparently cleaves the glycerophosphodiester formed by PLA to yield glycerol-3-phosphate. Possible roles of PLA and GPD in invading host eukaryotic cells and in forming mediators upon the interaction of M. hyorhinis with eukaryotic cells are suggested.

Mycoplasma pneumoniae infection induces a neutrophil-derived antimicrobial peptide, cathelin-related antimicrobial peptide

In innate immunity, cationic antimicrobial peptides including cathelin-related antimicrobial peptide (CRAMP) are known to play critical roles in protecting the host from infection by invasive microbes, including Gram-positive and -negative bacteria. However, little is known about the interactions between CRAMP and mycoplasmas. In the present study, the antimicrobial activity of CRAMP against M. pneumoniae and the expression of CRAMP in bronchoalveolar lavage fluid (BALF) of M. pneumoniae-infected mice was examined. CRAMP at 10-20 μg/mL reduced the growth of two strains of M. pneumoniae by 100 to 1000-fold. The amount of CRAMP in the BALF of M. pneumoniae-infected mice was 20∼25 ng/mL by ELISA. The presence of mature CRAMP in BALF was observed by Western blotting. Neutrophils in BALF showed a fair amount of CRAMP in their cytoplasm by immunofluorescence. Furthermore, the addition of M. pneumoniae resulted in the release of a large amount of CRAMP from neutrophils induced by thioglycolate. These results suggest that CRAMP from neutrophils may play an important role in protection against M. pneumoniae infection.

Binding of IgA by Mycoplasma penetrans

The current study shows that Mycoplasma penetrans strain GTU binds human serum immunoglobulin A (IgA) and secretory IgA but not IgG. Binding of IgA was associated almost exclusively with the lipoprotein fraction obtained by Triton X-114 fractionation of isolated M. penetrans membranes. Western immunoblot analysis of isolated membranes of M. penetrans strain GTU and of the Triton X-114 fraction showed that the major IgA-binding component was a lipoprotein with a molecular mass of 38 kDa, one of the major lipoproteins of this organism. The authors suggest that the high IgA-binding capacity of M. penetrans strain GTU may serve as a defense mechanism, conferring on this microorganism the ability to evade clearance mediated by specific IgA antibodies.

Polar lipid and fatty acid profiles – Re-vitalizing old approaches as a modern tool for the classification of mycoplasmas?

A set of 20 Mollicutes strains representing different lines of descent, including the type species of the genus Mycoplasma, Mycoplasma mycoides, Acholeplasma laidlawii and a strain of Mesoplasma, were subjected to polar lipid and fatty acid analyses in order to evaluate their suitability for classification purposes within members of this group. Complex polar lipid and fatty acid profiles were detected for each examined strain. All strains contained the polar lipids phosphocholine-6'-alpha-glucopyranosyl-(1'-3)-1, 2-diacyl-glycerol (MfGL-I), 1-O-alkyl/alkenyl-2-O-acyl-glycero-3-phosphocholine (MfEL), sphingomyelin (SphM), 1-O-alkyl/alkenyl-glycero-3-phosphocholine (lysoMfEL), the unknown aminophospholipid APL1 and the cholesterol Chol2. A total of 19 strains revealed the presence of phosphatidylethanolamine (PE) and/or phosphatidylglycerol (PG), and the presence of diphosphatidylglycerol (DPG) was detected in 13 strains. The unknown aminolipid AL1 was found in the extracts of 17 strains. Unbranched saturated and unsaturated compounds predominated in the fatty acid profiles. Major fatty acids were usually C16:0, C18:0, C18:1 omega9c and 'Summed feature 5' (C18:2 omega6, 9c/C18:0 anteiso). Our results demonstrated that members of the M. mycoides cluster showed rather homogenous polar lipid and fatty acid profiles. In contrast, each of the other strains was characterized by a unique polar lipid profile and significant quantitative differences in the presence of certain fatty acids. These results indicate that analyses of both polar lipid and fatty acid profiles could be a useful tool for classification of mycoplasmas.

