Haemozoin in Schistosoma mansoni

Role Transformation of HSPA8 to Heme-peroxidase After Binding Hemin to Catalyze Heme Polymerization

Hemin, a byproduct of hemoglobin degradation, inflicts oxidative insult to cells. Following its accumulation, several proteins are recruited for heme detoxification with heme oxygenase playing the key role. Chaperones play a protective role primarily by preventing protein degradation and unfolding. They also are known to have miscellaneous secondary roles during similar situations. To discover a secondary role of chaperones during heme stress we studied the role of the chaperone HSPA8 in the detoxification of hemin. In-silico studies indicated that HSPA8 has a well-defined biophoric environment to bind hemin. Through optical difference spectroscopy, we found that HSPA8 binds hemin through its N-terminal domain with a Kd value of 5.9 ± 0.04 µM and transforms into a hemoprotein. The hemoprotein was tested for exhibiting peroxidase activity using guaiacol as substrate. The complex formed reacts with H2O2 and exhibits classical peroxidase activity with an ability to oxidize aromatic and halide substrates. HSPA8 is dose-dependently catalyzing heme polymerization through its N-terminal domain. The IR results reveal that the polymer formed exhibits structural similarities to β-hematin suggesting its covalent nature. The polymerization mechanism was tested through optical spectroscopy, spin-trap, and activity inhibition experiments. The results suggest that the polymerization occurs through a peroxidase-H2O2 system involving a one-electron transfer mechanism, and the formation of free radical and radical-radical interaction. It highlights a possible role of the HSPA8-hemin complex in exhibiting cytoprotective function during pathological conditions like malaria, sickle cell disease, etc.

Targeting SmCB1: Perspectives and Insights to Design Antischistosomal Drugs

Neglected tropical diseases (NTDs) are prevalent in tropical and subtropical countries, and schistosomiasis is among the most relevant diseases worldwide. In addition, one of the two biggest problems in developing drugs against this disease is related to drug resistance, which promotes the demand to develop new drug candidates for this purpose. Thus, one of the drug targets most explored, Schistosoma mansoni Cathepsin B1 (SmCB1 or Sm31), provides new opportunities in drug development due to its essential functions for the parasite's survival. In this way, here, the latest developments in drug design studies targeting SmCB1 were approached, focusing on the most promising analogs of nitrile, vinyl sulphones, and peptidomimetics. Thus, it was shown that despite being a disease known since ancient times, it remains prevalent throughout the world, with high mortality rates. The therapeutic arsenal of antischistosomal drugs (ASD) consists only of praziquantel, which is widely used for this purpose and has several advantages, such as efficacy and safety. However, it has limitations, such as the impossibility of acting on the immature worm and exploring new targets to overcome these limitations. SmCB1 shows its potential as a cysteine protease with a catalytic triad consisting of Cys100, His270, and Asn290. Thus, design studies of new inhibitors focus on their catalytic mechanism for designing new analogs. In fact, nitrile and sulfonamide analogs show the most significant potential in drug development, showing that these chemical groups can be better exploited in drug discovery against schistosomiasis. We hope this manuscript guides the authors in searching for promising new antischistosomal drugs.

The common bed bug Cimex lectularius synthesizes hemozoin as an essential defense against the toxic effects of heme

The common bed bug Cimex lectularius (Linnaeus 1758) is an ectoparasite that feeds preferably on human blood, being considered an important public health issue. Blood-feeding is a challenging process for hematophagous organisms, and one of the inherent risks with this kind of diet is the liberation of high doses of free heme after the digestion of hemoglobin. In order to deal with this potent cytotoxic agent, such organisms have acquired different defense mechanisms. Here, we use UV-visible spectrophotometry and infrared spectroscopy to show that C. lectularius crystalizes free heme to form the much less dangerous compound, hemozoin. According to our results, the peak of formation of hemozoin in the intestinal contents occurred 4-5 days after the blood meal, primarily in the posterior midgut. The quantification of the rate of conversion of heme to hemozoin revealed that at the end of digestion all the heme was in the form of hemozoin. Inhibition of the synthesis of hemozoin using the anti-malarial drug quinine led to an increase in both catalase activity in the intestinal epithelium and the mortality of the bed bugs, indicating that the insects were unable to cope with the oxidative stress generated by the overload of free heme. The data presented here show for the first time how C. lectularius deals with free heme, and how the process of formation of hemozoin is essential for the survival of these insects. Since resistance to insecticides is a common feature among field populations of bed bugs, there is an urgent need to develop alternative control methods. Thus, targeting the synthesis of hemozoin emerges as a possible novel strategy to fight bed bugs.

Immunological mechanisms involved in macrophage activation and polarization in schistosomiasis

Human schistosomiasis is caused by helminths of the genus Schistosoma. Macrophages play a crucial role in the immune regulation of this disease. These cells acquire different phenotypes depending on the type of stimulus they receive. M1 macrophages can be ‘classically activated’ and can display a proinflammatory phenotype. M2 or ‘alternatively activated’ macrophages are considered anti-inflammatory cells. Despite the relevance of macrophages in controlling infections, the role of the functional types of these cells in schistosomiasis is unclear. This review highlights different molecules and/or macrophage activation and polarization pathways during Schistosoma mansoni and Schistosoma japonicum infection. This review is based on original and review articles obtained through searches in major databases, including Scopus, Google Scholar, ACS, PubMed, Wiley, Scielo, Web of Science, LILACS and ScienceDirect. Our findings emphasize the importance of S. mansoni and S. japonicum antigens in macrophage polarization, as they exert immunomodulatory effects in different stages of the disease and are therefore important as therapeutic targets for schistosomiasis and in vaccine development. A combination of different antigens can provide greater protection, as it possibly stimulates an adequate immune response for an M1 or M2 profile and leads to host resistance; however, this warrants in vitro and in vivo studies.

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.

Bazedoxifene, a Postmenopausal Drug, Acts as an Antimalarial and Inhibits Hemozoin Formation

Despite a remarkable improvement in health care and continued drug discovery efforts, malaria control efforts are continuously challenged by the emergence of drug-resistant parasite strains. Given a long and risky development path of new drugs, repurposing existing drugs for the treatment of malaria is an attractive and shorter path. Tamoxifen, a selective estrogen receptor modulator (SERM) for the treatment and prevention of estrogen receptor-positive breast cancer, possesses antibacterial, antifungal, and antiparasitic activities. Hence, we assessed tamoxifen, raloxifene, and bazedoxifene, which represent the first-, second-, and third-generation SERMs, respectively, for antimalarial activity. Raloxifene and bazedoxifene inhibited the erythrocytic development of Plasmodium falciparum with submicromolar 50% inhibitory concentration (IC₅₀) values. Among the three, bazedoxifene was the most potent and also decreased P. berghei infection in female mice but not in male mice. However, bazedoxifene similarly inhibited P. falciparum growth in erythrocytes of male and female origin, which highlights the importance of sex-specific host physiology in drug efficacy. Bazedoxifene was most potent on early ring-stage parasites, and about 35% of the treated parasites did not contain hemozoin in the food vacuole. Bazedoxifene-treated parasites had almost 34% less hemozoin content than the control parasites. However, both control and bazedoxifene-treated parasites had similar hemoglobin levels, suggesting that bazedoxifene inhibits hemozoin formation and that toxicity due to accumulation of free heme could be a mechanism of its antimalarial activity. Because bazedoxifene is in clinical use and bazedoxifene-chloroquine combination shows an additive antiparasitic effect, bazedoxifene could be an adjunctive partner of currently used antimalarial regimens.

