Haem transporter HRG-1 is essential in the barber's pole worm and an intervention target candidate

The mrp-3 gene is involved in haem efflux and detoxification in a blood-feeding nematode

BACKGROUND

Haem is essential but toxic for metazoan organisms. Auxotrophic nematodes can acquire sufficient haem from the environment or their hosts in the meanwhile eliminate or detoxify excessive haem through tightly controlled machinery. In previous work, we reported a role of the unique transporter protein HRG-1 in the haem acquisition and homeostasis of parasitic nematodes. However, little is known about the haem efflux and detoxification via ABC transporters, particularly the multiple drug resistance proteins (MRPs).

RESULTS

Here, we further elucidate that a member of the mrp family (mrp-3) is involved in haem efflux and detoxification in a blood-feeding model gastrointestinal parasite, Haemonchus contortus. This gene is haem-responsive and dominantly expressed in the intestine and inner membrane of the hypodermis of this parasite. RNA interference of mrp-3 resulted in a disturbance of genes (e.g. hrg-1, hrg-2 and gst-1) that are known to be involved in haem homeostasis and an increased formation of haemozoin in the treated larvae and lethality in vitro, particularly when exposed to exogenous haem. Notably, the nuclear hormone receptor NHR-14 appears to be associated the regulation of mrp-3 expression for haem homeostasis and detoxification. Gene knockdown of nhr-14 and/or mrp-3 increases the sensitivity of treated larvae to exogenous haem and consequently a high death rate (> 80%).

CONCLUSIONS

These findings demonstrate that MRP-3 and the associated molecules are essential for haematophagous nematodes, suggesting novel intervention targets for these pathogens in humans and animals.

A yeast-based high-throughput screen identifies inhibitors of trypanosomatid HRG heme transporters with potent leishmanicidal and trypanocidal activity

OBJECTIVES

New drugs are required to treat neglected diseases caused by trypanosomatid parasites such as Leishmania, Trypanosoma brucei and Trypanosoma cruzi. An Achilles' heel of these parasites is their heme auxotrophy; they have an absolute dependence on scavenging this molecule from the host, and trypanosomatid HRG heme transporters (TrypHRG) play an important role in this process. As these proteins are essential for the parasites and have low similarity with their human orthologue, they have been proposed as attractive therapeutic targets. Here, we have developed two yeast-based assays that allow an inexpensive high-throughput screening of TrypHRG inhibitors within a cellular context.

METHODS

We first assessed that Leishmania major, Leishmania donovani and T. brucei HRG proteins were heterologously expressed in the digestive vacuole membrane of a mutant heme auxotrophic yeast strain. Here, TrypHRG imports hemoglobinderived heme into the cytosol, allowing mutant yeast to grow in the presence of low hemoglobin concentrations and promoting the activity of hemeproteins such as catalase, which was used as a reporter of cytosolic heme levels.

RESULTS

In the presence of a TrypHRG inhibitor, both catalase activity (test 1) and yeast growth (test 2) were diminished, being easily monitored. The assays were then tested on a pilot scale for HTS purposes using a collection of repurposing drugs and food antioxidants. Some of the TrypHRG inhibitors identified in yeast presented strong trypanocidal and leishmanicidal activity in the submicromolar range, proving the potential of this approach.

CONCLUSIONS

Cumulatively, it was shown that the inhibition bioassays developed were robust and applicable to large-scale HTS.

A secreted BPTI/Kunitz inhibitor domain-containing protein of barber's pole worm interacts with host NLRP3 inflammasome activation-associated G protein subunit to inhibit IL-1β and IL-18 maturation in vitro

Protease inhibitors are major components of excretory/secretory products released by parasitic nematodes and have been proposed to play roles in host-parasite interactions. Haemonchus contortus (the barber's pole worm) encodes for several serine protease inhibitors, and in a previous study we identified a trypsin inhibitor-like serine protease inhibitor of this blood-feeding nematode, SPI-I8, as necessary for anticoagulation. Here, we demonstrated that a bovine pancreatic trypsin inhibitor/Kunitz-type serine protease inhibitor (BPTI/Kunitz) domain-containing protein highly expressed in parasitic stages, HCON_00133150, is involved in suppressing proinflammatory cytokine production in mammalian cells. Fluorescent labelling of HCON_00133150 revealed a punctate localisation at the inner hypodermal membrane of H. contortus, an organ closely related to the excretory column. Yeast two-hybrid screening and immunoprecipitation-mass spectrometry identified that the recombinant HCON_00133150 physically interacted with a range of host proteins including the G protein subunit beta 1 of sheep (Ovis aries; OaGNB1), a negative regulator of NLRP3 inflammasome activation. Interestingly, heterologous expression of HCON_00133150 enhanced the inhibitory effect of OaGNB1 on NLRP3 inflammasome and the maturation of proinflammatory cytokines IL-1β and IL-18 in transfected cells. 1-to-1 orthologues (n = 33) of BPTI/Kunitz inhibitor domain-containing proteins were predicted in clades III, IV and V (but not clade I) parasitic nematodes. Structural (tandem BPTI/Kunitz inhibitor domains inverted into the globular reticulation) and functional (a GNB1 enhancer) characterisation of HCON_00133150 and its orthologues elucidated that these molecules might contribute to immune suppression by parasitic nematodes in animals and humans.

