Entry of Spiroplasma citri into Circulifer haematoceps cells involves interaction between spiroplasma phosphoglycerate kinase and leafhopper actin

Integrated Information for Pathogenicity and Treatment of Spiroplasma

Spiroplasma, belonging to the class Mollicutes, is a small, helical, motile bacterium lacking a cell wall. Its host range includes insects, plants, and aquatic crustaceans. Recently, a few human cases of Spiroplasma infection have been reported. The diseases caused by Spiroplasma have brought about serious economic losses and hindered the healthy development of agriculture. The pathogenesis of Spiroplasma involves the ability to adhere, such as through the terminal structure of Spiroplasma, colonization, and invasive enzymes. However, the exact pathogenic mechanism of Spiroplasma remains a mystery. Therefore, we systematically summarize all the information about Spiroplasma in this review article. This provides a reference for future studies on virulence factors and treatment strategies of Spiroplasma.

Molecular functions of moonlighting proteins in cell metabolic processes

Moonlighting proteins have more than one physiologically significant role within one polypeptide chain. The multifunctionality of proteins was first described in 1987 by Joram Piatigorsky and Graeme Wistow. Cells can benefit from involvement of these proteins in biological processes in several ways, e.g. at the energy level. Furthermore, cells have developed a number of mechanisms to change these proteins' functions. Moonlighting proteins are found in all types of organisms, including prokaryotes, eukaryotes, and even viruses. These proteins include a variety of enzymes that serve as receptors, secreted cytokines, transcription factors, or proteasome components. Additionally, there are many combinations of functions, e.g. among receptors and transcription factors, chaperones and cytokines, as well as transcription factors within the ribosome. This work describes enzymes involved in several important metabolic processes in cells, namely cellular respiration, gluconeogenesis, the urea cycle, and pentose phosphate metabolism.

Development and application of the MIRA and MIRA-LFD detection methods of Spiroplasma eriocheiris

The tremor disease (TD) caused by Spiroplasma eriocheiris is the most destructive disease of the Chinese mitten crab, Eriocheir sinensis. This study attempts to construct Multienzyme Isothermal Rapid Amplification (MIRA), a quick and simple nucleic acid amplification method that operates at room temperature. Based on the gene sequences of S. eriocheiris, appropriate amplification primers were constructed and screened in this investigation. Both the relevant specific probe and the chosen specific amplification primers were designed and labeled. The MIRA and MIRA-LFD reaction conditions were then optimized. The result showed MIRA and MIRA-FFD could identify S. eriocheiris at 37 °C in 30 min and 15 min, respectively. To investigate the specificity of MIRA and MIRA-LFD, three Gram-negative bacteria (Bacillus subtilis, Bacillus thuringiensis, and Staphylococcus aureus), three Gram-positive bacteria (Escherichia coli, Aeromonas hydrophila, and Salmonella typhimurium) and S. eriocheiris were selected. The result showed MIRA and MIRA-LFD were highly specific to S. eriocheiris and did not react with other six pathogens. The sensitivities of PCR, MIRA, and MIRA-LFD were then evaluated. The result showed the detection limit of PCR is 1 ng/L whereas the detection limit of MIRA and MIRA-LFD is 10 pg/L. Finally, the established MIRA and MIRA-LFD detection methods had the advantages of being quick, sensitive, and specific for S. eriocheiris detection, as well as not requiring any specialized equipment.

Crossing the Gateless Barriers: Factors Involved in the Movement of Circulative Bacteria Within Their Insect Vectors

Plant bacterial pathogens transmitted by hemipteran vectors pose a large threat to the agricultural industry worldwide. Although virus-vector relationships have been widely investigated, a significant gap exists in our understanding of the molecular interactions between circulative bacteria and their insect vectors, mainly leafhoppers and psyllids. In this review, we will describe how these bacterial pathogens adhere, invade, and proliferate inside their insect vectors. We will also highlight the different transmission routes and molecular factors of phloem-limited bacteria that maintain an effective relationship with the insect host. Understanding the pathogen-vector relationship at the molecular level will help in the management of vector-borne bacterial diseases.

Secreted PGK1 and IGFBP2 contribute to the bystander effect of miR-10b gene editing in glioma

MicroRNA-10b (miR-10b) is an essential glioma driver and one of the top candidates for targeted therapies for glioblastoma and other cancers. This unique miRNA controls glioma cell cycle and viability via an array of established conventional and unconventional mechanisms. Previously reported CRISPR-Cas9-mediated miR-10b gene editing of glioma cells in vitro and established orthotopic glioblastoma in mouse models demonstrated the efficacy of this approach and its promise for therapy development. However, therapeutic gene editing in patients' brain tumors may be hampered, among other factors, by the imperfect delivery and distribution of targeting vectors. Here, we demonstrate that miR-10b gene editing in glioma cells triggers a potent bystander effect that leads to the selective cell death of the unedited glioma cells without affecting the normal neuroglial cells. The effect is mediated by the secreted miR-10b targets phosphoglycerate kinase 1 (PGK1) and insulin-like growth factor binding protein 2 (IGFBP2) that block cell-cycle progression and induce glioma cell death. These findings further support the feasibility of therapeutic miR-10b editing without the need to target every cell of the tumor.

