Targeted disruption of CK1α in Toxoplasma gondii increases acute virulence in mice

Released Parasite-Derived Kinases as Novel Targets for Antiparasitic Therapies

The efficient manipulation of their host cell is an essential feature of intracellular parasites. Most molecular mechanisms governing the subversion of host cell by protozoan parasites involve the release of parasite-derived molecules into the host cell cytoplasm and direct interaction with host proteins. Among these released proteins, kinases are particularly important as they govern the subversion of important host pathways, such as signalling or metabolic pathways. These enzymes, which catalyse the transfer of a phosphate group from ATP onto serine, threonine, tyrosine or histidine residues to covalently modify proteins, are involved in numerous essential biological processes such as cell cycle or transport. Although little is known about the role of most of the released parasite-derived kinases in the host cell, they are examples of kinases hijacking host cellular pathways such as signal transduction or apoptosis, which are essential for immune response evasion as well as parasite survival and development. Here we present the current knowledge on released protozoan kinases and their involvement in host-pathogen interactions. We also highlight the knowledge gaps remaining before considering those kinases - involved in host signalling subversion - as antiparasitic drug targets.

Comparative Phosphoproteomic Analysis of Sporulated Oocysts and Tachyzoites of Toxoplasma gondii Reveals Stage-Specific Patterns

Toxoplasma gondii is an obligate intracellular protozoan of severe threat to humans and livestock, whose life history harbors both gamic and apogamic stages. Chinese 1 (ToxoDB#9) was a preponderant genotype epidemic in food-derived animals and humans in China, with a different pathogenesis from the strains from the other nations of the world. Posttranslational modifications (PTMs) of proteins were critical mediators of the biology, developmental transforms, and pathogenesis of protozoan parasites. The phosphoprotein profiling and the difference between the developmental phases of T. gondii, contributing to development and infectivity, remain unknown. A quantitative phosphoproteomic approach using IBT integrated with TiO₂ affinity chromatography was applied to identify and analyze the difference in the phosphoproteomes between the sporulated oocysts and the tachyzoites of the virulent ToxoDB#9 (PYS) strain of T. gondii. A total of 4058 differential phosphopeptides, consisting of 2597 upregulated and 1461 downregulated phosphopeptides, were characterized between sporulated the oocysts and tachyzoites. Twenty-one motifs extracted from the upregulated phosphopeptides contained 19 serine motifs and 2 threonine motifs (GxxTP and TP), whereas 16 motifs identified from downregulated phosphopeptides included 13 serine motifs and 3 threonine motifs (KxxT, RxxT, and TP). Beyond the traditional kinases, some infrequent classes of kinases, including Ab1, EGFR, INSR, Jak, Src and Syk, were found to be corresponding to motifs from the upregulated and downregulated phosphopeptides. Remarkable functional properties of the differentially expressed phosphoproteins were discovered by GO analysis, KEGG pathway analysis, and STRING analysis. S8GFS8 (DNMT1-RFD domain-containing protein) and S8F5G5 (Histone kinase SNF1) were the two most connected peptides in the kinase-associated network. Out of these, phosphorylated modifications in histone kinase SNF1 have functioned in mitosis and interphase of T. gondii, as well as in the regulation of gene expression relevant to differentiation. Our study discovered a remarkable difference in the abundance of phosphopeptides between the sporulated oocysts and tachyzoites of the virulent ToxoDB#9 (PYS) strain of T. gondii, which may provide a new resource for understanding stage-specific differences in PTMs and may enhance the illustration of the regulatory mechanisms contributing to the development and infectivity of T. gondii.