Mycoplasma hyopneumoniae p65 surface lipoprotein is a lipolytic enzyme with a preference for shorter-chain fatty acids

Mycoplasma hyopneumoniae is the most significant bacterial pathogen of the respiratory tract of swine. p65 is an immunodominant surface lipoprotein of M. hyopneumoniae that is specifically recognized during disease. Analysis of the translated amino acid sequence of the gene encoding p65 revealed similarity to the GDSL family of lipolytic enzymes. To examine the lipolytic activity of p65, the gene was cloned and expressed in Escherichia coli after truncation of the prokaryotic lipoprotein signal sequence and mutagenesis of the mycoplasma TGA tryptophan codons. After treatment with thrombin, the recombinant glutathione S-transferase (GST)-p65 protein yielded a 66-kDa fusion protein cleavage product corresponding in size to the mature p65 protein. The esterase activity of recombinant GST-p65 was indicated by the formation of a cleared zone on tributyrin agar plates and the hydrolysis of p-nitrophenyl esters of caproate (pNPC) and p-nitrophenyl esters of palmitate (pNPP). Lipase activity was indicated by the hydrolysis of the artificial triglyceride 1,2-O-dilauryl-rac-glycero-3-glutaric acid resorufin ester. Using pNPC and pNPP as substrates, recombinant GST-p65 had optimal activity between pHs 9.2 and 10.2 and at a temperature higher than 39 degrees C. Calcium ions did not increase the activity of recombinant GST-p65. Rabbit anti-p65 antibodies inhibited the activity of recombinant GST-p65 and also inhibited the growth of M. hyopneumoniae in vitro. Examination of the kinetic parameters of recombinant GST-p65 for the hydrolysis of pNPC and pNPP indicated a preference for the shorter fatty acid chain of pNPC. The physiological and/or pathogenic role of mycoplasma lipolytic enzymes has not been determined, but they are likely to play an important role in mycoplasmas' nutritional requirements for long-chain fatty acids and may reduce the function of lung surfactants in mycoplasma-induced respiratory diseases. This is the first report of the lipolytic activity of a lipid-modified surface immunogen of a mycoplasma.

A continuous spectrophotometric assay for phospholipase A(2) activity

This paper describes a simple continuous spectrophotometric method for assaying phospholipase A(2) (PLA(2)) activity. The procedure is based on a coupled enzymatic assay, using dilinoleoyl phosphatidylcholine as phospholipase substrate and lipoxygenase as coupling enzyme. The linoleic acid released by phospholipase was oxidized by lipoxygenase and then phospholipase activity was followed spectrophotometrically by measuring the increase in absorbance at 234 nm due to the formation of the corresponding hydroperoxide from the linoleic acid. The optimal assay concentrations of hog pancreatic phospholipase A(2) and lipoxygenase were established. PLA(2) activity varied with pH, reaching its optimal value at pH 8.5. Scans of the deoxycholate concentration pointed to an optimal detergent concentration of 3mM. Phospholipid hydrolysis followed classical Michaelis-Menten kinetics (V(m)=1.8 microM/min, K(m)=4.5 microM, V(m)/K(m)=0.4 min(-1)). This assay also allows PLA(2) inhibitors, such as p-bromophenacyl bromide or dehydroabietylamine acetate, to be studied. This method was proved to be specific since there was no activity in the absence of phospholipase A(2). It also has the advantages of a short analysis time and the use of commercially nonradiolabeled and inexpensive substrates, which are, furthermore, natural substrates of phospholipase A(2).

Determination of the phospholipase activity of patatin by a continuous spectrophotometric assay