A New Perspective on the Co-Transmission of Plant Pathogens by Hemipterans

Co-infection of plants by pathogens is common in nature, and the interaction of the pathogens can affect the infection outcome. There are diverse ways in which viruses and bacteria are transmitted from infected to healthy plants, but insects are common vectors. The present review aims to highlight key findings of studies evaluating the co-transmission of plant pathogens by insects and identify challenges encountered in these studies. In this review, we evaluated whether similar pathogens might compete during co-transmission; whether the changes in the pathogen titer in the host, in particular associated with the co-infection, could influence its transmission; and finally, we discussed the pros and cons of the different approaches used to study co-transmission. At the end of the review, we highlighted areas of study that need to be addressed. This review shows that despite the recent development of techniques and methods to study the interactions between pathogens and their insect vectors, there are still gaps in the knowledge of pathogen transmission. Additional laboratory and field studies using different pathosystems will help elucidate the role of host co-infection and pathogen co-transmission in the ecology and evolution of infectious diseases.

Ubiquitin-modified proteome analysis of Eriocheir sinensis hemocytes during Spiroplasma eriocheiris infection

Spiroplasma eriocheiris, the pathogen of Eriocheir sinensis tremor disease (TD), has bring a huge economic loss to China aquaculture. The hemocytes of crab as the first target cells of S. eriocheiris, but the interactive relationship between the E. sinensis and this pathogen not particularly clear. The present study is the first time to analysis the role of protein ubiquitination in the process of E. sinensis hemocytes response S. eriocheiris infection. By applying label-free quantitative liquid chromatography with tandem mass spectrometry proteomics, 950 lysine ubiquitination sites and 803 ubiquitination peptides on 458 proteins were identified, of which 48 ubiquitination sites on 40 proteins were quantified as significantly changed after the S. eriocheiris infection. Bioinformatics analysis of ubiquitination different proteins suggested many biological process and pathways were participated in the interaction between S. eriocheiris and host cell, such as ubiquitin system, endocytosis, prophenoloxidase system (proPO system), cell apoptosis, glycolysis. Our study can enhance our understanding of interaction between the crab and S. eriocheiris, and also provides basis to study the role of protein ubiquitination in other crustacean innate immune system.

Functional genomics of a Spiroplasma associated with the carmine cochineals Dactylopius coccus and Dactylopius opuntiae

BACKGROUND

Spiroplasma is a widely distributed endosymbiont of insects, arthropods, and plants. In insects, Spiroplasma colonizes the gut, hemolymph, and reproductive organs of the host. Previous metagenomic surveys of the domesticated carmine cochineal Dactylopius coccus and the wild cochineal D. opuntiae reported sequences of Spiroplasma associated with these insects. However, there is no analysis of the genomic capabilities and the interaction of this Spiroplasma with Dactylopius.

RESULTS

Here we present three Spiroplasma genomes independently recovered from metagenomes of adult males and females of D. coccus, from two different populations, as well as from adult females of D. opuntiae. Single-copy gene analysis showed that these genomes were > 92% complete. Phylogenomic analyses classified these genomes as new members of Spiroplasma ixodetis. Comparative genome analysis indicated that they exhibit fewer genes involved in amino acid and carbon catabolism compared to other spiroplasmas. Moreover, virulence factor-encoding genes (i.e., glpO, spaid and rip2) were found incomplete in these S. ixodetis genomes. We also detected an enrichment of genes encoding the type IV secretion system (T4SS) in S. ixodetis genomes of Dactylopius. A metratranscriptomic analysis of D. coccus showed that some of these T4SS genes (i.e., traG, virB4 and virD4) in addition to the superoxide dismutase sodA of S. ixodetis were overexpressed in the ovaries.

CONCLUSION

The symbiont S. ixodetis is a new member of the bacterial community of D. coccus and D. opuntiae. The recovery of incomplete virulence factor-encoding genes in S. ixodetis of Dactylopius suggests that this bacterium is a non-pathogenic symbiont. A high number of genes encoding the T4SS, in the S. ixodetis genomes and the overexpression of these genes in the ovary and hemolymph of the host suggest that S. ixodetis use the T4SS to interact with the Dactylopius cells. Moreover, the transcriptional differences of S. ixodetis among the gut, hemolymph and ovary tissues of D. coccus indicate that this bacterium can respond and adapt to the different conditions (e.g., oxidative stress) present within the host. All this evidence proposes that there is a strong interaction and molecular signaling in the symbiosis between S. ixodetis and the carmine cochineal Dactylopius.

