Inducible glutathione S-transferase (IrGST1) from the tick Ixodes ricinus is a haem-binding protein

Abstract

Blood-feeding parasites are inadvertently exposed to high doses of potentially cytotoxic haem liberated upon host blood digestion. Detoxification of free haem is a special challenge for ticks, which digest haemoglobin intracellularly. Ticks lack a haem catabolic mechanism, mediated by haem oxygenase, and need to dispose of vast majority of acquired haem via its accumulation in haemosomes. The knowledge of individual molecules involved in the maintenance of haem homeostasis in ticks is still rather limited. RNA-seq analyses of the Ixodes ricinus midguts from blood- and serum-fed females identified an abundant transcript of glutathione S-transferase (gst) to be substantially up-regulated in the presence of red blood cells in the diet. Here, we have determined the full sequence of this encoding gene, ir-gst1, and found that it is homologous to the delta-/epsilon-class of GSTs. Phylogenetic analyses across related chelicerates revealed that only one clear IrGST1 orthologue could be found in each available transcriptome from hard and soft ticks. These orthologues create a well-supported clade clearly separated from other ticks' or mites' delta-/epsilon-class GSTs and most likely evolved as an adaptation to tick blood-feeding life style. We have confirmed that IrGST1 expression is induced by dietary haem(oglobin), and not by iron or other components of host blood. Kinetic properties of recombinant IrGST1 were evaluated by model and natural GST substrates. The enzyme was also shown to bind haemin in vitro as evidenced by inhibition assay, VIS spectrophotometry, gel filtration, and affinity chromatography. In the native state, IrGST1 forms a dimer which further polymerises upon binding of excessive amount of haemin molecules. Due to susceptibility of ticks to haem as a signalling molecule, we speculate that the expression of IrGST1 in tick midgut functions as intracellular buffer of labile haem pool to ameliorate its cytotoxic effects upon haemoglobin intracellular hydrolysis.

Sequence identification and expression profile of seven Dermacentor marginatus glutathione S-transferase genes

“…A prior study has noted the importance of a GST that showed heme-binding ability (Perner et al 2018 ). This GST was able to reduce the pressure caused by extra free heme in the tick digestive cell by binding to heme at the cost of losing its ability of detoxification.…”

“…The same team first found that the GST was the only GST gene which significantly up-regulated in response of heme presence by transcriptome analysis of I. ricinus (Perner et al 2016 ). However, recombinant IrGST1 did not reveal anti-tick effect (Perner et al 2018 ), although it might have a potential to be used as a vaccine candidate to target tick heme metabolic pathway by co-immunization with other tick protective antigens. A vaccine study on a hook worm, Necator americanus , using one of the heme-binding GSTs as antigen to immunization elicited 32 and 39% reductions in adult hookworm burdens in dog and hamster vaccine trials, respectively (Zhan et al 2010 ).…”

“…A more recent study on a delta-class GST of Ixodes ricinus (L.) revealed that the IrGST is induced by heme in blood meal digestion (Perner et al 2016 ). The same team also found that the IrGST is a heme-binding protein which sequesters free heme to ease the capacity pressure on hemosome in the digestive cell of the midgut (Perner et al 2018 ).…”

Comparative proteomics of the vector Dermacentor reticulatus revealed differentially regulated proteins associated with pathogen transmission in response to laboratory infection with Rickettsia slovaca

“…Detoxification of free haem liberated upon host blood digestion is a special challenge for ticks. Thus, it was suggested that GST in the tick midgut might function as an intracellular buffer of labile haem pool to ameliorate its cytotoxic effects upon intracellular hydrolysis of haemoglobin [61]. The authors showed by RNA-seq analysis of I. ricinus midguts that expression of this protein was upregulated in the presence of red blood cells in the diet.…”

Host gill attachment causes blood-feeding by the salmon louse (Lepeophtheirus salmonis) chalimus larvae and alters parasite development and transcriptome

“…A GST in the hard tick Ixodes ricinus (IrGST1) (GenBank: MF984398) was also found to be elevated in the midgut of blood-fed ticks compared with serum-fed ticks [34]. Further characterization of IrGST1 showed that it was heme-inducible and the recombinant protein was able to bind heme in vitro [63]. The authors speculated that IrGST1 is important for detoxifying excess heme to avoid cytotoxicity in the tick [63].…”