Solution structure of a PAN module from the apicomplexan parasite Eimeria tenella

Diverse Roles of TgMIC1/4/6 in the Toxoplasma Infection

Toxoplasma gondii microneme is a specialized secretory organelle that discharges its contents at the apical tip of this apicomplexan parasite in a sequential and regulated manner. Increasing number of studies on microneme proteins (MICs) have shown them as a predominant and important role in host cell attachment, invasion, motility and pathogenesis. In this review, we summarize the research advances in one of the most important MICs complexes, TgMIC1/4/6, which will contribute to improve the understanding of the molecular mechanism of T. gondii infection and provide a theoretical basis for the effective control against T. gondii.

Genomic and transcriptional analysis of genes containing fibrinogen and IgSF domains in the schistosome vector Biomphalaria glabrata, with emphasis on the differential responses of snails susceptible or resistant to Schistosoma mansoni

Achieving a deeper understanding of the factors controlling the defense responses of invertebrate vectors to the human-infecting pathogens they transmit will provide needed new leads to pursue for control. Consequently, we provide new genomic and transcriptomic insights regarding FReDs (containing a fibrinogen domain) and FREPs (fibrinogen domain and one or two IgSF domains) from the planorbid snail Biomphalaria glabrata, a Neotropical vector of Schistosoma mansoni, causative agent of human intestinal schistosomiasis. Using new bioinformatics approaches to improve annotation applied to both genome and RNA-Seq data, we identify 73 FReD genes, 39 of which are FREPs. We provide details of domain structure and consider relationships and homologies of B. glabrata FBG and IgSF domains. We note that schistosome-resistant (BS-90) snails mount complex FREP responses following exposure to S. mansoni infection whereas schistosome-susceptible (M line) snails do not. We also identify several coding differences between BS-90 and M line snails in three FREPs (2, 3.1 and 3.2) repeatedly implicated in other studies of anti-schistosome responses. In combination with other results, our study provides a strong impetus to pursue particular FREPs (2, 3.1, 3.2 and 4) as candidate resistance factors to be considered more broadly with respect to schistosome control efforts, including involving other Biomphalaria species vectoring S. mansoni in endemic areas in Africa.

Site-restricted plasminogen activation mediated by group A streptococcal streptokinase variants

By examining the roles of bacterial and host-derived cofactors in streptokinase-mediated plasminogen activation, we find that phenotypic streptokinase variation functionally underpins a pathogenic mechanism whereby streptokinase variants differentially focus plasminogen activation, leading to specific niche adaption within the host.

Ss-Sl2, a novel cell wall protein with PAN modules, is essential for sclerotial development and cellular integrity of Sclerotinia sclerotiorum

The sclerotium is an important dormant body for many plant fungal pathogens. Here, we reported that a protein, named Ss-Sl2, is involved in sclerotial development of Sclerotinia sclerotiorum. Ss-Sl2 does not show significant homology with any protein of known function. Ss-Sl2 contains two putative PAN modules which were found in other proteins with diverse adhesion functions. Ss-Sl2 is a secreted protein, during the initial stage of sclerotial development, copious amounts of Ss-Sl2 are secreted and accumulated on the cell walls. The ability to maintain the cellular integrity of RNAi-mediated Ss-Sl2 silenced strains was reduced, but the hyphal growth and virulence of Ss-Sl2 silenced strains were not significantly different from the wild strain. Ss-Sl2 silenced strains could form interwoven hyphal masses at the initial stage of sclerotial development, but the interwoven hyphae could not consolidate and melanize. Hyphae in these interwoven bodies were thin-walled, and arranged loosely. Co-immunoprecipitation and yeast two-hybrid experiments showed that glyceraldehyde-3-phosphate dehydrogenase (GAPDH), Woronin body major protein (Hex1) and elongation factor 1-alpha interact with Ss-Sl2. GAPDH-knockdown strains showed a similar phenotype in sclerotial development as Ss-Sl2 silenced strains. Hex1-knockdown strains showed similar impairment in maintenance of hyphal integrity as Ss-Sl2 silenced strains. The results suggested that Ss-Sl2 functions in both sclerotial development and cellular integrity of S. sclerotiorum.