Phosphoglycerate kinase: structural aspects and functions, with special emphasis on the enzyme from Kinetoplastea

Phosphoglycerate kinase (PGK) is a glycolytic enzyme that is well conserved among the three domains of life. PGK is usually a monomeric enzyme of about 45 kDa that catalyses one of the two ATP-producing reactions in the glycolytic pathway, through the conversion of 1,3-bisphosphoglycerate (1,3BPGA) to 3-phosphoglycerate (3PGA). It also participates in gluconeogenesis, catalysing the opposite reaction to produce 1,3BPGA and ADP. Like most other glycolytic enzymes, PGK has also been catalogued as a moonlighting protein, due to its involvement in different functions not associated with energy metabolism, which include pathogenesis, interaction with nucleic acids, tumorigenesis progression, cell death and viral replication. In this review, we have highlighted the overall aspects of this enzyme, such as its structure, reaction kinetics, activity regulation and possible moonlighting functions in different protistan organisms, especially both free-living and parasitic Kinetoplastea. Our analysis of the genomes of different kinetoplastids revealed the presence of open-reading frames (ORFs) for multiple PGK isoforms in several species. Some of these ORFs code for unusually large PGKs. The products appear to contain additional structural domains fused to the PGK domain. A striking aspect is that some of these PGK isoforms are predicted to be catalytically inactive enzymes or ‘dead’ enzymes. The roles of PGKs in kinetoplastid parasites are analysed, and the apparent significance of the PGK gene duplication that gave rise to the different isoforms and their expression in Trypanosoma cruzi is discussed.

Spiroplasma eriocheiris Invasion Into Macrobrachium rosenbergii Hemocytes Is Mediated by Pathogen Enolase and Host Lipopolysaccharide and β-1, 3-Glucan Binding Protein

Spiroplasma eriocheiris is a crustacean pathogen, without a cell wall, that causes enormous economic loss. Macrobrachium rosenbergii hemocytes are the major targets during S. eriocheiris infection. As wall-less bacteria, S. eriocheiris, its membrane protein should interact with host membrane protein directly and firstly when invaded in host cell. In this investigation, six potential hemocyte receptor proteins were identified firstly that mediate interaction between S. eriocheiris and M. rosenbergii. Among these proteins, lipopolysaccharide and β-1, 3-glucan binding protein (MrLGBP) demonstrated to bind to S. eriocheiris using bacterial binding assays and confocal microscopy. Four spiroplasma ligand proteins for MrLGBP were isolated and identified. But, competitive assessment demonstrated that only enolase of S. eriocheiris (SeEnolase) could be a candidate ligand for MrLGBP. Subsequently, the interaction between MrLGBP and SeEnolase was confirmed by co-immunoprecipitation and co-localization in vitro. After the interaction between MrLGBP and SeEnolase was inhibited by antibody neutralization test, the virulence ability of S. eriocheiris was effectively reduced. The quantity of S. eriocheiris decreased in Drosophila S2 cells after overexpression of MrLGBP, compared with the controls. In addition, RNA interference (RNAi) knockdown of MrLGBP made M. rosenbergii more sensitive to S. eriocheiris infection. Further studies found that the immune genes, including MrLGBP and prophenoloxidase (MrproPO), MrRab7A, and Mrintegrin α1 were significantly up-regulated by SeEnolase stimulation. After SeEnolase pre-stimulation, the ability of M. rosenbergii resistance to S. eriocheiris was significantly improved. Collectively, this investigation demonstrated that MrLGBP and pathogen SeEnolase involved in mediating S. eriocheiris invasion into M. rosenbergii hemocytes.

iTRAQ-based quantitative proteomic analysis of Procambarus clakii hemocytes during Spiroplasma eriocheiris infection