Protein kinases in Toxoplasma gondii

Toxoplasma gondii is an obligatory intracellular pathogen that causes life threatening illness in immunodeficient individuals, miscarriage in pregnant woman, and blindness in newborn children. Similar to any other eukaryotic cell, protein kinases play critical and essential roles in the Toxoplasma life cycle. Accordingly, many studies have focused on identifying and defining the mechanism of function of these signalling proteins with a long-term goal to develop anti-Toxoplasma therapeutics. In this review, we briefly discuss classification and key components of the catalytic domain which are critical for functioning of kinases, with a focus on domains, families, and groups of kinases within Toxoplasma. More importantly, this article provides a comprehensive, current overview of research on kinase groups in Toxoplasma including the established eukaryotic AGC, CAMK, CK1, CMGC, STE, TKL families and the apicomplexan-specific FIKK, ROPK and WNG family of kinases. This work provides an overview and discusses current knowledge on Toxoplasma kinases including their localization, function, signalling network and role in acute and chronic pathogenesis, with a view towards the future in probing kinases as viable drug targets.

Dangerous Duplicity: The Dual Functions of Casein Kinase 1 in Parasite Biology and Host Subversion

Casein Kinase 1 (CK1) family members are serine/threonine protein kinases that are involved in many biological processes and highly conserved in eukaryotes from protozoan to humans. Even though pathogens exploit host CK1 signaling pathways to survive, the role of CK1 in infectious diseases and host/pathogen interaction is less well characterized compared to other diseases, such as cancer or neurodegenerative diseases. Here we present the current knowledge on CK1 in protozoan parasites highlighting their essential role for parasite survival and their importance for host-pathogen interactions. We also discuss how the dual requirement of CK1 family members for parasite biological processes and host subversion could be exploited to identify novel antimicrobial interventions.

iTRAQ-based comparative proteomic analysis in different developmental stages of Echinococcus granulosus

Cystic echinococcosis, caused by infection with the larval stage of the cestode Echinococcus granulosus, is a chronic zoonosis. The lifecycle of the E. granulosus parasite includes three consecutive stages that require specific gene regulation or protein expression to survive environmental shifts between definitive hosts and intermediate hosts. The aim of the present study is to screen and analyze the stage differential antigens to be considered for vaccine development against E. granulosus. By using the iTRAQ (isobaric tags for relative and absolute quantification) method, the differentially expressed proteins were selected from the three consecutive developmental stages of E. granulosus: oncosphere, adult tapeworms, and protoscolex. Through a bioinformatics analysis including Clusters of Orthologous Groups (COG), Gene Ontology (GO), and pathway metabolic annotation, we identified some proteins of interest from each stage. The results showed that a large number of differentially expressed proteins (375: oncosphere vs. adult, 346: oncosphere vs. protoscolex, and 391: adult vs. protoscolex) were identified from the three main lifecycle stages. Analysis of the differential protein pathways showed that these differential proteins are mainly enriched in metabolic pathways, Huntington’s diseases, Alzheimer’s diseases, and ribosome metabolic pathways. Interestingly, among these differential proteins, expression levels of paramyosin, HSP60, HSP70, HSP90, cathepsin L1, cathepsin D, casein kinase, and calmodulin were significantly higher in the oncosphere than in the adult or protoscolex (p < 0.05). We hope our findings will help to identify potential targets for diagnosis or for therapeutic and prophylactic intervention.

Mapping immune variation and var gene switching in naive hosts infected with Plasmodium falciparum

Falciparum malaria is clinically heterogeneous and the relative contribution of parasite and host in shaping disease severity remains unclear. We explored the interaction between inflammation and parasite variant surface antigen (VSA) expression, asking whether this relationship underpins the variation observed in controlled human malaria infection (CHMI). We uncovered marked heterogeneity in the host response to blood challenge; some volunteers remained quiescent, others triggered interferon-stimulated inflammation and some showed transcriptional evidence of myeloid cell suppression. Significantly, only inflammatory volunteers experienced hallmark symptoms of malaria. When we tracked temporal changes in parasite VSA expression to ask whether variants associated with severe disease rapidly expand in naive hosts, we found no transcriptional evidence to support this hypothesis. These data indicate that parasite variants that dominate severe malaria do not have an intrinsic growth or survival advantage; instead, they presumably rely upon infection-induced changes in their within-host environment for selection.