IMPORTANCE The emergence and spread of drug-resistant strains of the human malaria parasite Plasmodium falciparum has necessitated new drugs. Selective estrogen receptor modulators are in clinical use for the prevention and treatment of breast cancer and postmenopausal osteoporosis. We demonstrate that bazedoxifene, a third-generation selective estrogen receptor modulator, has potent inhibitory activity against both susceptible and drug-resistant strains of Plasmodium falciparum. It also blocked the development of Plasmodium berghei in mice. The inhibitory effect was strongest on the ring stage and resulted in the inhibition of hemozoin formation, which could be the major mechanism of bazedoxifene action. Hemozoin is a nontoxic polymer of heme, which is a by-product of hemoglobin degradation by the malaria parasite during its development within the erythrocyte. Because bazedoxifene is already in clinical use for the treatment of postmenopausal osteoporosis, our findings support repurposing of bazedoxifene as an antimalarial.

Adsorption to the surface of hemozoin crystals: Structure-based design and synthesis of new amino-phenoxazine β-hematin inhibitors

In silico adsorption of eight antimalarials that inhibit β-hematin (synthetic hemozoin) formation identified a primary binding site on the (001) face, which accommodates inhibitors via formation of predominantly π-π interactions. A good correlation (r2 = 0.64, P = 0.017) between adsorption energies and the logarithm of β-hematin inhibitory activity was found for this face. Of 53 monocyclic, bicyclic and tricyclic scaffolds, the latter yielded the most favorable adsorption energies. Five new amino-phenoxazine compounds were pursued as β-hematin inhibitors based on adsorption behaviour. The 2-substituted phenoxazines show good to moderate β-hematin inhibitory activity (< 100 μM) and Plasmodium falciparum blood stage activity against the 3D7 strain. N1,N1-diethyl-N4-(10H-phenoxazin-2-yl)pentane-1,4-diamine (P2a) is the most promising hit with IC50 values of 4.7 ± 0.6 and 0.64 ± 0.05 μM, respectively. Adsorption energies are predictive of β-hematin inhibitory activity, and thus the in silico approach is a beneficial tool for structure-based development of new non-quinoline inhibitors.

Review of Microdevices for Hemozoin-Based Malaria Detection

Despite being preventable and treatable, malaria still puts almost half of the world's population at risk. Thus, prompt, accurate and sensitive malaria diagnosis is crucial for disease control and elimination. Optical microscopy and immuno-rapid tests are the standard malaria diagnostic methods in the field. However, these are time-consuming and fail to detect low-level parasitemia. Biosensors and lab-on-a-chip devices, as reported to different applications, usually offer high sensitivity, specificity, and ease of use at the point of care. Thus, these can be explored as an alternative for malaria diagnosis. Alongside malaria infection inside the human red blood cells, parasites consume host hemoglobin generating the hemozoin crystal as a by-product. Hemozoin is produced in all parasite species either in symptomatic and asymptomatic individuals. Furthermore, hemozoin crystals are produced as the parasites invade the red blood cells and their content relates to disease progression. Hemozoin is, therefore, a unique indicator of infection, being used as a malaria biomarker. Herein, the so-far developed biosensors and lab-on-a-chip devices aiming for malaria detection by targeting hemozoin as a biomarker are reviewed and discussed to fulfil all the medical demands for malaria management towards elimination.

The Future in Sensing Technologies for Malaria Surveillance: A Review of Hemozoin-Based Diagnosis

Early and effective malaria diagnosis is vital to control the disease spread and to prevent the emergence of severe cases and death. Currently, malaria diagnosis relies on optical microscopy and immuno-rapid tests; however, these require a drop of blood, are time-consuming, or are not specific and sensitive enough for reliable detection of low-level parasitaemia. Thus, there is an urge for simpler, prompt, and accurate alternative diagnostic methods. Particularly, hemozoin has been increasingly recognized as an attractive biomarker for malaria detection. As the disease proliferates, parasites digest host hemoglobin, in the process releasing toxic haem that is detoxified into an insoluble crystal, the hemozoin, which accumulates along with infection progression. Given its magnetic, optical, and acoustic unique features, hemozoin has been explored for new label-free diagnostic methods. Thereby, herein, we review the hemozoin-based malaria detection methods and critically discuss their challenges and potential for the development of an ideal diagnostic device.

Drug Discovery and Target Identification against Schistosomiasis: a Reality Check on Progress and Future Prospects

Schistosomiasis ranks among the most important infectious diseases, with over 200 million people currently being infected and > 280,000 deaths reported annually. Chemotherapeutic treatment has relied on one drug, praziquantel, for four decades, while other drugs, such as oxamniquine and metrifonate, are no longer preferred for clinical use due to their narrow spectrum of activity - these are only active against S. mansoni and S. haematobium, respectively. Despite being cheap, safe, and effective against all schistosome species, praziquantel is ineffective against immature worms, which may lead to reinfections and treatment failure in endemic areas; a situation that necessitates repeated administration besides other limitations. Therefore, novel drugs are urgently needed to overcome this situation. In this paper, an up to date review of drug targets identified and validated against schistosomiasis while also encompassing promising clinical and preclinical candidate drugs is presented. While there are considerable efforts aimed at identifying and validating drug targets, the pipeline for new antischistosomals is dry. Moreover, the majority of compounds evaluated preclinically are not really advanced because most of them were evaluated in very small preclinical species such as mice alone. Overall, it appears that although a lot of research is going on at discovery phases, unfortunately, it does not translate to advanced preclinical and clinical evaluation.

Validation of a human-serum-based in vitro growth method for drug screening on juvenile development stages of Schistosoma mansoni

Background

Schistosomiasis affects over 200 million people worldwide but only praziquantel is available for treatment and control. Drug discovery is often based on phenotypic drug screening, involving different parasite stages retrieved from infected mice. Aiming to reduce animal use, we validated an in vitro growth method for juvenile Schistosoma mansoni for the purpose of drug sensitivity assays.

Methodology/Principal findings

We compared inter–batch variability of serum, worm size and organ development, gender distribution, and drug sensitivity between in vitro and in vivo grown worms over different life stages. In vitro developed S. mansoni in Hybridoma medium supplemented with 20% human serum were similar in size as in vivo worms until 28 days of incubation (males 1.4 ± 0.2 mm, females 1.1 ± 0.5 mm long). qPCR analysis revealed similar gender distribution both on newly transformed schistosomula and worms grown for 21 days. Worms developed in vitro and in vivo were similarly sensitive to praziquantel from 7 to 35 days of development with the exception of 21 days of development, where a slightly lower activity was observed for the in vitro grown worms (IC₅₀: 0.54 μM in vitro, 0.14 μM in vivo 72 hours post-incubation). The evaluation of five additional drugs revealed a similar sensitivity on worms developed for 21 days, with the exception of mefloquine, where we observed a 10-fold lower sensitivity on in vitro developed schistosomes when compared to in vivo grown (IC₅₀: 4.43 μM in vitro, 0.48 μM in vivo).

Conclusion

A large number of juvenile S. mansoni worms can be grown in vitro, which show similar drug sensitivity, gender distribution, size and morphology as the worms recovered from rodents, supporting the use of this method in drug screening efforts.