As a new-found aquaculture pathogen, Spiroplasma eriocheiris, has resulted in inconceivable economic losses in aquaculture. In the infection of S. eriocheiris, the Procambarus clakii hemocytes have indicated to be major target cells. What was designed to examine in our study is the hemocytes' immune response at the protein levels. Before the pathogen was injected and after 192 h of post-injection, the differential proteomes of the crayfish hemocytes were analyzed immediately by isobaric tags for relative and absolute quantization (iTRAQ) labeling, followed by liquid chromatogramphytandem mass spectrometry (LC-MS/MS). This research had identified a total of 285 differentially expressed proteins. Eighty-three and 202 proteins were up-regulated and down-regulated, respectively, caused by the S. eriocheiris infection. Up-regulated proteins included alpha-2-macroglobulin (α2M), vitellogenin, ferritin, etc. Down-regulated proteins, involved with serine protease, peroxiredoxin 6, 14-3-3-like protein, C-type lectin, cdc42 homolog precursor, etc. The prophenoloxidase-activating system, antimicrobial action involved in the immune responses of P. clarkii is considered to be damaged due to S. eriocheiris infection. The present work could lay the foundation for future research on the proteins related to the susceptibility/resistance of P. clarkii to S. eriocheiris. In addition, it is helpful for our understanding molecular mechanism of disease processes in crayfishes.

Infection Function of Adhesin-Like Protein ALP609 from Spiroplasma melliferum CH-1

Spiroplasma melliferum is the causative agent of spiroplasmosis in honeybees. During infection, adhesion of spiroplasmas to the host cells through adhesion factors is a crucial step. In this study, we identified an adhesin-like protein (ALP609) in S. melliferum CH-1 and investigated its role in the infection. To determine whether ALP609 is an adhesion factor, we performed indirect immunofluorescence microscopy to visualize its adhesion properties. Subsequently, an infection model of S. melliferum CH-1 was established using primary midgut cells of Apis mellifera to examine the adhesion and invasion of spiroplasma using anti-ALP609 antibodies inhibition assays and competition assays with recombinant ALP609 in vitro. We found that anti-ALP609 antibodies could inhibit the adhesion and invasion of spiroplasma to the midgut cells of A. mellifera and reduce midgut cell invasion on increased exposure to recombinant ALP609. To the best of our knowledge, this is the first report identifying adhesion-related factors in S. melliferum. Our results suggested that ALP609 is an adhesin-like protein critical for invasion of S. melliferum CH-1 into midgut cells of A. mellifera.

Integrated analysis of mRNA-seq in the haemocytes of Eriocheir sinensis in response to Spiroplasma eriocheiris infection

The Chinese mitten crab Eriocheir sinensis is an important economic crustacean that has been exposed to various diseases. Spiroplasma eriocheiris, isolated from tremor-diseased E. sinensis, was first identified as a lethal pathogen of freshwater crustaceans. To understand the pathogenesis of S. eriocheiris to E. sinensis, the transcriptomic profiles of haemocytes in the experimental and control groups at 1 d and 7 d post-injection were obtained using Illumina HiSeq 2500. These results showed that 40,358,724, 44,462,112, 45,516,576 and 37,713,728 paired-end clean reads were obtained from the cDNA libraries of DZ1 (the control group at 1 d), DZ7 (the control group at 7 d), SY1 (the experimental group at 1 d) and SY7 (the experimental group at 7 d), respectively. In total, 106,641 unique transcript fragments (unigenes) were assembled, with an average length of 710 bp. On the first day of stimulation, 33,084 up-regulated transcripts and 19,208 down-regulated transcripts were found in the experimental group compared with those in the control group. On the seventh day of stimulation, 40,198 up-regulated transcripts and 12,032 down-regulated transcripts were found in the experimental group compared with those in the control group. Some canonical immune-related pathways were identified via KEGG pathway analysis, including complement and coagulation cascades, the VEGF signalling pathway, the Wnt signalling pathway, natural killer cell-mediated cytotoxicity, the MAPK signalling pathway, neuroactive ligand-receptor interactions, and the Lysosome pathway. We found important immune-related genes (GNPTAB, MASP2, F7, F5, NFATC, TRAF6, MAP3K5, and TRa) in the KEGG pathway, and those genes were confirmed by qRT-PCR analysis. In addition, the significantly enriched neuroactive ligand-receptor interaction pathway was associated with intense paroxysmal tremors of infected crabs. Our results provide valuable information for the further analysis of the mechanisms of E. sinensis defence against S. eriocheiris invasion.

