Expression and immunological characteristics of the surface-localized pyruvate kinase in Mycoplasma gallisepticum

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.

Characterization of Mycoplasma gallisepticum pyruvate dehydrogenase alpha and beta subunits and their roles in cytoadherence

Mycoplasma gallisepticum is a causative agent of chronic respiratory disease in chickens, typically causing great economic losses. Cytoadherence is the critical stage for mycoplasma infection, and the associated proteins are important for mycoplasma pathogenesis. Many glycolytic enzymes are localized on the cell surface and can bind the extracellular matrix of host cells. In this study, the M. gallisepticum pyruvate dehydrogenase E1 alpha subunit (PDHA) and beta subunit (PDHB) were expressed in Escherichia coli, and their enzymatic activities were identified based on 2,6-dichlorophenol indophenol reduction. When recombinant PDHA (rPDHA) and recombinant PDHB (rPDHB) were mixed at a 1:1 molar ratio, they exhibited strong enzymatic activity. Alone, rPDHA and rPDHB exhibited no or weak enzymatic activity. Further experiments indicated that both PDHA and PDHB were surface-exposed immunogenic proteins of M. gallisepticum. Bactericidal assays showed that the mouse anti-rPDHA and anti-rPDHB sera killed 48.0% and 75.1% of mycoplasmas respectively. A combination of rPDHA and rPDHB antisera had a mean bactericidal rate of 65.2%, indicating that rPDHA and rPDHB were protective antigens, and combining the two sera did not interfere with bactericidal activity. Indirect immunofluorescence and surface display assays showed that both PDHA and PDHB adhered to DF-1 chicken embryo fibroblast cells and adherence was significantly inhibited by antisera against PDHA and PDHB. Adherence inhibition of M. gallisepticum to DF-1 chicken embryo fibroblast cells was 30.2% for mouse anti-rPDHA serum, 45.1% for mouse anti-rPDHB serum and 72.5% for a combination of rPDHA and rPDHB antisera, suggesting that rPDHA and rPDHB antisera may have synergistically interfered with M. gallisepticum cytoadherence. Plasminogen (Plg)-binding assays further demonstrated that both PDHA and PDHB were Plg-binding proteins, which may have contributed to bacterial colonization. Our results clarified the enzymatic activity of M. gallisepticum PDHA and PDHB and demonstrated these compounds as Plg-binding proteins involved in cytoadherence.

Identification of proteome, antigen protein and antigen membrane protein from Spiroplasma eriocheiris

Spiroplasma eriocheiris, which causes tremor disease in Chinese mitten crab Eriocheir sinensis, has led to huge economic losses in aquaculture. Immunoproteomics, a new scientific technique combining proteomics and immunological analytical methods, provided the direction of our research on S. eriocheiris. The aim of our study was to identify the proteome, antigen proteins and antigen membrane proteins of S. eriocheiris. A total of 780 S. eriocheiris proteins were identified by the LC-MS/MS technique. Based on immunoproteomics, 51 proteins and 7 proteins in S. eriocheiris were identified by anti-S. eriocheiris serum and negative serum respectively (six proteins in common). Thus, 45 antigenic proteins in S. eriocheiris were identified; among them, molecular chaperone DnaK, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), ATP synthase subunit beta and enolase can be considered as immunogenic proteins. Similarly, 32 membrane proteins and 6 membrane proteins were identified by anti-S. eriocheiris serum and negative serum respectively (two proteins in common). Thus, 30 antigenic membrane proteins in S. eriocheiris were identified; three of them have been reported as surface proteins including pyruvate kinase, enolase and GAPDH. All of these proteins may play key roles in the pathogeny and can be used in the future for diagnoses and prevention.

SIGNIFICANCE AND IMPACT OF THE STUDY

Spiroplasma eriocheiris is a novel pathogen causing the tremor disease in Chinese mitten crab Eriocheir sinensis. This is the first time LC-MS/MS was used to identify the proteome, antigen protein and antigen membrane protein of S. eriocheiris. The results can certainly provide valuable information towards the identification of virulent proteins or diagnosis of pathogenic mechanisms.

Interactions of surface-displayed glycolytic enzymes of Mycoplasma pneumoniae with components of the human extracellular matrix

Mycoplasma pneumoniae is a major cause of community-acquired respiratory infections worldwide. Due to the strongly reduced genome, the number of virulence factors expressed by this cell wall-less pathogen is limited. To further understand the processes during host colonization, we investigated the interactions of the previously confirmed surface-located glycolytic enzymes of M. pneumoniae (pyruvate dehydrogenase A-C [PdhA-C], glyceraldehyde-3-phosphate dehydrogenase [GapA], lactate dehydrogenase [Ldh], phosphoglycerate mutase [Pgm], pyruvate kinase [Pyk] and transketolase [Tkt]) to the human extracellular matrix (ECM) proteins fibrinogen (Fn), fibronectin (Fc), lactoferrin (Lf), laminin (Ln) and vitronectin (Vc), respectively. Concentration-dependent interactions between Fn and Vc and all eight recombinant proteins derived from glycolytic enzymes, between Ln and PdhB-C, GapA, Ldh, Pgm, Pyk and Tkt, between Lf and PdhA-C, GapA and Pyk, and between Fc and PdhC and GapA were demonstrated. In most cases, these associations are significantly influenced by ionic forces and by polyclonal sera against recombinant proteins. In immunoblotting, the complex of human plasminogen, activator (tissue-type or urokinase plasminogen activator) and glycolytic enzyme was not able to degrade Fc, Lf and Ln, respectively. In contrast, degradation of Vc was confirmed in the presence of all eight enzymes tested. Our data suggest that the multifaceted associations of surface-localized glycolytic enzymes play a potential role in the adhesion and invasion processes during infection of human respiratory mucosa by M. pneumoniae.