Schistosomiasis is a water-borne disease affecting over 200 million people worldwide and praziquantel is the only drug available for treatment. Although this drug is effective on all Schistosoma species on the adult stage of development, its activity is only partial against early infection stages. In addition, this has been the only drug in use for over 40 years, a situation that poses selective pressure towards resistant worms and highlights the urgent need of new drugs. The discovery and development of new drugs is often based on drug screenings, performed on worms collected from infected mammals. In line with animal welfare recommendations and aiming to increase throughput and reduce the assay costs, we validated a recently established in vitro method to grow juvenile S. mansoni as a drug screening tool. The in vitro established worms have similar gender distribution, size and drug sensitivity as worms collected from mice.

Host-Parasite Interaction between Parasitic Cymothoid Ceratothoa oestroides and Its Host, Farmed European Sea Bass (Dicentrarchus labrax)

Parasitic isopod Ceratothoa oestroides (Cymothoidea, Isopoda) is a common and generalist buccal cavity-dweller in marine fish, recognised for its detrimental effect in fingerling and juvenile farmed European sea bass (Dicentrarchus labrax). Although distributed throughout the Mediterranean, the isopod provokes acute outbreaks mainly limited to particular endemic areas in Croatia (Adriatic Sea) and Greece (Aegean Sea). While numerous studies have previously evidenced its gross effect on farmed fish (i.e. decreased condition index, slower growth rate, lethargy and mortality), details on the host-parasite interaction are still lacking. Therefore, using a multimethodological approach, we closely examined the structure and appearance of isopod body parts acting in the attachment and feeding (stereomicroscopy, scanning and transmission electron microscopy), and the extent of host tissues damage (histology, immunohistochemistry, micro-computational tomography) induced by parasitation. Interestingly, while hematophagous nature of the parasite has been previously postulated we found no unambiguous data to support this; we observed host tissues fragmentation and extensive hyperplasia at the parasitation site, and no structures indicative of heme detoxifying mechanisms in the parasite gut, or other traces of a blood meal. The bacterial biofilm covering C. oestroides mouthparts and pereopods suggests that the isopod may play a role in conveying secondary pathogens to the infected host, or alternatively, it serves the parasite in normal interaction with its environment.

Effect of a newly synthesized quinoline-based compound (PPQ-8) on murine schistosomiasis mansoni

Schistosomiasis represents a public health problem and praziquantel is the only drug used for treatment of all forms of the disease. Thus, the development of new anti-schistosomal agents is of utmost importance to increase the effectiveness, reduce side effects and delay the emergence of resistance. The present study was conducted to report the therapeutic efficacy of PPQ-8, a new synthetic quinoline-based compound against Schistosoma mansoni. Mice were treated with PPQ-8 at day 49 post infection using two treatment regimens (20 and 40 mg/kg). Significant reductions were recorded in hepatic (62.9% and 83.6%) and intestinal tissue egg load (57.4% and 73.5%), granuloma count (75.4% and 89.1%) and diameter (26.2% and 47.3%), in response to the drug regimens, respectively. In addition, both treatment regimens induced significant decrease in liver (23.3% and 32.8%) and spleen (37.5% and 45.3%) indices. Also, there were significant reductions in mature ova, total worm and female count, which were more prominent with the higher dose. The reduction in the level of nitric oxide in the liver by both therapeutic regimens to 22.5% and 47.2% indicates the anti-oxidant activity of PPQ-8. Bright field microscopic examination of worms recovered from infected and PPQ-8-treated mice showed nearly empty intestinal caeca with no observable changes in the tegument. Our findings hold promise for the development of a novel anti-schistosomal drug using PPQ-8, but further in vitro and in vivo studies are needed to elucidate the possible mechanism/s of action and to study the effect of PPQ-8 on other human schistosomes.

Long-term ethanol intake causes morphological changes in Schistosoma mansoni adult worms in mice

Schistosoma mansoni adult worms are extensively challenged by reactive oxygen species from intrinsic sources. However, the effects of extrinsic sources such as ethanol have not been looked at in schistosomes. We examined adult worms recovered from ethanol-consuming mice by light (LM), confocal (CM) and scanning electron microscopy (SEM) to address this question. Schistosomiasis-infected mice were orally gavaged with 18% (v/v) ethanol from 35 to 63 days post-infection, when they were euthanized. CM examination revealed reduced germ cells density (-36%, p = 0.0001) and sperm density (-58%, p = 0.0001) in testicular lobes, and immature cells in seminal vesicle compared to unexposed control worms. Female worms showed reduced density of vitellin glands (-71%, p = 0.0001), maturation of oocytes (-7%, p = 0.0071) and reduced spermatozoa density (-23%, p = 0.0002) within the seminal receptacle. SEM revealed remarkable damages in male's tegument, including tubercles flattening, tegumental peeling and erosive lesions. Given that lipids are present in reproductive system and tegument, our results suggest that phenotypic changes are due to ethanol-induced lipid peroxidation. To the best of our knowledge, this is the first report revealing the biological action of ethanol intake on adult schistosomes in vivo.

A proteomic comparison of excretion/secretion products in Fasciola hepatica newly excysted juveniles (NEJ) derived from Lymnaea viatrix or Pseudosuccinea columella

The characteristics of parasitic infections are often tied to host behavior. Although most studies have investigated definitive hosts, intermediate hosts can also play a role in shaping the distribution and accumulation of parasites. This is particularly relevant in larval stages, where intermediate host's behavior could potentially interfere in the molecules secreted by the parasite into the next host during infection. To investigate this hypothesis, we used a proteomic approach to analyze excretion/secretion products (ESP) from Fasciola hepatica newly excysted juveniles (NEJ) derived from two intermediate host species, Lymnaea viatrix and Pseudosuccinea columella. The two analyzed proteomes showed differences in identity, abundance, and functional classification of the proteins. This observation could be due to differences in the biological cycle of the parasite in the host, environmental aspects, and/or host-dependent factors. Categories such as protein modification machinery, protease inhibitors, signal transduction, and cysteine-rich proteins showed different abundance between samples. More specifically, differences in abundance of individual proteins such as peptidyl-prolyl cis-trans isomerase, thioredoxin, cathepsin B, cathepsin L, and Kunitz-type inhibitors were identified. Based on the differences identified between NEJ ESP samples, we can conclude that the intermediate host is a factor influencing the proteomic profile of ESP in F. hepatica.

Schistosome Egg Migration: Mechanisms, Pathogenesis and Host Immune Responses

Many parasitic worms possess complex and intriguing life cycles, and schistosomes are no exception. To exit the human body and progress to their successive snail host, Schistosoma mansoni eggs must migrate from the mesenteric vessels, across the intestinal wall and into the feces. This process is complex and not always successful. A vast proportion of eggs fail to leave their definite host, instead becoming lodged within intestinal or hepatic tissue, where they can evoke potentially life-threatening pathology. Thus, to maximize the likelihood of successful egg passage whilst minimizing host pathology, intriguing egg exit strategies have evolved. Notably, schistosomes actively exert counter-inflammatory influences on the host immune system, discreetly compromise endothelial and epithelial barriers, and modulate granuloma formation around transiting eggs, which is instrumental to their migration. In this review, we discuss new developments in our understanding of schistosome egg migration, with an emphasis on S. mansoni and the intestine, and outline the host-parasite interactions that are thought to make this process possible. In addition, we explore the potential immune implications of egg penetration and discuss the long-term consequences for the host of unsuccessful egg transit, such as fibrosis, co-infection and cancer development.