Vector-Borne Bacterial Plant Pathogens: Interactions with Hemipteran Insects and Plants

Hemipteran insects are devastating pests of crops due to their wide host range, rapid reproduction, and ability to transmit numerous plant-infecting pathogens as vectors. While the field of plant-virus-vector interactions has flourished in recent years, plant-bacteria-vector interactions remain poorly understood. Leafhoppers and psyllids are by far the most important vectors of bacterial pathogens, yet there are still significant gaps in our understanding of their feeding behavior, salivary secretions, and plant responses as compared to important viral vectors, such as whiteflies and aphids. Even with an incomplete understanding of plant-bacteria-vector interactions, some common themes have emerged: (1) all known vector-borne bacteria share the ability to propagate in the plant and insect host; (2) particular hemipteran families appear to be incapable of transmitting vector-borne bacteria; (3) all known vector-borne bacteria have highly reduced genomes and coding capacity, resulting in host-dependence; and (4) vector-borne bacteria encode proteins that are essential for colonization of specific hosts, though only a few types of proteins have been investigated. Here, we review the current knowledge on important vector-borne bacterial pathogens, including Xylella fastidiosa, Spiroplasma spp., Liberibacter spp., and 'Candidatus Phytoplasma spp.'. We then highlight recent approaches used in the study of vector-borne bacteria. Finally, we discuss the application of this knowledge for control and future directions that will need to be addressed in the field of vector-plant-bacteria interactions.

Differential expression of Spiroplasma citri surface protein genes in the plant and insect hosts

Background

Spiroplasma citri is a cell wall-less, plant pathogenic bacteria that colonizes two distinct hosts, the leafhopper vector and the host plant. Given the absence of a cell wall, surface proteins including lipoproteins and transmembrane polypeptides are expected to play key roles in spiroplasma/host interactions. Important functions in spiroplasma/insect interactions have been shown for a few surface proteins such as the major lipoprotein spiralin, the transmembrane S. citri adhesion-related proteins (ScARPs) and the sugar transporter subunit Sc76. S. citri efficient transmission from the insect to the plant is expected to rely on its ability to adapt to the different environments and more specifically to regulate the expression of genes encoding surface-exposed proteins.

Results

Genes encoding S. citri lipoproteins and ScARPs were investigated for their expression level in axenic medium, in the leafhopper vector Circulifer haematoceps and in the host plant (periwinkle Catharanthus roseus) either insect-infected or graft-inoculated. The vast majority of the lipoprotein genes tested (25/28) differentially responded to the various host environments. Considering their relative expression levels in the different environments, the possible involvement of the targeted genes in spiroplasma host adaptation was discussed. In addition, two S. citri strains differing notably in their ability to express adhesin ScARP2b and pyruvate dehydrogenase E1 component differed in their capacity to multiply in the two hosts, the plant and the leafhopper vector.

Conclusions

This study provided us with a list of genes differentially expressed in the different hosts, leading to the identification of factors that are thought to be involved in the process of S. citri host adaptation. The identification of such factors is a key step for further understanding of S. citri pathogenesis. Moreover the present work highlights the high capacity of S. citri in tightly regulating the expression level of a large set of surface protein genes, despite the small size of its genome.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-016-0666-y) contains supplementary material, which is available to authorized users.

iTRAQ-based quantitative proteomic analysis of Macrobrachium rosenbergii hemocytes during Spiroplasma eriocheiris infection

Spiroplasma eriocheiris, as a novel aquaculture pathogen, has led into catastrophic economic losses in aquaculture. The Macrobrachium rosenbergii hemocytes were major target cells in S. eriocheiris infection. Our study was designed to examine the hemocytes' immune response at the protein levels. The differential proteomes of the prawn hemocytes were analyzed immediately prior to injection with the pathogen, and at 192h post-injection by isobaric tags for relative and absolute quantization (iTRAQ) labeling, followed by liquid chromatogramphytandem mass spectrometry (LC-MS/MS). A total of 69 differentially expressed proteins were identified. Forty-nine proteins were up-regulated and 20 proteins were down-regulated resulting from a S. eriocheiris infection. Up-regulated proteins included vertebrate gliacolin-like protein, vitellogenin, Gram-negative binding protein 1, alpha2 macroglobulin isoform 2 (a2M), etc. Down-regulated proteins, involved with beta-1,3-glucan-binding protein (BGBP), immunoglobulin like, Rab7, lipopolysaccharide and β-1,3-glucan (LGBP), actin-related protein, etc. Selected bioactive factors (tachylectin, α2M and vitellogenin, BGBP, C-type lectin, LGBP and Rab7) were verified by their immune roles in the S. eriocheiris infection using real-time PCR. The present work could serve as a basis for future studies on the proteins implicated in the susceptibility/resistance of M. rosenbergii to S. eriocheiris, as well as contribute to our understanding of disease processes in prawns.

BIOLOGICAL SIGNIFICANCE

This is the first time using an iTRAQ approach to analyze proteomes of M. rosenbergii mobilized against S. eriocheiris infection and substantiated the hemocytes' proteomic changes in M. rosenbergii using an infection model. The results reported here can provide a significant step forward toward a more complete elucidation of the immune relationship between M. rosenbergii and the pathogen S. eriocheiris.