Galactose recognition by the apicomplexan parasite Toxoplasma gondii

Toxosplasma gondii is the model parasite of the phylum Apicomplexa, which contains numerous obligate intracellular parasites of medical and veterinary importance, including Eimeria, Sarcocystis, Cryptosporidium, Cyclospora, and Plasmodium species. Members of this phylum actively enter host cells by a multistep process with the help of microneme protein (MIC) complexes that play important roles in motility, host cell attachment, moving junction formation, and invasion. T. gondii (Tg)MIC1-4-6 complex is the most extensively investigated microneme complex, which contributes to host cell recognition and attachment via the action of TgMIC1, a sialic acid-binding adhesin. Here, we report the structure of TgMIC4 and reveal its carbohydrate-binding specificity to a variety of galactose-containing carbohydrate ligands. The lectin is composed of six apple domains in which the fifth domain displays a potent galactose-binding activity, and which is cleaved from the complex during parasite invasion. We propose that galactose recognition by TgMIC4 may compromise host protection from galectin-mediated activation of the host immune system.

Engineering streptokinase for generation of active site-labeled plasminogen analogs

We previously demonstrated that streptokinase (SK) can be used to generate active site-labeled fluorescent analogs of plasminogen (Pg) by virtue of its nonproteolytic activation of the zymogen. The method is versatile and allows stoichiometric and active site-specific incorporation of any one of many molecular probes. The limitation of the labeling approach is that it is both time-consuming and low yield. Here we demonstrate an improved method for the preparation of labeled Pg analogs by the use of an engineered SK mutant fusion protein with both COOH- and NH(2)-terminal His(6) tags. The NH(2)-terminal tag is followed by a tobacco etch virus proteinase cleavage site to ensure that the SK Ile(1) residue, essential for conformational activation of Pg, is preserved. The SK COOH-terminal Lys(414) residue and residues Arg253-Leu260 in the SK β-domain were deleted to prevent cleavage by plasmin (Pm) and to disable Pg substrate binding to the SK·Pg(∗)/Pm catalytic complexes, respectively. Near elimination of Pm generation with the SKΔ(R253-L260)ΔK414-His(6) mutant increased the yield of labeled Pg 2.6-fold and reduced the time required more than 2-fold. The versatility of the labeling method was extended to the application of Pg labeled with a near-infrared probe to quantitate Pg receptors on immune cells by flow cytometry.

Eimeria tenella: effects of diclazuril treatment on microneme genes expression in second-generation merozoites and pathological changes of caeca in parasitized chickens

The effects of diclazuril on mRNA expression levels of invasion-related microneme genes were examined in second-generation merozoites of Eimeria tenella (E. tenella) by quantitative real-time (QRT) PCR. Diclazruil treatment of infected chickens significantly decreased the number of second-generation merozoites by 65.13%, and resulted in downregulation of EtMIC genes: EtMIC1 by 65.63%, EtMIC2 by 64.12%, EtMIC3 by 56.82%, EtMIC4 by 73.48%, and EtMIC5 by 78.17%. SEM images of caecum tissue from uninfected chickens showed regular intestinal villus structure. In infected chickens, a distinct loss of the superficial epithelium, with a flattened mucosa and large-area necrosis and anabrosis, was evident. In diclazruil-treated chickens, a decrease in merozoite number and a visibly improved appearance of the caeca were noted. These improvements appeared to be mediated in part by downregulation of the expression of invasion-related EtMIC genes in response to diclazuril.