Heme crystallization in a Chagas disease vector acts as a redox-protective mechanism to allow insect reproduction and parasite infection

Heme crystallization as hemozoin represents the dominant mechanism of heme disposal in blood feeding triatomine insect vectors of the Chagas disease. The absence of drugs or vaccine for the Chagas disease causative agent, the parasite Trypanosoma cruzi, makes the control of vector population the best available strategy to limit disease spread. Although heme and redox homeostasis regulation is critical for both triatomine insects and T. cruzi, the physiological relevance of hemozoin for these organisms remains unknown. Here, we demonstrate that selective blockage of heme crystallization in vivo by the antimalarial drug quinidine, caused systemic heme overload and redox imbalance in distinct insect tissues, assessed by spectrophotometry and fluorescence microscopy. Quinidine treatment activated compensatory defensive heme-scavenging mechanisms to cope with excessive heme, as revealed by biochemical hemolymph analyses, and fat body gene expression. Importantly, egg production, oviposition, and total T. cruzi parasite counts in R. prolixus were significantly reduced by quinidine treatment. These effects were reverted by oral supplementation with the major insect antioxidant urate. Altogether, these data underscore the importance of heme crystallization as the main redox regulator for triatomine vectors, indicating the dual role of hemozoin as a protective mechanism to allow insect fertility, and T. cruzi life-cycle. Thus, targeting heme crystallization in insect vectors represents an innovative way for Chagas disease control, by reducing simultaneously triatomine reproduction and T. cruzi transmission.

Chagas disease is a fatal illness caused by Trypanosoma cruzi parasites, which are transmitted by blood sucking triatomine insect vectors. Although blood is a natural food source for these insects, its digestion releases toxic products, which poses a dietary challenge for both triatomine insects and trypanosomes. To overcome this, triatomines eliminate these toxic blood products by a unique process of heme crystallization into hemozoin that take place in their digestive tract. Here we describe that this detoxification process represents the major mechanism for redox balance control, and is necessary to allow triatomine insect reproduction, and Trypanosoma cruzi infection. Disruption of heme crystallization in triatomine insects thus represents a new venue for Chagas disease control, by targeting at the same time insect reproduction and parasite transmission.

Modulation of mitochondrial metabolism as a biochemical trait in blood feeding organisms: the redox vampire hypothesis redux

Hematophagous organisms undergo remarkable metabolic changes during the blood digestion process, increasing fermentative glucose metabolism, and reducing respiratory rates, both consequence of functional mitochondrial remodeling. Here, we review the pathways involved in energy metabolism and mitochondrial functionality in a comparative framework across different hematophagous species, and consider how these processes regulate redox homeostasis during blood digestion. The trend across distinct species indicate that a switch in energy metabolism might represent an important defensive mechanism to avoid the potential harmful interaction of oxidants generated from aerobic energy metabolism with products derived from blood digestion. Indeed, in insect vectors, blood feeding transiently reduces respiratory rates and oxidant production, irrespective of tissue and insect model. On the other hand, a different scenario is observed in several unrelated parasite species when exposed to blood digestion products, as respiratory rates reduce and mitochondrial oxidant production increase. The emerging picture indicates that re-wiring of energy metabolism, through reduced mitochondrial function, culminates in improved tolerance to redox insults and seems to represent a key step for hematophagous organisms to cope with the overwhelming and potentially toxic blood meal.

Design, synthesis and bioactivities of phenithionate analogues or derivatives for anti-schistosomiasis

A novel series of phenithionate analogues or derivatives were designed and synthesized using phenithionate as the lead compound, and their bioactivities were studied. Their structures were confirmed by ¹H NMR, ¹³C NMR, HR-ESI-MS, and elemental analysis, respectively. The results of in vitro inhibitory activity measurement proved that compounds 5a, 5c, 5g, 5i, 5m and 5o had a better inhibitory effect on larva and imago schistosoma. Among them, the inhibitory activity of compound 5i for larva schistosoma was IC₅₀ = 5.21 ± 0.04 μg mL^-1, and for imago schistosoma it was IC₅₀ = 6.35 ± 0.08 μg mL^-1. Moreover, the experimental results of in vivo anti-schistosomiasis activity measurement showed that they had good anti-schistosomiasis activity. Therefore, these compounds had better drugability.

Antischistosomal Activity of Pyrido[1,2-a]benzimidazole Derivatives and Correlation with Inhibition of β-Hematin Formation

The extensive use of praziquantel against schistosomiasis raises concerns about drug resistance. New therapeutic alternatives targeting critical pathways within the parasite are therefore urgently needed. Hemozoin formation in Schistosoma presents one such target. We assessed the in vitro antischistosomal activity of pyrido[1,2-a]benzimidazoles (PBIs) and investigated correlations with their ability to inhibit β-hematin formation. We further evaluated the in vivo efficacy of representative compounds in experimental mice and conducted pharmacokinetic analysis on the most potent. At 10 μM, 48/57 compounds resulted in >70% mortality of newly transformed schistosomula, whereas 37 of these maintained >60% mortality of adult S. mansoni. No correlations were observed between β-hematin inhibitory and antischistosomal activities against both larval and adult parasites, suggesting possible presence of other target(s) or a mode of inhibition of crystal formation that is not adequately modeled by the assay. The most active compound in vivo showed 58.7 and 61.3% total and female worm burden reduction, respectively. Pharmacokinetic analysis suggested solubility-limited absorption and high hepatic clearance as possible contributors to the modest efficacy despite good in vitro activity. The PBIs evaluated in this report thus merit further optimization to improve their efficacy and to elucidate their possible mode of action.

Novel carbazole aminoalcohols as inhibitors of β-hematin formation: Antiplasmodial and antischistosomal activities

Malaria and schistosomiasis are two of the most socioeconomically devastating parasitic diseases in tropical and subtropical countries. Since current chemotherapeutic options are limited and defective, there is an urgent need to develop novel antiplasmodials and antischistosomals. Hemozoin is a disposal product formed from the hemoglobin digestion by some blood-feeding parasites. Hemozoin formation is an essential process for the parasites to detoxify free heme, which is a reliable therapeutic target for identifying novel antiparasitic agents. A series of novel carbazole aminoalcohols were designed and synthesized as potential antiplasmodial and antischistosomal agents, and several compounds showed potent in vitro activities against Plasmodium falciparum 3D7 and Dd2 strains and adult and juvenile Schistosoma japonicum. Investigations on the dual antiparasitic mechanisms showed the correlation between inhibitory activity of β-hematin formation and antiparasitic activity. Inhibiting hemozoin formation was identified as one of the mechanisms of action of carbazole aminoalcohols. Compound 7 displayed potent antiplasmodial (Pf3D7 IC₅₀ = 0.248 μM, PfDd2 IC₅₀ = 0.091 μM) and antischistosomal activities (100% mortality of adult and juvenile schistosomes at 5 and 10 μg/mL, respectively) and exhibited low cytotoxicity (CC₅₀ = 7.931 μM), which could be considered as a promising lead for further investigation. Stoichiometry determination and molecular docking studies were also performed to explain the mode of action of compound 7.

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Carbazole aminoalcohol was confirmed as a novel antiplasmodial and antischistosomal scaffold.

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The mechanism of action relied on β-hematin formation inhibition.