Role of the major antigenic membrane protein in phytoplasma transmission by two insect vector species

Background

Phytoplasmas are bacterial plant pathogens (class Mollicutes), transmitted by phloem feeding leafhoppers, planthoppers and psyllids in a persistent/propagative manner. Transmission of phytoplasmas is under the control of behavioral, environmental and geographical factors, but molecular interactions between membrane proteins of phytoplasma and vectors may also be involved. The aim of the work was to provide experimental evidence that in vivo interaction between phytoplasma antigenic membrane protein (Amp) and vector proteins has a role in the transmission process. In doing so, we also investigated the topology of the interaction at the gut epithelium and at the salivary glands, the two barriers encountered by the phytoplasma during vector colonization.

Methods

Experiments were performed on the ‘Candidatus Phytoplasma asteris’ chrysanthemum yellows strain (CYP), and the two leafhopper vectors Macrosteles quadripunctulatus Kirschbaum and Euscelidius variegatus Kirschbaum.

To specifically address the interaction of CYP Amp at the gut epithelium barrier, insects were artificially fed with media containing either the recombinant phytoplasma protein Amp, or the antibody (A416) or both, and transmission, acquisition and inoculation efficiencies were measured.

An abdominal microinjection protocol was employed to specifically address the interaction of CYP Amp at the salivary gland barrier. Phytoplasma suspension was added with Amp or A416 or both, injected into healthy E. variegatus adults and then infection and inoculation efficiencies were measured.

An internalization assay was developed, consisting of dissected salivary glands from healthy E. variegatus exposed to phytoplasma suspension alone or together with A416 antibody. The organs were then either observed in confocal microscopy or subjected to DNA extraction and phytoplasma quantification by qPCR, to visualize and quantify possible differences among treatments in localization/presence/number of CYP cells.

Results

Artificial feeding and abdominal microinjection protocols were developed to address the two barriers separately. The in vivo interactions between Amp of ‘Candidatus Phytoplasma asteris’ Chrysanthemum yellows strain (CYP) and vector proteins were studied by evaluating their effects on phytoplasma transmission by Euscelidius variegatus and Macrosteles quadripunctulatus leafhoppers. An internalization assay was developed, consisting of dissected salivary glands from healthy E. variegatus exposed to phytoplasma suspension alone or together with anti-Amp antibody. To visualize possible differences among treatments in localization/presence of CYP cells, the organs were observed in confocal microscopy. Pre-feeding of E. variegatus and M. quadripunctulatus on anti-Amp antibody resulted in a significant decrease of acquisition efficiencies in both species. Inoculation efficiency of microinjected E. variegatus with CYP suspension and anti-Amp antibody was significantly reduced compared to that of the control with phytoplasma suspension only. The possibility that this was due to reduced infection efficiency or antibody-mediated inhibition of phytoplasma multiplication was ruled out. These results provided the first indirect proof of the role of Amp in the transmission process.

Conclusion

Protocols were developed to assess the in vivo role of the phytoplasma native major antigenic membrane protein in two phases of the vector transmission process: movement through the midgut epithelium and colonization of the salivary glands. These methods will be useful also to characterize other phytoplasma-vector combinations. Results indicated for the first time that native CYP Amp is involved in vivo in specific crossing of the gut epithelium and salivary gland colonization during early phases of vector infection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0522-5) contains supplementary material, which is available to authorized users.

An overview of protein moonlighting in bacterial infection

We are rapidly returning to a world in which bacterial infections are a major health issue. Pathogenic bacteria are able to colonize and cause pathology due to the possession of virulence factors such as adhesins, invasins, evasins and toxins. These are generally specifically evolved proteins with selective actions. It is, therefore, surprising that most human bacterial pathogens employ moonlighting proteins as virulence factors. Currently, >90 bacterial species employ one or more moonlighting protein families to aid colonization and induce disease. These organisms employ 90 moonlighting bacterial protein families and these include enzymes of the glycolytic pathway, tricarboxylic acid (TCA) cycle, hexosemonophosphate shunt, glyoxylate cycle and a range of other metabolic enzymes, proteases, transporters and, also, molecular chaperones and protein-folding catalysts. These proteins have homologues in eukaryotes and only a proportion of the moonlighting proteins employed are solely bacterial in origin. Bacterial moonlighting proteins can be divided into those with single moonlighting functions and those with multiple additional biological actions. These proteins contribute significantly to the population of virulence factors employed by bacteria and some are obvious therapeutic targets. Where examined, bacterial moonlighting proteins bind to target ligands with high affinity. A major puzzle is the evolutionary mechanism(s) responsible for bacterial protein moonlighting and a growing number of highly homologous bacterial moonlighting proteins exhibit widely different moonlighting actions, suggesting a lack in our understanding of the mechanism of evolution of protein active sites.