Plasminogen substrate recognition by the streptokinase-plasminogen catalytic complex is facilitated by Arg253, Lys256, and Lys257 in the streptokinase beta-domain and kringle 5 of the substrate

Streptokinase (SK) conformationally activates the central zymogen of the fibrinolytic system, plasminogen (Pg). The SK.Pg* catalytic complex binds Pg as a specific substrate and cleaves it into plasmin (Pm), which binds SK to form the SK.Pm complex that propagates Pm generation. Catalytic complex formation is dependent on lysine-binding site (LBS) interactions between a Pg/Pm kringle and the SK COOH-terminal Lys(414). Pg substrate recognition is also LBS-dependent, but the kringle and SK structural element(s) responsible have not been identified. SK mutants lacking Lys(414) with Ala substitutions of charged residues in the SK beta-domain 250-loop were evaluated in kinetic studies that resolved conformational and proteolytic Pg activation. Activation of [Lys]Pg and mini-Pg (containing only kringle 5 of Pg) by SK with Ala substitutions of Arg(253), Lys(256), and Lys(257) showed decreases in the bimolecular rate constant for Pm generation, with nearly total inhibition for the SK Lys(256)/Lys(257) double mutant. Binding of bovine Pg (BPg) to the SK.Pm complex containing fluorescently labeled Pm demonstrated LBS-dependent assembly of a SK.labeled Pm.BPg ternary complex, whereas BPg did not bind to the complex containing the SK Lys(256)/Lys(257) mutant. BPg was activated by SK.Pm with a K(m) indistinguishable from the K(D) for BPg binding to form the ternary complex, whereas the SK Lys(256)/Lys(257) mutant did not support BPg activation. We conclude that SK residues Arg(253), Lys(256), and Lys(257) mediate Pg substrate recognition through kringle 5 of the [Lys]Pg and mini-Pg substrates. A molecular model of the SK.kringle 5 complex identifies the putative interactions involved in LBS-dependent Pg substrate recognition.

Microneme proteins in apicomplexans

Microneme secretion supports several key cellular processes including gliding motility, active cell invasion and migration through cells, biological barriers, and tissues. The modular design of microneme proteins enables these molecules to assist each other in folding and passage through the quality control system, accurately target to the micronemes, oligimerizing with other parasite proteins, and engaging a variety of host receptors for migration and cell invasion. Structural and biochemical analyses of MIC domains is providing new perspectives on how adhesion is regulated and the potentially distinct roles MICs might play in long or short range interactions during parasite attachment and entry. New access to complete genome sequences and ongoing advances in genetic manipulation should provide fertile ground for refining current models and defining exciting new roles for MICs in apicomplexan biology.

The Microneme Proteins EtMIC4 and EtMIC5 of Eimeria tenella Form a Novel, Ultra-high Molecular Mass Protein Complex That Binds Target Host Cells

Eimeria tenella, in common with other parasitic protozoa of the phylum Apicomplexa, invades host cells using an actinomyosin-powered "glideosome" complex and requires the secretion of adhesive proteins from the microneme organelles onto the parasite surface. Microneme proteins of E. tenella include EtMIC4, a transmembrane protein that has multiple thrombospondin type I domains and calcium-binding epidermal growth factor-like domains in its extracellular domain, and EtMIC5, a soluble protein composed of 11 tandemly repeated domains that belong to the plasminogen-apple-nematode superfamily. We show here that EtMIC4 and EtMIC5 interact to form an oligomeric, ultrahigh molecular mass protein complex. The complex was purified from lysed parasites by non-denaturing techniques, and the stoichiometry was shown to be [EtMIC4](2):[EtMIC5](1), with an octamer of EtMIC4 bound non-covalently to a tetramer of EtMIC5. The complex is formed within the parasite secretory pathway and is maintained after secretion onto the surface of the parasite. The purified complex binds to a number of epithelial cell lines in culture. Identification and characterization of this complex contributes to an overall understanding of the role of multimolecular protein complexes in specific interactions between pathogens and their hosts during infection.

Functional and immunological insights from the three-dimensional structures of Plasmodium surface proteins

Malaria is a major global health problem and is caused by the unicellular parasite Plasmodium. Plasmodial surface proteins have important roles in host cell invasion and are responsible for antigenic diversity in this organism. Knowledge of the three-dimensional structure of surface proteins can facilitate our understanding their biological function, and contribute to the development of therapeutic and vaccine strategies against malaria. Structural studies allow rational drug design when ligand- or receptor-binding sites are identified and characterized. Analysis of the three-dimensional distribution of protective antibody epitopes and polymorphic residues can facilitate vaccine candidate optimization. With this in mind, some Plasmodium surface-protein structures have determined by X-ray crystallography or nuclear magnetic resonance.