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The carbazole aminoalcohols interacted with hematin through forming a 1:1 complex.

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Compound 7 showed potent antiplasmodial ability (Pf3D7 IC₅₀ = 0.248 μM, PfDd2 IC₅₀ = 0.091 μM).

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In vitro antischistosomal effect of 7 meets the WHO's criterion of “hit” for schistosomiasis control.

Schistosoma hemozoin and its possible roles

More than 95years ago Schistosoma pigment had been deemed as a degradation product of haemoglobin. Until the 1950s, scientists initiated to pay attention to understand the hematophagous habit of schistosomes, and to study the degradation of haemoglobin as well as the formation of hemozoin inside the gut of the worms. For a long time, the formation of hemozoin in both Plasmodium and in Schistosoma was considered to be the major route of heme detoxification, and hemozoin served a role in waste disposal. At the beginning of this century, the chemical structure of Schistosoma pigment was confirmed to be identical to that of malarial pigment (hemozoin) and its synthetic analogue, β-hematin. Since then, studies on Schistosoma hemozoin have been investigated by some workers and the results showed that Schistosoma hemozoin may play important roles in pathogenicity, immune modulation, iron supply for egg formation, and interaction with some anti-schistosomal drugs. In this review, we briefly review and discuss the hematophagous habit of schistosomes, degradation of haemoglobin, formation of hemozoin in the worm gut, and possible roles of hemozoin.

Sexual Preferences in Nutrient Utilization Regulate Oxygen Consumption and Reactive Oxygen Species Generation in Schistosoma mansoni: Potential Implications for Parasite Redox Biology

Schistosoma mansoni, one of the causative agents of human schistosomiasis, has a unique antioxidant network that is key to parasite survival and a valuable chemotherapeutic target. The ability to detoxify and tolerate reactive oxygen species increases along S. mansoni development in the vertebrate host, suggesting that adult parasites are more exposed to redox challenges than young stages. Indeed, adult parasites are exposed to multiple redox insults generated from blood digestion, activated immune cells, and, potentially, from their own parasitic aerobic metabolism. However, it remains unknown how reactive oxygen species are produced by S. mansoni metabolism, as well as their biological effects on adult worms. Here, we assessed the contribution of nutrients and parasite gender to oxygen utilization pathways, and reactive oxygen species generation in whole unpaired adult S. mansoni worms. We also determined the susceptibilities of both parasite sexes to a pro-oxidant challenge. We observed that glutamine and serum importantly contribute to both respiratory and non-respiratory oxygen utilization in adult worms, but with different proportions among parasite sexes. Analyses of oxygen utilization pathways revealed that respiratory rates were high in male worms, which contrast with high non-respiratory rates in females, regardless nutritional sources. Interestingly, mitochondrial complex I-III activity was higher than complex IV specifically in females. We also observed sexual preferences in substrate utilization to sustain hydrogen peroxide production towards glucose in females, and glutamine in male worms. Despite strikingly high oxidant levels and hydrogen peroxide production rates, female worms were more resistant to a pro-oxidant challenge than male parasites. The data presented here indicate that sexual preferences in nutrient metabolism in adult S. mansoni worms regulate oxygen utilization and reactive oxygen species production, which may differently contribute to redox biology among parasite sexes.

Detergent-Mediated Formation of β-Hematin: Heme Crystallization Promoted by Detergents Implicates Nanostructure Formation for Use as a Biological Mimic

Hemozoin is a unique biomineral that results from the sequestration of toxic free heme liberated as a consequence of hemoglobin degradation in the malaria parasite. Synthetic neutral lipid droplets (SNLDs) and phospholipids were previously shown to support the rapid formation of β-hematin, abiological hemozoin, under physiologically relevant pH and temperature, though the mechanism by which heme crystallization occurs remains unclear. Detergents are particularly interesting as a template because they are amphiphilic molecules that spontaneously organize into nanostructures and have been previously shown to mediate β-hematin formation. Here, 11 detergents were investigated to elucidate the physicochemical properties that best recapitulate crystal formation in the parasite. A strong correlation between the detergent's molecular structure and the corresponding kinetics of β-hematin formation was observed, where higher molecular weight polar chains promoted faster reactions. The larger hydrophilic chains correlated to the detergent's ability to rapidly sequester heme into the lipophilic core, allowing for crystal nucleation to occur. The data presented here suggest that detergent nanostructures promote β-hematin formation in a similar manner to SNLDs and phospholipids. Through understanding mediator properties that promote optimal crystal formation, we are able to establish an in vitro assay to probe this drug target pathway.

Acquisition of exogenous haem is essential for tick reproduction

Haem and iron homeostasis in most eukaryotic cells is based on a balanced flux between haem biosynthesis and haem oxygenase-mediated degradation. Unlike most eukaryotes, ticks possess an incomplete haem biosynthetic pathway and, together with other (non-haematophagous) mites, lack a gene encoding haem oxygenase. We demonstrated, by membrane feeding, that ticks do not acquire bioavailable iron from haemoglobin-derived haem. However, ticks require dietary haemoglobin as an exogenous source of haem since, feeding with haemoglobin-depleted serum led to aborted embryogenesis. Supplementation of serum with haemoglobin fully restored egg fertility. Surprisingly, haemoglobin could be completely substituted by serum proteins for the provision of amino-acids in vitellogenesis. Acquired haem is distributed by haemolymph carrier protein(s) and sequestered by vitellins in the developing oocytes. This work extends, substantially, current knowledge of haem auxotrophy in ticks and underscores the importance of haem and iron metabolism as rational targets for anti-tick interventions.

DOI:http://dx.doi.org/10.7554/eLife.12318.001

Ticks are small blood-feeding parasites that transmit a range of diseases through their bites, including Lyme disease and encephalitis in humans. Like other blood-feeders, ticks acquire essential nutrients from their host in order to develop and reproduce.

Iron and haem (the iron-containing part of haemoglobin) are essential for the metabolism of every breathing animal on Earth. Most organisms obtain iron by degrading haem and, reciprocally, most of the iron in cells is used to make haem. However, an initial search of existing genome databases revealed that ticks lack the genes required to make the proteins that make and degrade haem.

Perner et al. wanted to find out if ticks can steal haem from the host and use it for their own development. To achieve this, Perner et al. exploited a method of tick membrane feeding that simulates natural feeding on a host by using a silicone imitation of a skin and cow smell extracts (“l´odeur de vache”). Ticks were fed either a haemoglobin-rich (whole blood) or a haemoglobin-poor (serum) diet. This experiment revealed that ticks can develop normally without haemoglobin, but female ticks fed a haemoglobin-poor diet lay sterile eggs out of which no offspring can hatch.

Further investigation showed that haemoglobin is vitally important as a source of haem but not as a source of the amino acids needed to produce the vitellin proteins that nourish embryos. As ticks are not armed with the ability to degrade haem, they do not acquire iron from the host haem but rather from a serum transferrin, a major iron transporter protein found in mammalian blood. Further experiments revealed that ticks have evolved proteins that can transport and store haem and so make the obtained haem available across the whole tick body.

Overall, Perner et al.’s findings suggest that targeting the mechanisms by which ticks metabolise haem and iron could lead to the design of new “anti-tick” strategies.