Invasion of insect cells by Spiroplasma citri involves spiralin relocalization and lectin/glycoconjugate-type interactions

Spiroplamas are helical, cell wall-less bacteria belonging to the Class Mollicutes, a group of microorganisms phylogenetically related to low G+C, Gram-positive bacteria. Spiroplasma species are all found associated with arthropods and a few, including Spiroplasma citri are pathogenic to plant. Thus S. citri has the ability to colonize cells of two very distinct hosts, the plant and the insect vector. While spiroplasmal factors involved in transmission by the leafhopper Circulifer haematoceps have been identified, their specific contribution to invasion of insect cells is poorly understood. In this study we provide evidence that the lipoprotein spiralin plays a major role in the very early step of cell invasion. Confocal laser scanning immunomicroscopy revealed a relocalization of spiralin at the contact zone of adhering spiroplasmas. The implication of a role for spiralin in adhesion to insect cells was further supported by adhesion assays showing that a spiralin-less mutant was impaired in adhesion and that recombinant spiralin triggered adhesion of latex beads. We also showed that cytochalasin D induced changes in the surface-exposed glycoconjugates, as inferred from the lectin binding patterns, and specifically improved adhesion of S. citri wild-type but not of the spiralin-less mutant. These results indicate that cytochalasin D exposes insect cell receptors of spiralin that are masked in untreated cells. In addition, competitive adhesion assays with lectins strongly suggest spiralin to exhibit glycoconjugate binding properties similar to that of the Vicia villosa agglutinin (VVA) lectin.

The repetitive domain of ScARP3d triggers entry of Spiroplasma citri into cultured cells of the vector Circulifer haematoceps

Spiroplasma citri is a plant pathogenic mollicute transmitted by the leafhopper vector Circulifer haematoceps. Successful transmission requires the spiroplasmas to cross the intestinal epithelium and salivary gland barriers through endocytosis mediated by receptor-ligand interactions. To characterize these interactions we studied the adhesion and invasion capabilities of a S. citri mutant using the Ciha-1 leafhopper cell line. S. citri GII3 wild-type contains 7 plasmids, 5 of which (pSci1 to 5) encode 8 related adhesins (ScARPs). As compared to the wild-type strain GII3, the S. citri mutant G/6 lacking pSci1 to 5 was affected in its ability to adhere and enter into the Ciha-1 cells. Proteolysis analyses, Triton X-114 partitioning and agglutination assays showed that the N-terminal part of ScARP3d, consisting of repeated sequences, was exposed to the spiroplasma surface whereas the C-terminal part was anchored into the membrane. Latex beads cytadherence assays showed the ScARP3d repeat domain (Rep3d) to be involved, and internalization of the Rep3d-coated beads to be actin-dependent. These data suggested that ScARP3d, via its Rep3d domain, was implicated in adhesion of S. citri GII3 to insect cells. Inhibition tests using anti-Rep3d antibodies and competitive assays with recombinant Rep3d both resulted in a decrease of insect cells invasion by the spiroplasmas. Unexpectedly, treatment of Ciha-1 cells with the actin polymerisation inhibitor cytochalasin D increased adhesion and consequently entry of S. citri GII3. For the ScARPs-less mutant G/6, only adhesion was enhanced though to a lesser extent following cytochalasin D treatment. All together these results strongly suggest a role of ScARPs, and particularly ScARP3d, in adhesion and invasion of the leafhopper cells by S. citri.

A multidisciplinary study on the effects of phloem-limited viruses on the agronomical performance and berry quality of Vitis vinifera cv. Nebbiolo

Viral infections are known to have a detrimental effect on grapevine yield and performance, but there is still a lack of knowledge about their effect on the quality and safety of end products. Vines of Vitis vinifera cv. Nebbiolo clone 308, affected simultaneously by Grapevine leafroll-associated virus 1 (GLRaV-1), Grapevine virus A (GVA), and Rupestris stem pitting associated virus (RSPaV), were subjected to integrated analyses of agronomical performance, grape berry characteristics, instrumental texture profile, and proteome profiling. The comparison of performance and grape quality of healthy and infected vines cultivated in a commercial vineyard revealed similar shoot fertility, number of clusters, total yield, with significant differences in titratable acidity, and resveratrol content. Also some texture parameters such as cohesiveness and resilience were altered in berries of infected plants. The proteomic analysis of skin and pulp visualized about 400 spots. The ANOVA analysis on 2D gels revealed significant differences among healthy and virus-infected grape berries for 12 pulp spots and 7 skin spots. Virus infection mainly influenced proteins involved in the response to oxidative stress in the berry skin, and proteins involved in cell structure metabolism in the pulp.