Notes from the Underground: Heme Homeostasis in C. elegans

Heme is an iron-containing tetrapyrrole that plays a critical role in various biological processes, including oxygen transport, electron transport, signal transduction, and catalysis. However, free heme is hydrophobic and potentially toxic to cells. Organisms have evolved specific pathways to safely transport this essential but toxic macrocycle within and between cells. The bacterivorous soil-dwelling nematode Caenorhabditis elegans is a powerful animal model for studying heme-trafficking pathways, as it lacks the ability to synthesize heme but instead relies on specialized trafficking pathways to acquire, distribute, and utilize heme. Over the past 15 years, studies on this microscopic animal have led to the identification of a number of heme-trafficking proteins, with corresponding functional homologs in vertebrates. In this review, we provide a comprehensive overview of the heme-trafficking proteins identified in C. elegans and their corresponding homologs in related organisms.

Fatty acid- and retinol-binding protein 6 does not control worm fatty acid content in Caenorhabditis elegans but might play a role in Haemonchus contortus parasitism

BACKGROUND

Nematodes have lost the ability to synthesise necessary lipids de novo and have complementally evolved the capacity to acquire fatty acids and their derivatives from a diet or host animal. Nematode-specific fatty acid- and retinol-binding protein (FAR) family is one approach that facilitates lipid acquisition, representing an Achilles heel and potential target against roundworms of socioeconomic significance. However, little is known about their detailed functional roles in either free-living or parasitic nematodes.

METHODS

A genome-wide identification and curation were performed to screen the FAR family members of Haemonchus contortus. Their transcription patterns in worms were also analysed to identify the targets. Ligand binding assay and molecular docking were conducted to verify the fatty acid binding activities of FAR proteins of interest. RNA interference (RNAi) and heterologous expression (rescuing) experiments were designed to explore the potential roles of the selected FAR protein in nematodes. Localisation of the protein was shown in sections of paraffin-embedded worms after an immunohistochemistry (IHC) assay.

RESULTS

Here, an orthologue of far-6 in the model organism Caenorhabditis elegans (Ce-far-6) was functionally characterised in a parasitic nematode, H. contortus (Hc-far-6). It is demonstrated that knockdown of Ce-far-6 gene did not affect worm fat content, reproduction, or lifespan, but decreased worm body length at an early life stage of C. elegans. In particular, the Ce-far-6 mutant associated phenotype was completely rescued by Hc-far-6, suggesting a conserved functional role. Surprisingly, there were distinct tissue expression patterns of FAR-6 in the free-living C. elegans and parasitic H. contortus. High transcriptional level of Hc-far-6 and dominant expression of FAR-6 in the intestine of the parasitic stage of H. contortus link this gene/protein to nematode parasitism.

CONCLUSIONS

These findings substantially enhance our understanding of far genes and the associated lipid biology of this important parasitic nematode at a molecular level, and the approaches established are readily applicable to the studies of far genes in a broad range of parasites.

Identification and characterisation of the haemozoin of Haemonchus contortus

BACKGROUND

Most haematophagous organisms constantly suck the host's haemoglobin, which produces toxic free haem. This toxic haem aggregation into the nontoxic crystallisation complex known as haemozoin represents one of the most important detoxification pathways in living organisms, but very little is known about the features of haemozoin in parasitic nematodes. Here, we identified and characterised the haemozoin of an economically significant blood-sucking nematode, Haemonchus contortus.

METHODS

Using electron microscopy, spectrophotometry analyses and biochemical approaches, haemozoin crystallisation was identified and characterised in parasitic fourth-stage larvae (L4s) and/or adult worms as well as L4s of in vitro culture.

RESULTS

The haemozoin was formed in intestinal lipid droplets of the parasitic L4s and adult worms. The characterisation of the haemozoin showed regularly spherical structures and had a 400-nm absorption peak. Furthermore, the haemozoin in in vitro cultured L4s was associated with the culture time and concentration of red blood cells added into the medium, and its formation could be inhibited by chloroquine-derived drugs.

CONCLUSIONS

This work provides detailed insight into the haemozoin formation of H. contortus and should have important implications for developing novel therapeutic targets against this parasite or related haematophagous organisms.