DOI:http://dx.doi.org/10.7554/eLife.12318.002

Hemozoin is a product of heme detoxification in the gut of the most medically important species of the family Opisthorchiidae

Many species of trematodes such as Schistosoma spp., Fasciola hepatica and Echinostoma trivolvis are blood-feeding parasites. Nevertheless, there is no consensus on the feeding habits of the family Opisthorchiidae (Opisthorchis felineus, Opisthorchis viverrini and Clonorchis sinensis). Previously, histological studies of O. felineus and C. sinensis revealed some dark stained material in their gut lumen. In this study we conducted a comprehensive analysis of the gut contents of three members of the family Opisthorchiidae (O. felineus, O. viverrini and C. sinensis). Using transmission electron microscopy, we demonstrated for the first known time the presence of disintegrating blood cells in the gut of O. felineus as well as electron-dense crystals in the gut of O. felineus and C. sinensis. Electron energy loss spectroscopy revealed iron atoms in these crystals, and mass spectrometry of the purified pigment demonstrated the presence of heme. Fourier-transform infrared spectroscopy identified the signature peaks of the common iron-carboxylate bond characteristic in crystals isolated from O. felineus and C. sinensis. Scanning electron microscopy showed layered ovoid crystals of various sizes from 50 nm to 2 μm. Morphological, chemical and paramagnetic properties of these crystals were similar to those of hemozoin from Schistosoma mansoni. Crystal formation occurs on the surface of lipid droplets in O. felineus and C. sinensis guts. Our results suggest that the diet of O. felineus and C. sinensis includes blood. Detoxification of the free heme produced during the digestion proceeds via formation of insoluble crystals that contain iron and heme dimers, i.e. crystals of hemozoin. Furthermore, we believe that biocrystallisation of hemozoin takes place on the surface of the lipid droplets, similar to S. mansoni. Hemozoin was not detected in the closely related species O. viverrini.

Insights into the initial stages of lipid-mediated haemozoin nucleation

Lipid-mediated haemozoin nucleation, as probed by molecular dynamics, proceeds via aggregation of ferrihaem π–π dimers at a lipid–aqueous interface.

Hemozoin "knobs" in Opisthorchis felineus infected liver

Background

Hemozoin is the pigment produced by some blood-feeding parasites. It demonstrates high diagnostic and therapeutic potential. In this work the formation of co-called hemozoin “knobs” – the bile duct ectasia filled up by hemozoin pigment - in Opisthorhis felineus infected hamster liver has been observed.

Methods

The O. felineus infected liver was examined by histological analysis and magnetic resonance imaging (MRI). The pigment hemozoin was identified by Fourier transform infrared spectroscopy and high resolution electrospray ionization mass spectrometry analysis. Hemozoin crystals were characterised by high resolution transmission electron microscopy.

Results

Hemozoin crystals produced by O. felineus have average length 403 nm and the length-to-width ratio equals 2.0. The regurgitation of hemozoin from parasitic fluke during infection leads to formation of bile duct ectasia. The active release of hemozoin from O. felineus during in vitro incubation has also been evidenced. It has been shown that the hemozoin knobs can be detected by magnetic resonance imaging.

Conclusions

In the paper for the first time the characterisation of hemozoin pigment extracted from liver fluke O. felineus has been conducted. The role of hemozoin in the modification of immune response by opisthorchiasis is assumed.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-1061-5) contains supplementary material, which is available to authorized users.

Significance of higher drug concentration in erythrocytes of mice infected with Schistosoma japonicum and treated orally with mefloquine at single doses

The purpose of the present study is to understand the pharmacokinetic feature of mefloquine measured by erythrocytes and plasma in Schistosoma japonicum (S. j.)-infected mice and non-infected mice after oral administration of the drug at single doses. A high-performance liquid chromatography (HPLC) method was used to measure the plasma and erythrocyte concentrations of mefloquine at varying intervals posttreatment. Our results demonstrated that in non-infected mice treated orally with mefloquine at an ineffective dose of 50 mg/kg or effective dose of 200 mg/kg for 2-72 h, the erythrocyte-to-plasma ratios of mefloquine were 5.8-11.2 or 2-14.2. On the other hand, in S. j.-infected mice treated with the same single doses of the drug, the erythrocyte and plasma drug concentration ratios were 3.1-4.6 or 2.9-8.5, manifesting that either in infected mice or in non-infected mice that received oral mefloquine resulted in higher concentration of mefloquine in erythrocytes than that in plasma. Unexpectedly, under oral administration of mefloquine at a higher single dose of 200 mg/kg, the pharmacokinetic parameter C max values for plasma from S. j.-infected and non-infected mice were 1.6 ± 0.3 and 2.0 ± 0.4 μg/mL, respectively, which were below the determined in vitro LC50 (50 % lethal concentration) value of 4.93 μg/mL. Therefore, the plasma concentration of mefloquine may display a little effect against schistosomes during the treatment. Although the values of T 1/2 and AUC0-∞ for erythrocytes were significantly longer and higher in infected mice than those of corresponding non-infect mice that received the same single mefloqine dose of 50 mg/kg, the C max value was only 2.6 ± 0.4 μg/mL lower than the determined in vitro LC50, which may explain why this low single dose is ineffective against schistosomes in vivo. After administration of higher mefloquine dose of 200 mg/kg, the C max value for erythrocytes in infected mice was 30 % (7.4 ± 0.7 versus 10.7 ± 2.7 μg/mL) lower than that in the corresponding non-infected mice, but its level was above the determined in vitro LC95 (95 % lethal concentration) value of 6.12 μg/mL. Meanwhile, longer T 1/2 value of 159.2 ± 129.3 h in infected mice led to significant increase in AUC0-∞ value (1969.3 ± 1057.7 vs 486.4 ± 53.0 μg/mL·h), relative to corresponding non-infected mice. In addition, the mean residence time (MRT0-∞) in infected mice was also significantly longer than that in non-infected mice. All these results may beneficial for the treatment. According to the results, we suggest that higher ratios of mefloquine concentration in erythrocytes to plasma may offer a way to transport mefloquine to the worm gut through ingestion of erythrocytes by the worms, where the gut is the site for displaying the effect by mefloquine.

Haem uptake is essential for egg production in the haematophagous blood fluke of humans, Schistosoma mansoni

Schistosomes ingest host erythrocytes, liberating large quantities of haem. Despite its toxicity, haem is an essential factor for numerous biological reactions, and may be an important iron source for these helminths. We used a fluorescence haem analogue, palladium mesoporphyrin, to investigate pathways of haem acquisition, and showed that palladium mesoporphyrin accumulates in the vitellaria (eggshell precursor glands) and ovary of female Schistosoma mansoni. Furthermore, incubation of adult females in 10-100 μm cyclosporin A (IC50 = 2.3 μm) inhibits the uptake of palladium mesoporphyrin to these tissues, with tenfold reductions in fluorescence intensity of the ovary. In vitro exposure to cyclosporin A resulted in significant perturbation of egg production, reducing egg output from 34 eggs per female to 5.7 eggs per female over the incubation period, and retardation of egg development. We characterized a S. mansoni homologue of the haem-responsive genes of Caenorhabditis elegans. The gene (Smhrg-1) encodes a protein with a molecular weight of approximately 17 kDa. SmHRG-1 was able to rescue growth in haem transport-deficient HEM1Δ yeast. Transcriptional suppression of Smhrg-1 in adult S. mansoni worms resulted in significant delay in egg maturation, with 47% of eggs from transcriptionally suppressed worms being identified as immature compared with only 27% of eggs laid by control worms treated with firefly luciferase. Our findings indicate the presence of transmembrane haem transporters in schistosomes, with a high abundance of these molecules being present in tissues involved in oogenesis.

