Echinococcus granulosus: Evidence of a heterodimeric glutathione transferase built up by phylogenetically distant subunits

Immunization with a Mu-class glutathione transferase from Echinococcus granulosus induces efficient antibody responses and confers long-term protection against secondary cystic echinococcosis

Cystic echinococcosis, a zoonosis caused by cestodes belonging to the Echinococcus granulosus sensu lato (s.l.) genetic complex, affects humans and diverse livestock species. Although a veterinary vaccine exhibiting high levels of antibody-mediated protection has successfully reached the market, the large genetic diversity among parasite isolates and their particular host preferences, makes still necessary the search for novel vaccine candidates. Glutathione transferases (GSTs) constitute attractive targets for immunoprophylaxis due to their outstanding relevance in helminth detoxification processes, against both exogenous and endogenous stressors. Among the six GSTs known to be expressed in E. granulosus s.l., EgGST1 (Mu-class), EgGST2 (Sigma-class), and EgGST3 (a still non-classifiable isoenzyme), show the highest proteomic expression. Therefore, their recombinant forms -rEgGST1, rEgGST2 and rEgGST3- were herein analyzed regarding their potential to induce long-term antiparasite protection in mice. Only immunization with rEgGST1 induced long-lasting protection; and accordingly, rEgGST1-specific antibodies enhanced the parasite killing through both the classical activation of the host complement system and the antibody-dependent cellular cytotoxicity by macrophages. These results support further testing of rEgGST1 as a vaccine candidate in diverse hosts due to the broad expression of EgGST1 in different parasite stages and tissues.

Sigma-Class Glutathione Transferases (GSTσ): A New Target with Potential for Helminth Control

Glutathione transferases (GSTs EC 2.5.1.18) are critical components of phase II metabolism, instrumental in xenobiotics' metabolism. Their primary function involves conjugating glutathione to both endogenous and exogenous toxic compounds, which increases their solubility and enables their ejection from cells. They also play a role in the transport of non-substrate compounds and immunomodulation, aiding in parasite establishment within its host. The cytosolic GST subfamily is the most abundant and diverse in helminths, and sigma-class GST (GSTσ) belongs to it. This review focuses on three key functions of GSTσ: serving as a detoxifying agent that provides drug resistance, functioning as an immune system modulator through its involvement in prostaglandins synthesis, and acting as a vaccine antigen.

Expression and distribution of glutathione transferases in protoscoleces of Echinococcus granulosus sensu lato

Glutathione transferases (GSTs) belong to a diverse superfamily of multifunctional proteins involved in metabolic detoxification. In helminth parasite, GSTs are particularly relevant since they are also involved in host immunomodulation. Echinococcus granulosus sensu lato (s.l.) is a cestode parasite known to express at least three phylogenetically distant cytosolic GSTs: EgGST1 and EgGST2 previously grouped within Mu and Sigma classes, respectively; and EgGST3 related to both Omega and Sigma classes. To better characterize E. granulosus s.l. GSTs, herein their expression and distribution were assessed in the pre-adult protoscolex (PSC) parasite stage. Potential transcriptional regulatory mechanisms of the corresponding EgGST genes were also explored. Firstly, the transcription of the three EgGSTs was significantly induced during the early stages of the murine model of infection, suggesting a potential role during parasite establishment. EgGST1 was detected in the parenchyma of PSCs and its expression increased after H₂O₂ exposure, supporting its role in detoxification. EgGST2 was mainly detected on the PSCs tegument, strategically localized for potential immunoregulation functions due to its Sigma-class characteristics. In addition, its expression increased after anthelmintic treatment, suggesting a role in chemotherapy resistance. Finally, the Omega-related EgGST3 was localized throughout the entire PSC body, including suckers and tegument, and since its expression also increased after H₂O₂ treatment, a potential role in oxidative stress response could also be ascribed. On the other hand, known cis-acting regulatory motifs were detected in EgGST genes, suggesting similar transcription processes to other eukaryotes. The results herein reported provide additional data regarding the roles of EgGSTs in E. granulosus s.l. biology, contributing to a better understanding of its host-parasite interaction.

