Measurement of binding strength between prey proteins interacting with Toxoplasma gondii SAG1 and SAG2 using isothermal titration calorimetry (ITC)

Toxoplasma gondii SAG1 targeting host cell S100A6 for parasite invasion and host immunity

Toxoplasma gondii surface antigen 1 (TgSAG1) is a surface protein of tachyzoites, which plays a crucial role in toxoplasma gondii infection and host cell immune regulation. However, how TgSAG1 regulates these processes remains elucidated. We utilized the biotin ligase -TurboID fusion with TgSAG1 to identify the host proteins interacting with TgSAG1, and identified that S100A6 was co-localized with TgSAG1 when T. gondii attached to the host cell. S100A6, either knocking down or blocking its functional epitopes resulted in inhibited parasites invasion. Meanwhile, S100A6 overexpression in host cells promoted T. gondii infection. We further verified that TgSAG1 could inhibit the interaction of host cell vimentin with S100A6 for cytoskeleton organization during T. gondii invasion. As an immunogen, TgSAG1 could promote the secretion of tumor necrosis factor alpha (TNF-α) through S100A6-Vimentin/PKCθ-NF-κB signaling pathway. In summary, our findings revealed a mechanism for how TgSAG1 functioned in parasitic invasion and host immune regulation.

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TgSAG1 interacts with host protein S100A6 then regulates T. gondii infection

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TgSAG1 could regulate binding vimentin with S100A6 during T. gondii infection

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TgSAG1 regulate TNFα secretion through S100A6-vimentin/PKCθ-NF-κB signaling pathway

A discussion on the affecting factors of the fitting procedures' reliability in isothermal titration calorimetry analysis

The standard isothermal titration calorimetry (ITC) curve, characterized as a typical sigmoid is strictly confined by the so-called c value, which is a ratio of titrand concentration to K_D. The proper c value with a range from 5 to 500 is commonly recommended as a standard protocol in routine detection process for acquiring the reliable fitting results in 1:1 binding mode. However, if the c value is less than "1" due to the weak binding or low concentration of analyte, fitting precision gets unstable and susceptible to the data noise. Herein, we first got a deep discussion into the reliability of the fitting procedure for 1:1 binding mode by data simulation, then quantized the effect of several affecting factors on the precision of parameters estimation through mathematical analysis. Finally, we proposed the value of 2~4 times K_D for final ligand concentration is optimal for the ITC titration in low c system (c < 1). All the theoretical derivations were further verified by a practical experiment of Magnesium-EDTA binding test.

Identification of Host Proteins Interacting with Toxoplasma gondii SAG1 by Yeast Two-Hybrid Assay

Toxoplasma gondii is one of the most successful human pathogens. To eliminate the infection, identification of receptors or binding partners from humans is indeed urgent. T. gondii surface antigen is the ultimate component involved during the attachment of parasite into host cell. However, mechanism of invasion between SAG and host-cell membrane remains unclear. Yeast two-hybrid experiment was used to identify the binding partners from cDNA human library by using T. gondii SAG1 as bait. Mated yeast cells were plated on DDO/X plates to confirm only prey plasmid that expressing interacting protein was selected. We detected 39 clones interacted with SAG1 based on a series of the selection procedures. After colony PCR, only 29 clones were positive and subsequently sent for sequencing. The yeast plasmids for true positive clones were rescued by transformation into E. coli TOP 10F' cells. Twenty-two clones were further examined by small-scale Y2H experiment. The results indicated that a strong interaction existed between Homo sapiens lysine-rich coil-coiled and SAG1 protein, which could activate the expressions of the reporter genes in diploid yeast. Co-immunoprecipitation experiment result indicated the binding between this prey and SAG1 protein was significant (Mann-Whitney U test, Z = - 1.964, P = 0.05). H. sapiens lysine-rich coil-coiled protein was found to be interacted with SAG1. This prey protein may serve as the potential drug target in vaccination study.

Genotyping and virulence analysis of Toxoplasma gondii isolates from a dead human fetus and dead pigs in Jiangsu province, Eastern China

Toxoplasma gondii is an obligate intracellular parasite with worldwide distribution. Virulence of T. gondii is a multigenic trait. Genetic and virulence data for T. gondii isolates from humans and animals in China have been reported. However, almost all biological materials used for genotyping of T. gondii from humans and pigs were DNA samples prepared from tissues, and T. gondii strains used for virulence analysis were isolated mainly from cats. In this study, one isolate from a dead human fetus was identified as type I (ToxoDB #10) while the two isolates from dead pigs were type Chinese I (ToxoDB #9) with PCR-restriction fragment length polymorphism using 10 markers (SAG1, SAG2, SAG3, BTUB, GRA6, c22-8, c29-2, L358, PK1 and Apico). Three isolates were comfirmed as virulent strains in mice. By cloning and sequences analysis, all isolates contained a Pvu II restriction site (572-577 bp) in the KHB fragment and five tandem repeats in the 5' UTR region of SAG1, which were associated with T. gondii virulence. The type Chinese I isolates contained two deletions of 15 and 3 bp at positions 635 to 649 and 658 to 660 in the GRA6, which were correlated with genotype, but not with virulence. To our knowledge, this is the first report on the systematic analysis of murine virulence of type Chinese I strain from pigs, and the associations of sequences of the KHB fragment and SAG1 with virulence of type Chinese I strain. The Chinese I genotype was more closely related to type II strains.