Synthesis and evaluation of 1,4-naphthoquinone ether derivatives as SmTGR inhibitors and new anti-schistosomal drugs

Investigations regarding the chemistry and mechanism of action of 2-methyl-1,4-naphthoquinone (or menadione) derivatives revealed 3-phenoxymethyl menadiones as a novel anti-schistosomal chemical series. These newly synthesized compounds (1-7) and their difluoromethylmenadione counterparts (8, 9) were found to be potent and specific inhibitors of Schistosoma mansoni thioredoxin-glutathione reductase (SmTGR), which has been identified as a potential target for anti-schistosomal drugs. The compounds were also tested in enzymic assays using both human flavoenzymes, i.e. glutathione reductase (hGR) and selenium-dependent human thioredoxin reductase (hTrxR), to evaluate the specificity of the inhibition. Structure-activity relationships as well as physico- and electro-chemical studies showed a high potential for the 3-phenoxymethyl menadiones to inhibit SmTGR selectively compared to hGR and hTrxR enzymes, in particular those bearing an α-fluorophenol methyl ether moiety, which improves anti-schistosomal action. Furthermore, the (substituted phenoxy)methyl menadione derivative (7) displayed time-dependent SmTGR inactivation, correlating with unproductive NADPH-dependent redox cycling of SmTGR, and potent anti-schistosomal action in worms cultured ex vivo. In contrast, the difluoromethylmenadione analog 9, which inactivates SmTGR through an irreversible non-consuming NADPH-dependent process, has little killing effect in worms cultured ex vivo. Despite ex vivo activity, none of the compounds tested was active in vivo, suggesting that the limited bioavailability may compromise compound activity. Therefore, future studies will be directed toward improving pharmacokinetic properties and bioavailability.

Efficacy of Synriam™, a new antimalarial combination of OZ277 and piperaquine, against different developmental stages of Schistosoma mansoni

Control of schistosomiasis relies on a single drug, praziquantel (PZQ). Given the rising concerns about the potential emergence of PZQ-resistant strains, it has now become necessary to search for novel therapeutics. However, the current pace for anti-schistosomal drug discovery is slow; hence, repositioning of existing approved drugs can offer a safe, rapid and cost-effective solution. The anti-malarial synthetic artemisinin-derivatives trioxolanes demonstrated anti-schistosomal efficacies against the three major species infecting humans and, unlike PZQ, showed activities against both juvenile and adult worm stages. The 1,2,4-trioxolane/OZ277 (arterolane maleate) in combination with a partner drug: piperaquine phosphate was recently developed as an anti-malarial drug and manufactured by Ranbaxy (India) as Synriam™ (SYN). Herein, the in vivo activities of SYN were investigated in a mouse model of Schistosoma mansoni (S. mansoni), compared to PZQ. We show that a single fixed dose of 240mg/kg SYN (40mg/kg arterolane and 200mg/kg piperaqine) induced significant protective effects in mice, in terms of reduction in worm and tissue egg burdens, which were evident against all schistosome developmental stages. Extensive alterations in the tegument and subtegumental tissues of SYN-exposed worms were revealed by both scanning and transmission electron microscopes. Progressive decrease in worm activity and occurrence of death were noticed in vitro upon exposure to the drug - more pronounced in the presence of haemin. This report provides the first evidence of the efficacy of a combination of 1,2,4-trioxolane and piperaquine against S. mansoni in mice. Being effective against young stages, SYN could be used to prevent early Schistosoma infection.

Natural products as leads in schistosome drug discovery

Schistosomiasis is a neglected parasitic tropical disease that claims around 200,000 human lives every year. Praziquantel (PZQ), the only drug recommended by the World Health Organization for the treatment and control of human schistosomiasis, is now facing the threat of drug resistance, indicating the urgent need for new effective compounds to treat this disease. Therefore, globally, there is renewed interest in natural products (NPs) as a starting point for drug discovery and development for schistosomiasis. Recent advances in genomics, proteomics, bioinformatics, and cheminformatics have brought about unprecedented opportunities for the rapid and more cost-effective discovery of new bioactive compounds against neglected tropical diseases. This review highlights the main contributions that NP drug discovery and development have made in the treatment of schistosomiasis and it discusses how integration with virtual screening (VS) strategies may contribute to accelerating the development of new schistosomidal leads, especially through the identification of unexplored, biologically active chemical scaffolds and structural optimization of NPs with previously established activity.

Evaluation of a novel magneto-optical method for the detection of malaria parasites

Improving the efficiency of malaria diagnosis is one of the main goals of current malaria research. We have recently developed a magneto-optical (MO) method which allows high-sensitivity detection of malaria pigment (hemozoin crystals) in blood via the magnetically induced rotational motion of the hemozoin crystals. Here, we evaluate this MO technique for the detection of Plasmodium falciparum in infected erythrocytes using in-vitro parasite cultures covering the entire intraerythrocytic life cycle. Our novel method detected parasite densities as low as ∼ 40 parasites per microliter of blood (0.0008% parasitemia) at the ring stage and less than 10 parasites/µL (0.0002% parasitemia) in the case of the later stages. These limits of detection, corresponding to approximately 20 pg/µL of hemozoin produced by the parasites, exceed that of rapid diagnostic tests and compete with the threshold achievable by light microscopic observation of blood smears. The MO diagnosis requires no special training of the operator or specific reagents for parasite detection, except for an inexpensive lysis solution to release intracellular hemozoin. The devices can be designed to a portable format for clinical and in-field tests. Besides testing its diagnostic performance, we also applied the MO technique to investigate the change in hemozoin concentration during parasite maturation. Our preliminary data indicate that this method may offer an efficient tool to determine the amount of hemozoin produced by the different parasite stages in synchronized cultures. Hence, it could eventually be used for testing the susceptibility of parasites to antimalarial drugs.

Mefloquine in combination with hemin causes severe damage to adult Schistosoma japonicum in vitro

In order to explore the interaction of mefloquine with hemin against adult Schistosoma japonicum in vitro, the 50% and 95% lethal concentration (LC50 and LC95) of mefloquine and hemin against schistosomes, some factors, such as other iron providing agents, iron chelaters, zinc protoporphyrin-IX, and biological relevant reductants, that might impact on antischistosomal activity induced by interaction of mefloquine with hemin, and preliminary analysis of chemical interaction of both compounds were undertaken. The LC50 and LC95 of mefloquine and hemin alone against schistosomes were determined to be 6.5μg/ml and 7.8μg/ml as well as 232μg/ml and 355μg/ml, respectively. The LC50 and LC95 of mefloquine in the presence of hemin 100μg/ml was 0.24μg/ml and 0.59μg/ml, respectively. On the other hand the LC50 and LC95 of hemin in the presence of mefloquine 1μg/ml was 23.2μg/ml and 77.2μg/ml, respectively. Meanwhile, mefloquine/hemin combinations showed potential synergistic effects against adult S. japonicum, with combination index (CI) values <1. Apart from hemin, zinc protoporphyrin-IX, and other iron providing agents such as ferrous sulfate and ferriammonium sulfate combined with mefloquine exhibited no toxic effect against schistosomes. On the other hand, addition of iron chelators (deferiprone, desferrioxamine mesylate, or 2,2'-bipyridine) to the medium containing mefloquine-hemin resulted in no protective effect on the worms. Furthermore, biological reductants like glutathione, vitamine C or cysteine showed no apparent worm protection effect from toxic mefloquine-hemin even at higher concentrations (242.3-614.6μg/ml, i.e., 6.4-17.8-fold higher than the concentration of hemin). Chemical interaction of mefloquine with hemin was studied in 40% DMSO-Tris buffer solution. Both UV-Vis spectrum and mass spectrum demonstrated the strong interaction of mefloquine with hemin, which resulted in a reduction of hemin color and emergence of an adduct formed by mefloquine and hemin. The results confirm that mefloquine combined with hemin exhibits potential synergistic effect against adult S. japonicum in vitro.