Patatin is a family of glycoproteins that accounts for 30-40% of the total soluble protein in potato (Solanum tuberosum L.) tubers. This protein has been reported to serve as a storage protein and also to exhibit lipid phospholipase activity. This paper describes a simple continuous spectrophotometric method for assaying patatin phospholipase activity. The procedure is based on a coupled enzymatic assay using [1,2-dilinoleoyl] PC as the phospholipase substrate and lipoxygenase as the coupling enzyme. In the procedure developed in this work, lipoxygenase oxidizes the linoleic acid released by the phospholipase activity of patatin. This activity can then be followed spectrophotometrically by recording the increase in absorbance at 234 nm that results from the formation of the corresponding hydroperoxide from linoleic acid by the action of lipoxygenase. The optimal assay concentrations of patatin and lipoxygenase were established. Phospholipase activity varied with pH, reaching its optimal value at pH 9.5. Scans of the deoxycholate concentration pointed to an optimal detergent concentration of 3 mM. Phospholipid hydrolysis followed classical Michaelis-Menten kinetics (Vm = 9.8 x 10(-3) micromol/min x microg protein, Km = 7.8 microM, Vm/Km = 1.3 min(-1) x microg protein). This method proved to be specific since there was no activity in the absence of patatin. It also had the advantages of a short analysis time and the use of commercially nonradiolabeled and inexpensive substrates, which are, furthermore, natural substrates of phospholipase.

Interaction of mycoplasmas with host cells

The mycoplasmas form a large group of prokaryotic microorganisms with over 190 species distinguished from ordinary bacteria by their small size, minute genome, and total lack of a cell wall. Owing to their limited biosynthetic capabilities, most mycoplasmas are parasites exhibiting strict host and tissue specificities. The aim of this review is to collate present knowledge on the strategies employed by mycoplasmas while interacting with their host eukaryotic cells. Prominant among these strategies is the adherence of mycoplasma to host cells, identifying the mycoplasmal adhesins as well as the mammalian membrane receptors; the invasion of mycoplasmas into host cells including studies on the role of mycoplasmal surface molecules and signaling mechanisms in the invasion; the fusion of mycoplasmas with host cells, a novel process that raises intriguing questions of how microinjection of mycoplasma components into eukaryotic cells subvert and damage the host cells. The observations of diverse interactions of mycoplasmas with cells of the immune system and their immunomodulatory effects and the discovery of genetic systems that enable mycoplasmas to rapidly change their surface antigenic composition have been important developments in mycoplasma research over the past decade, showing that mycoplasmas possess an impressive capability of maintaining a dynamic surface architecture that is antigenically and functionally versatile, contributing to the capability of the mycoplasmas to adapt to a large range of habitats and cause diseases that are often chronic in nature.

Plasminogen binding and activation by Mycoplasma fermentans

The binding of plasminogen to Mycoplasma fermentans was studied by an immunoblot analysis and by a binding assay using iodine-labeled plasminogen. The binding of 125I-labeled plasminogen was inhibited by unlabeled plasminogen, lysine, and lysine analog epsilon-aminocaproic acid. Partial inhibition was obtained by a plasminogen fragment containing kringles 1 to 3 whereas almost no inhibition was observed with a fragment containing kringle 4. Scatchard analysis revealed a dual-phase interaction, one with a dissociation constant (kd) of 0.5 microM and the second with a kd of 7.5 microM. The estimated numbers of plasminogen molecules bound were calculated to be 110 and 790 per cell, respectively. Autoradiograms of ligand blots containing M. fermentans membrane proteins incubated with 125I-labeled plasminogen identified two plasminogen-binding proteins of about 32 and 55 kDa. The binding of plasminogen to M. fermentans enhances the activation of plasminogen to plasmin by the urokinase-type plasminogen activator (uPA), as monitored by measuring the breakdown of chromogenic substrate S-2251. Enhancement was more pronounced with the low-molecular-weight and the single-chain uPA variants, known to have low plasminogen activator activities. The binding of plasminogen also promotes the invasion of HeLa cells by M. fermentans. Invasion was more pronounced in the presence of uPA, suggesting that the ability of the organism to invade host cells stems not only from its potential to bind plasminogen but also from the activation of plasminogen to plasmin.

Novel lysophospholipase A secreted by Legionella pneumophila

We show that Legionella pneumophila possesses lysophospholipase A activity, which releases fatty acids from lysophosphatidylcholine. The NH2-terminal sequence of the enzyme contained FGDSLS, corresponding to a catalytic domain in a recently described group of lipolytic enzymes. Culture supernatants of a L. pneumophila pilD mutant lost the ability to cleave lysophosphatidylcholine.