Structural interpretation of 42 mutations causing factor XI deficiency using homology modeling

BACKGROUND

Factor (F)XI is important in the consolidation phase of blood coagulation. The structural effects of mutations causing FXI deficiency have not been well described due to the lack of a structure for FXI.

OBJECTIVES

To develop molecular models of the four apple (Ap) and serine protease (SP) domains in FXI in order to assess the structural effects of published FXI mutations in the light of their phenotypes.

METHODS

The Ap domains were modeled using the NMR structure of an adhesin from Eimeria tenella. The SP domain was modeled using the crystal structure of beta-tryptase.

RESULTS

The effect of 42 mutations causing FXI deficiency was analyzed using homology models for the Ap and SP domains in FXI. Protein misfolding was implicated as the likely structural mechanism of disease in six of 14 mutations in the four Ap domains with Type I phenotypes. Likewise, misfolding was implicated in eight of 14 mutations in the SP domain with Type I phenotypes. Unlike other coagulation factor deficiencies, Type II phenotypes based on a catalytically dysfunctional FXI are uncommon. The structural models indicated that two known Type II mutations in the Ap domains could be correlated with functional defects in substrate or cofactor binding, and likewise four Type II mutations in the SP domain would disrupt the active site.

CONCLUSIONS

New FXI disease-causing mutations can now be structurally characterized to complement phenotypic data, and expression studies can be designed to verify the molecular basis of each deficiency.

The Biology of Avian Eimeria with an Emphasis on their Control by Vaccination

Studies on the biology of the avian species of Eimeria are currently benefiting from the availability of a comprehensive sequence for the nuclear genome of Eimeria tenella. Allied to some recent advances in transgenic technologies and genetic approaches to identify protective antigens, some elements are now being assembled that should be helpful for the development of a new generation of vaccines. In the meantime, control of avian coccidiosis by vaccination represents a major success in the fight against infections caused by parasitic protozoa. Live vaccines that comprise defined populations of oocysts are used routinely and this form of vaccination is based upon the long-established fact that chickens infected with coccidial parasites rapidly develop protective immunity against challenge infections with the same species. Populations of wild-type Eimeria parasites were the basis of the first live vaccines introduced around 50 years ago and the more recent introduction of safer, live-attenuated, vaccines has had a significant impact on coccidiosis control in many areas of the world. In Europe the introduction of vaccination has coincided with declining drug efficacy (on account of drug resistance) and increasing concerns by consumers about the inclusion of in-feed medication and prospects for drug residues in meat. The use of attenuated vaccines throughout the world has also stimulated a greater interest in the vaccines that comprise wild-type parasites and, during the past 3 years worldwide, around 3x10(9) doses of each type of vaccine have been used. The need for only small numbers of live parasites to induce effective protective immunity and the recognition that Eimeria spp. are generally very potent immunogens has stimulated efforts to develop other types of vaccines. None has succeeded except for the licensing, within several countries in 2002, of a vaccine (CoxAbic vaccine; Abic, Israel) that protects via the maternal transfer of immunoglobulin to the young chick. Building on the success of viral vaccines that are delivered via the embryonating egg, an in ovo coccidiosis vaccine (Inovocox, Embrex Inc.) is currently in development. Following successful field trials in 2001, the product will be ready for Food and Drug Administration approval in 2005 and a manufacturing plant will begin production for sale in late 2005. Limited progress has been achieved towards the development of subunit or recombinant vaccines. No products are available and studies to identify potential antigens remain compromised by an absence of effective in vitro assays that correlate with the induction of protective immunity in the host. To date, only a relatively small portfolio of molecules has been evaluated for an ability to induce protection in vivo. Although Eimeria are effective immunogens, it is probable that to date none of the antigens that induce potent protective immune responses during the course of natural infection has been isolated.