Characterisation of a Teladorsagia circumcincta glutathione transferase

A 615 bp full length cDNA encoding a Teladorsagia circumcincta glutathione transferase (TcGST) was cloned, expressed in Escherichia coli and the recombinant protein purified and its kinetic properties determined. The predicted protein consisted of 205 amino acids and was present as a single band of about 24 kDa on SDS-PAGE. Multiple alignments of the protein sequence of TcGST with homologues from other helminths showed that the highest identity of 53-68% with haem-binding nematode proteins designated as members of the nu class of GSTs. Substrate binding sites and conserved regions were identified and were generally conserved. The predicted 3-dimensional structures of TcGST and HcGST revealed highly open binding cavities typical of this class of GST, considered to allow greater accessibility to diverse ligands compared with other classes of GST. At 25 °C, the optimum pH for TcGST activity was pH 7, the V_max was 1535 ± 33 nmoles.min^-1. mg^-1 protein and the apparent K_m for the substrate 1-chloro-2,4-dinitrobenzene (CDNB) was 0.22 ± 0.01 mM (mean ± SD, n = 2). Antibodies in both serum and saliva from field-immune, but not nematode-naïve, sheep, recognised recombinant TcGST in enzyme-linked immunosorbent assays. The recognition of the recombinant protein by antibodies generated by exposure of sheep to the native enzyme indicates similar antigenicity of the two proteins. These findings could aid in the design of novel drugs and vaccine antigens for economically important parasites of livestock.

Unraveling oxidative stress response in the cestode parasite Echinococcus granulosus

Cystic hydatid disease (CHD) is a worldwide neglected zoonotic disease caused by Echinococcus granulosus. The parasite is well adapted to its host by producing protective molecules that modulate host immune response. An unexplored issue associated with the parasite's persistence in its host is how the organism can survive the oxidative stress resulting from parasite endogenous metabolism and host defenses. Here, we used hydrogen peroxide (H₂O₂) to induce oxidative stress in E. granulosus protoescoleces (PSCs) to identify molecular pathways and antioxidant responses during H₂O₂ exposure. Using proteomics, we identified 550 unique proteins; including 474 in H₂O₂-exposed PSCs (H-PSCs) samples and 515 in non-exposed PSCs (C-PSCs) samples. Larger amounts of antioxidant proteins, including GSTs and novel carbonyl detoxifying enzymes, such as aldo-keto reductase and carbonyl reductase, were detected after H₂O₂ exposure. Increased concentrations of caspase-3 and cathepsin-D proteases and components of the 26S proteasome were also detected in H-PSCs. Reduction of lamin-B and other caspase-substrate, such as filamin, in H-PSCs suggested that molecular events related to early apoptosis were also induced. We present data that describe proteins expressed in response to oxidative stress in a metazoan parasite, including novel antioxidant enzymes and targets with potential application to treatment and prevention of CHD.

Characterization of catalytic and non-catalytic activities of EgGST2-3, a heterodimeric glutathione transferase from Echinococcus granulosus

Glutathione transferases (GSTs) perform several catalytic and non-catalytic roles in the defense against toxicities of electrophile compounds and oxidative stress, and therefore are involved in stress-response and cell detoxification. Previously, we have provided evidence indicating that EgGST2 and EgGST3, two phylogenetically distant Echinococcus granulosus GSTs, can naturally form a heterodimeric structure (EgGST2-3). In the present work, the recombinant heterodimer GST (rEgGST2-3) is characterized. Hence, rEgGST2-3 was able to conjugate GSH to three substrates: 1-chloro-2,4-dinitrobenzene (CDNB, general substrate for GSTs), 1,2-dichloro-4-nitrobenzene (specific substrate for mammalian Mu class) and trans,trans-deca-2,4-dienal (reactive carbonyl). The canonical activity was considerably reduced by all the conventional inhibitors (cybacron blue, triphenylthin chloride and bromosulfophthalein) and by other inhibitors (ellagic acid, alizarin and chenodeoxycholic acid). Besides this, rEgGST2-3 activity was inhibited by a number of anthelmintic drugs, where the halogenated phenolic drugs (mainly bithionol and hexachlorophene) acted as stronger inhibitors, suggesting they may bind to the EgGST2-3. Moreover, rEgGST2-3 exhibited glutathione-peroxidase activity, and its specific constant (k_cat/K_M) was calculated. Finally, rEgGST2-3 displayed the ability to bind non-substrate molecules, particularly anthelmintic drugs, suggesting that ligandin activity may have potential to act as a passive protection parasite mechanism. Overall, the rEgGST2-3 behavior was shown to be both complementary and redundant to that reported for rEgGST1, another characterized GST from E. granulosus. It would be appropriate that different enzymes in the same organism do not have exactly the same functional properties to develop a better adaptation to life in the host.