Characterization of phospholipase A1, A2, C activity in Ureaplasma urealyticum membranes

The presence of endogenous phospholipase A (PL-A) activity of U. urealyticum hydrolyzing the acyl ester bond and phospholipase C (PL-C) activity hydrolyzing the phosphodiester bond is primarily localized in the membranes of ureaplasmas. Characterization of the membrane PL-A and PL-C activity in exponential growing cells of serovars 3, 4, and 8 was investigated. The pH optimum was about 8.5-9 for phospholipase A1 (PL-A1) in the three serovars. A more acidic pH optimum of 6 was observed for phospholipase A2 (PL-A2) enzymes in serovars 3 and 4. However, a very significant stimulation of PL-A2 activity in serovar 8 occurred around pH 7. The specific activity of PL-A2 was always 50-100 fold higher than PL-A1 activity in the pH range studied. Ca2+ ions only slightly stimulated PL-A1 activity in all 3 serovars. PL-A1 activity was stimulated about 6-fold from 0.5-0.8 mM Ca2+ ion concentrations for serovar 3 and 12-fold for serovar 8. Only lower concentrations (0.2-0.4 mM) of calcium stimulated PL-A2 activity in serovar 4. EDTA inhibition corresponded to Ca2+ stimulation for PL-A1 activity for serovars 3 and 8. A general stimulation of PL-A1 activity by diethyl ether was evident but the degree of stimulation varied with the serovar. Sodium deoxycholate enhanced PL-A activity of serovars 4 and 3, but partially inhibited that of serovar 8. PL-A activity in the three serovars were not significantly affected by p-hydroxymercuribenzoate, a marker of -SH groups in the enzyme. All 3 serovars were inactivated by heat. A broad pH optimum for PL-C activity was evident around 7-8. Diethyl ether enhanced PL-C activity of serovar 8. Sodium deoxycholate and heat were inhibitory to PL-C activity. The results demonstrate that the major characteristics of ureaplasma membrane bound PL-A and PL-C are basically similar to those of other mollicutes and bacteria. However, the major differences in the specific characteristics of specially PL-A1 and PL-A2 suggest that the ureaplasma phospholipases are unique enzymes different from the phospholipases of bacteria. Both the PL-A and PL-C enzymes function over the broad range at which ureaplasma can grow, pH 5-9 essential for survival. The ureaplasma PL-As are also markedly different from one serovar to another. This variation in specific activity could contribute significantly to differences in virulence among serovars in specific host milieus. There is significant variation from acidic pH of the vagina and alveolar surface of the lung to a more neutral pH of the endometrium and placenta. There are marked differences in calcium concentrations under specific circumstances in various host tissues. Thus the differences in specific activity among the phospholipases of the serovars of U. urealyticum may be of physiological importance in interactions with host tissues and pathogenesis of disease.

Role of Mycoplasma penetrans endonuclease P40 as a potential pathogenic determinant

Recently, we reported the purification to homogeneity and characterization of Ca(2+)- and Mg(2+)-dependent endonuclease P40 produced by Mycoplasma penetrans (M. Bendjennat, A. Blanchard, M. Loutfi, L. Montagnier, and E. Bahraoui, J. Bacteriol. 179; 2210-2220, 1997), a mycoplasma which was isolated for the first time from the urine of human immunodeficiency virus-infected patients. To evaluate how this nuclease could interact with host cells, we tested its effect on CEM and Molt-4 lymphocytic cell lines and on peripheral blood mononuclear cells. We observed that 10(-7) to 10(-9) M P40 is able to mediate a cytotoxic effect. We found that 100% of cells were killed after 24 h of incubation with 10(-7) M P40 while only 40% cytotoxicity was obtained after 72 h of incubation with 10(-9) M P40. Phase-contrast microscopy observations of P40-treated cells revealed morphological changes, including pronounced blebbing of the plasma membrane and cytoplasmic shrinkage characteristic of programmed cell death, which is in agreement with the internucleosomal fragmentation of P40-treated cell DNA as shown by agarose gel electrophoresis. We showed that (125)I-radiolabeled or fluorescein isothiocyanate-labeled P40 was able to bind specifically in a dose-dependent manner to the cell membrane of CEM cells, which suggested that the cytotoxicity of P40 endonuclease was mediated by its interaction with the cell surface receptor(s). The concentration of unlabeled P40 required to inhibit by 50% the formation of (125)I-P40-CEM complexes was about 3 x 10(-9) M, indicating a high-affinity interaction. Both P40 interaction and cytotoxicity are Ca(2+) dependent. Our results suggest that the cytotoxicity of M. penetrans observed in vitro is mediated at least partially by secreted P40, which, after interaction with host cells, can induce an apoptosis-like death. These results strongly suggest a major role of mycoplasmal nucleases as potential pathogenic determinants.