The major antigenic membrane protein of "Candidatus Phytoplasma asteris" selectively interacts with ATP synthase and actin of leafhopper vectors

Phytoplasmas, uncultivable phloem-limited phytopathogenic wall-less bacteria, represent a major threat to agriculture worldwide. They are transmitted in a persistent, propagative manner by phloem-sucking Hemipteran insects. Phytoplasma membrane proteins are in direct contact with hosts and are presumably involved in determining vector specificity. Such a role has been proposed for phytoplasma transmembrane proteins encoded by circular extrachromosomal elements, at least one of which is a plasmid. Little is known about the interactions between major phytoplasma antigenic membrane protein (Amp) and insect vector proteins. The aims of our work were to identify vector proteins interacting with Amp and to investigate their role in transmission specificity. In controlled transmission experiments, four Hemipteran species were identified as vectors of “Candidatus Phytoplasma asteris”, the chrysanthemum yellows phytoplasmas (CYP) strain, and three others as non-vectors. Interactions between a labelled (recombinant) CYP Amp and insect proteins were analysed by far Western blots and affinity chromatography. Amp interacted specifically with a few proteins from vector species only. Among Amp-binding vector proteins, actin and both the α and β subunits of ATP synthase were identified by mass spectrometry and Western blots. Immunofluorescence confocal microscopy and Western blots of plasma membrane and mitochondrial fractions confirmed the localisation of ATP synthase, generally known as a mitochondrial protein, in plasma membranes of midgut and salivary gland cells in the vector Euscelidius variegatus. The vector-specific interaction between phytoplasma Amp and insect ATP synthase is demonstrated for the first time, and this work also supports the hypothesis that host actin is involved in the internalization and intracellular motility of phytoplasmas within their vectors. Phytoplasma Amp is hypothesized to play a crucial role in insect transmission specificity.

Involvement of a minimal actin-binding region of Spiroplasma citri phosphoglycerate kinase in spiroplasma transmission by its leafhopper vector

Background

Spiroplasma citri is a wall-less bacterium that colonizes phloem vessels of a large number of host plants. Leafhopper vectors transmit S. citri in a propagative and circulative manner, involving colonization and multiplication of bacteria in various insect organs. Previously we reported that phosphoglycerate kinase (PGK), the well-known glycolytic enzyme, bound to leafhopper actin and was unexpectedly implicated in the internalization process of S. citri into Circulifer haematoceps cells.

Methodology/Principal Findings

In an attempt to identify the actin-interacting regions of PGK, several overlapping PGK truncations were generated. Binding assays, using the truncations as probes on insect protein blots, revealed that the actin-binding region of PGK was located on the truncated peptide designated PGK-FL5 containing amino acids 49–154. To investigate the role of PGK-FL5-actin interaction, competitive spiroplasma attachment and internalization assays, in which His₆-tagged PGK-FL5 was added to Ciha-1 cells prior to infection with S. citri, were performed. No effect on the efficiency of attachment of S. citri to leafhopper cells was observed while internalization was drastically reduced. The in vivo effect of PGK-FL5 was confirmed by competitive experimental transmission assays as injection of PGK-FL5 into S. citri infected leafhoppers significantly affected spiroplasmal transmission.

Conclusion

These results suggest that S. citri transmission by its insect vector is correlated to PGK ability to bind actin.

Sequences essential for transmission of Spiroplasma citri by its leafhopper vector, Circulifer haematoceps, revealed by plasmid curing and replacement based on incompatibility

Spiroplasma citri GII3 contains highly related low-copy-number plasmids pSci1 to -6. Despite the strong similarities between their replication regions, these plasmids coexist in the spiroplasma cells, indicating that they are mutually compatible. The pSci1 to -6 plasmids encode the membrane proteins known as S. citri adhesion-related proteins (ScARPs) (pSci1 to -5) and the hydrophilic protein P32 (pSci6), which had been tentatively associated with insect transmission, as they were not detected in non-insect-transmissible strains. With the aim of further investigating the role of plasmid-encoded determinants in insect transmission, we have constructed S. citri mutant strains that differ in their plasmid contents by developing a plasmid curing/replacement strategy based on the incompatibility of plasmids having identical replication regions. Experimental transmission of these S. citri plasmid mutants through injection into the leafhopper vector Circulifer haematoceps revealed that pSci6, more precisely, the pSci6_06 coding sequence, encoding a protein of unknown function, was essential for transmission. In contrast, ScARPs and P32 were dispensable for both acquisition and transmission of the spiroplasmas by the leafhopper vector, even though S. citri mutants lacking pSci1 to -5 (encoding ScARPs) were acquired and transmitted at lower efficiencies than the wild-type strain GII3.