Casein kinase 1α: biological mechanisms and theranostic potential

Casein kinase 1α (CK1α) is a multifunctional protein belonging to the CK1 protein family that is conserved in eukaryotes from yeast to humans. It regulates signaling pathways related to membrane trafficking, cell cycle progression, chromosome segregation, apoptosis, autophagy, cell metabolism, and differentiation in development, circadian rhythm, and the immune response as well as neurodegeneration and cancer. Given its involvement in diverse cellular, physiological, and pathological processes, CK1α is a promising therapeutic target. In this review, we summarize what is known of the biological functions of CK1α, and provide an overview of existing challenges and potential opportunities for advancing theranostics.

Targeted overexpression of cyclic AMP-dependent protein kinase subunit in Toxoplasma gondii promotes replication and virulence in host cells

Toxoplasma gondii (T. gondii) is one of the most common parasite that can infect almost any warm-blooded animals including humans. The cyclic nucleotide-dependent protein kinase (PKA) regulates a spectrum of intracellular signal pathways in many organisms. Protein kinase catalytic subunit (PKAC) is the core of the whole protein, and plays an important role in the life cycle of T.gondii. Here, T.gondii PKAC (TgPKAC) overexpression strain (TgPKAC-OE) was constructed. The growth of the TgPKAC-OE, RH△Ku80, and TgPKAC inhibition strains (TgPKAC-H89) were analysed by SYBR-green real-time PCR, and the ultrastructure was observed by transmission electron microscopy. The survival rate in mice was also recorded to analyse the virulence of the parasites. We also investigated the subcellular localization of TgPKAC in Vero cells by laser scanning microscope. We found that TgPKAC-OE strain exhibited obviously increased growth rate in Vero cells in vitro, and infected mice survived for a shorter time compared to wild type strain. Ultrastructural analysis found more autophagosomes-like structures in TgPKAC-H89 parasite compared to RH△Ku80 strain, and the relative expression level of Toxoplasma gondii autophagy-related protein (ATG8) in TgPKAC-H89 parasite was higher than wild type parasite. Laser confocal results showed that TgPKAC was mainly expressed in the cytoplasm of Vero cells. In conclusion, we hypothesized that inhibition of TgPKAC could cause autophagy of Toxoplasma gondii and then influence the replication of the parasite. TgPKAC plays an important role in parasite virulence in vivo, and the subcellular localization was successfully detected in Vero cells. Our data will provide a basis for further study of TgPKAC function and help screen drug targets of T. gondii.

Genome-Wide Identification and Evolutionary Analysis of Sarcocystis neurona Protein Kinases

The apicomplexan parasite Sarcocystis neurona causes equine protozoal myeloencephalitis (EPM), a degenerative neurological disease of horses. Due to its host range expansion, S. neurona is an emerging threat that requires close monitoring. In apicomplexans, protein kinases (PKs) have been implicated in a myriad of critical functions, such as host cell invasion, cell cycle progression and host immune response evasion. Here, we used various bioinformatics methods to define the kinome of S. neurona and phylogenetic relatedness of its PKs to other apicomplexans. We identified 97 putative PKs clustering within the various eukaryotic kinase groups. Although containing the universally-conserved PKA (AGC group), S. neurona kinome was devoid of PKB and PKC. Moreover, the kinome contains the six-conserved apicomplexan CDPKs (CAMK group). Several OPK atypical kinases, including ROPKs 19A, 27, 30, 33, 35 and 37 were identified. Notably, S. neurona is devoid of the virulence-associated ROPKs 5, 6, 18 and 38, as well as the Alpha and RIO kinases. Two out of the three S. neurona CK1 enzymes had high sequence similarities to Toxoplasma gondii TgCK1-α and TgCK1-β and the Plasmodium PfCK1. Further experimental studies on the S. neurona putative PKs identified in this study are required to validate the functional roles of the PKs and to understand their involvement in mechanisms that regulate various cellular processes and host-parasite interactions. Given the essentiality of apicomplexan PKs in the survival of apicomplexans, the current study offers a platform for future development of novel therapeutics for EPM, for instance via application of PK inhibitors to block parasite invasion and development in their host.