Phosphatidylinositol synthesis in mycobacteria

The metabolism and synthesis of an important mycobacterial lipid component, phosphatidylinositol (PI), and its metabolites, was studied in Mycobacterium smegmatis and M. smegmatis subcellular fractions. Little is known about the synthesis of PI in prokaryotic cells. Only a cell wall fraction (P60) in M. smegmatis was shown to possess PI synthase activity. Product was identified as PI by migration on TLC, treatment with phospholipase C and ion exchange chromatography. PI was the only major product (92.3%) when both cells and P60 fraction were labeled with [3H]inositol. Also, a neutral lipid inositol-containing product (4.1% of the total label) was identified in the P60 preparations. Strangely, PI synthase substrates, CDP-dipalmitoyl-DAG and CDP-NBD-DAG, added to the assay did not stimulate [3H]PI and NBD-PI yield by M. smegmatis. At the same time, addition of both substrates to rat liver and Saccharomyces cerevisiae PI synthase assays resulted in an increase in the product yield. Upon addition of CHAPS to the mycobacterial PI synthase assay, both substrates were utilized in a dose-dependent manner for the synthesis of NBD-PI and [3H]PI. These results demonstrate a strict substrate specificity of mycobacterial PI synthase toward endogenous substrates. K(m) of the enzyme toward inositol was shown to be 25 microM; Mg2+ stimulated the enzyme to a greater degree than Mn2+. Structural analogs of myo-inositol, epi-inositol and scyllo-inositol and Zn2+ were shown to be more potent inhibitors of mycobacterial PI synthase than of mammalian analogs. Lack of sequence homology with mammalian PI synthases, different kinetic characteristics, existence of selective inhibitors and an important physiological role in mycobacteria, suggest that PI synthase may be a good potential target for antituberculosis therapy.

Mycoplasma penetrans infection of Molt-3 lymphocytes induces changes in the lipid composition of host cells

The AIDS-associated Mycoplasma penetrans is capable of inducing its own uptake by non-phagocytic cells. The ability of M. penetrans to both adhere to and invade Molt-3 lymphocytes was markedly increased in the presence of polyethylene glycol 8000 (PEG). The effect of PEG was more pronounced in the more alkaline pH range, where the binding kinetics were much faster and almost unaffected by temperature (4-37 degrees C). Incubation of [14C]oleic-acid-labelled Molt-3 cells with viable M. penetrans resulted in a substantial release of radioactive fatty acids, whereas treating the host cells with heat-inactivated mycoplasmas, isolated M. penetrans membrane preparations, or M. penetrans growth medium, had no effect. Total lipid analysis of Molt-3 lymphocytes infected by M. penetrans revealed an augmented level of the neutral lipid fraction that was associated with a decrease in the relative amounts of polar lipids, mainly a decrease in the amount of phosphatidylserine and diphosphatidylglycerol. Analysis of the neutral lipid fraction in the infected Molt-3 cells revealed a fivefold increase in the relative amount of diacylglycerol and a marked increase in the free fatty acid (FFA) fraction. The profile of the FFAs released was dominated by a relatively high concentration of the polyunsaturated fatty acid docosahexaenoic acid. The release of lipid intermediates suggests that the degradation of Molt-3 cell phospholipids induced by M. penetrans may initiate a signal transmission cascade in the host cell.

Subversion and exploitation of host cells by mycoplasmas

Mycoplasmas are minute wall-less bacterial parasites that exhibit strict host and tissue specificities. They enter, multiply and survive within the host for extended periods by circumventing host defenses. Their intimate interaction with eukaryotic cells, and in some cases the subsequent invasion into or fusion with these cells, mediates cell damage. Mycoplasmas also modulate the activity of host cells by a variety of direct mechanisms and/or indirectly by cytokine-mediated effects.

Mycoplasma pulmonis inhibits electrogenic ion transport across murine tracheal epithelial cell monolayers

Murine chronic respiratory disease is characterized by persistent colonization of tracheal and bronchial epithelial cell surfaces by Mycoplasma pulmonis, submucosal and intraluminal immune and inflammatory cells, and altered airway activity. To determine the direct effect of M. pulmonis upon transepithelial ion transport in the absence of immune and inflammatory cell responses, primary mouse tracheal epithelial cell monolayers (MTEs) were apically infected and assayed in Ussing chambers. M. pulmonis-infected MTEs, but not those infected with a nonmurine mycoplasma, demonstrated reductions in amiloride-sensitive Na+ absorption, cyclic AMP, and cholinergic-stimulated Cl- secretion and transepithelial resistance. These effects were shown to require interaction of viable organisms with the apical surface of the monolayer and to be dependent upon organism number and duration of infection. Altered transport due to M. pulmonis was not merely a result of epithelial cell death as evidenced by the following: (i) active transport of Na+ and Cl-, albeit at reduced rates; (ii) normal cell morphology, including intact tight junctions, as demonstrated by electron microscopy; (iii) maintenance of a mean transepithelial resistance of 440 omega/cm2; and (iv) lack of leakage of fluid from the basolateral to the apical surface of the monolayer. Alteration in epithelial ion transport in vitro is consistent with impaired pulmonary clearance and altered airway function in M. pulmonis-infected animals. Furthermore, the ability of M. pulmonis to alter transport without killing the host cell may explain its successful parasitism and long-term persistence in the host. Further study of the MTE-M. pulmonis model should elucidate the molecular mechanisms which mediate this reduction in transepithelial ion transport.

Purification and characterization of Mycoplasma penetrans Ca2+/Mg2+-dependent endonuclease

The major nuclease from Mycoplasma penetrans has been purified to homogeneity. The enzyme seems to be present as a membrane-associated precursor of 50 kDa and as a peripheral membrane monomeric polypeptide of 40 kDa that is easily removed by washing of cells with isotonic buffers and in the aqueous phase upon Triton partitioning of Triton X-114-solubilized protein. The 40-kDa nuclease was extracted from M. penetrans cells by Triton X-114 and phase fractionation and was further purified by chromatography on Superdex 75 and chelating Sepharose (Zn2+ form) columns. By gel filtration, the apparent molecular mass was 40 kDa. The purified enzyme exhibits both a nicking activity on superhelical and linear double-stranded DNA and a nuclease activity on RNA and single-stranded DNA. No exonuclease activity was found for this enzyme. This nuclease required both Mg2+ (optimum, 5 mM) and Ca2+ (optimum, 2 mM) for activity and exhibited a pH optimum between pH 7 and 8 for DNase activity. It was inhibited by Zn2+, Mn2+, heparin, sodium dodecyl sulfate, and chelator agents such EDTA and EGTA, but no effect was observed with ATP, 2-mercaptoethanol, N-ethylmaleimide, dithiothreitol, nonionic detergents, phenylmethylsulfonyl fluoride, and iodoacetamide. Nuclease activity was inhibited by diethylpyrocarbonate at both pH 6 and 8 and by pepstatin, suggesting the involvement of a histidine and an aspartate in the active site. When added to human lymphoblast nuclei, the purified M. penetrans endonuclease induced internucleosomal fragmentation of the chomatin into oligonucleosomal fragments. On the basis of this result, and taking into account the fact that M. penetrans has the capacity to invade eucaryotic cells, one can suggest, but not assert, that produced Ca2+/Mg2+-dependent endonuclease may alter the nucleic acid metabolism of host cells by DNA and/or RNA degradation and may act as a potential pathogenic determinant.