Unsaturated glycerophospholipids mediate heme crystallization: biological implications for hemozoin formation in the kissing bug Rhodnius prolixus

Hemozoin (Hz) is a heme crystal produced by some blood-feeding organisms, as an efficient way to detoxify heme derived from hemoglobin digestion. In the triatomine insect Rhodnius prolixus, Hz is essentially produced by midgut extracellular phospholipid membranes known as perimicrovillar membranes (PMVM). Here, we investigated the role of commercial glycerophospholipids containing serine, choline and ethanolamine as headgroups and R. prolixus midgut lipids (RML) in heme crystallization. All commercial unsaturated forms of phospholipids, as well as RML, mediated fast and efficient β-hematin formation by means of two kinetically distinct mechanisms: an early and fast component, followed by a late and slow one. The fastest reactions observed were induced by unsaturated forms of phosphatidylethanolamine (uPE) and phosphatidylcholine (uPC), with half-lives of 0.04 and 0.7 minutes, respectively. β-hematin crystal morphologies were strikingly distinct among groups, with uPE producing homogeneous regular brick-shaped crystals. Interestingly, uPC-mediated reactions resulted in two morphologically distinct crystal populations: one less representative group of regular crystals, resembling those induced by uPE, and the other largely represented by crystals with numerous sharp edges and tapered ends. Heme crystallization reactions induced by RML were efficient, with a heme to β-hematin conversion rate higher than 70%, but clearly slower (t1/2 of 9.9-17.7 minutes) than those induced by uPC and uPE. Interestingly, crystals produced by RML were homogeneous in shape and quite similar to those mediated by uPE. Thus, β-hematin formation can be rapidly and efficiently induced by unsaturated glycerophospholipids, particularly uPE and uPC, and may play a role on biological heme crystallization in R. prolixus midgut.

Characterization of a gene family encoding SEA (sea-urchin sperm protein, enterokinase and agrin)-domain proteins with lectin-like and heme-binding properties from Schistosoma japonicum

Background

We previously identified a novel gene family dispersed in the genome of Schistosoma japonicum by retrotransposon-mediated gene duplication mechanism. Although many transcripts were identified, no homolog was readily identifiable from sequence information.

Methodology/Principal Findings

Here, we utilized structural homology modeling and biochemical methods to identify remote homologs, and characterized the gene products as SEA (sea-urchin sperm protein, enterokinase and agrin)-domain containing proteins. A common extracellular domain in this family was structurally similar to SEA-domain. SEA-domain is primarily a structural domain, known to assist or regulate binding to glycans. Recombinant proteins from three members of this gene family specifically interacted with glycosaminoglycans with high affinity, with potential implication in ligand acquisition and immune evasion. Similar approach was used to identify a heme-binding site on the SEA-domain. The heme-binding mode showed heme molecule inserted into a hydrophobic pocket, with heme iron putatively coordinated to two histidine axial ligands. Heme-binding properties were confirmed using biochemical assays and UV-visible absorption spectroscopy, which showed high affinity heme-binding (K_D = 1.605×10⁻⁶ M) and cognate spectroscopic attributes of hexa-coordinated heme iron. The native proteins were oligomers, antigenic, and are localized on adult worm teguments and gastrodermis; major host-parasite interfaces and site for heme detoxification and acquisition.

Conclusions

The results suggest potential role, at least in the nucleation step of heme crystallization (hemozoin formation), and as receptors for heme uptake. Survival strategies exploited by parasites, including heme homeostasis mechanism in hemoparasites, are paramount for successful parasitism. Thus, assessing prospects for application in disease intervention is warranted.

While isolating membrane-bound and secreted proteins as targets for Schistosoma japonicum vaccine, we identified a novel potentially functional gene family which had originated by a gene duplication mechanism. Here, we integrated structural homology modeling and biochemical methods to show that this gene family encodes proteins with sea-urchin sperm protein, enterokinase and agrin (SEA) –domain, with heme-binding properties. Typical of SEA-structural domains, the characterized proteins specifically interacted with glycosaminoglycans (GAGs), with implication in ligand gathering and immune-evasion. Consistent with modeled heme-binding pocket, we observed high affinity heme-binding and spectroscopic attributes of hexa-coordinated heme iron. Localization of the native gene-products on adult worm tegument and gastrodermis, host interfaces for heme-sequestration and acquisition, suggests potential roles for this gene family in heme-detoxification and heme-iron uptake.

The schistosome oesophageal gland: initiator of blood processing

Background

Although the ultrastructure of the schistosome esophageal gland was described >35 years ago, its role in the processing of ingested blood has never been established. The current study was prompted by our identification of MEG-4.1 expression in the gland and the observation of erythrocyte uncoating in the posterior esophagus.

Methodology/Principal Findings

The salient feature of the posterior esophagus, characterized by confocal and electron microscopy, is the enormous increase in membrane surface area provided by the plate-like extensions and basal invaginations of the lining syncytium, with unique crystalloid vesicles releasing their contents between the plates. The feeding process was shown by video microscopy to be divided into two phases, blood first accumulating in the anterior lumen before passing as a bolus to the posterior. There it streamed around a plug of material revealed by confocal microscopy as tethered leucocytes. These were present in far larger numbers than predicted from the volume of the lumen, and in varying states of damage and destruction. Intact erythrocytes were detected in the anterior esophagus but not observed thereafter, implying that their lysis occurred rapidly as they enter the posterior. Two further genes, MEGs 4.2 and 14, were shown to be expressed exclusively in the esophageal gland. Bioinformatics predicted that MEGs 4.1 and 4.2 possessed a common hydrophobic region with a shared motif, while antibodies to SjMEG-4.1 showed it was bound to leucocytes in the esophageal lumen. It was also predicted that MEGs 4.1 and 14 were heavily O-glycosylated and this was confirmed for the former by 2D-electrophoresis and Western blotting.

Conclusions/Significance

The esophageal gland and its products play a central role in the processing of ingested blood. The binding of host antibodies in the esophageal lumen shows that some constituents are antibody targets and could provide a new source of vaccine candidates.

Host blood ingested by adult schistosomes resident in the bloodstream, passes down a short esophagus to reach the gut where proteolysis of its constituents occurs. The esophageal gland surrounds the posterior half of the esophagus, which has a membrane surface area enormously expanded into plates that must be crucial for its functioning. The contents of a unique crystalloid vesicle, manufactured in the gland, are released into the lumen and accumulate between the plates. Incoming host leucocytes are tethered in the lumen of the posterior esophagus and damaged or destroyed there. Erythrocytes are also lysed as they enter the posterior esophagus. We have identified two further protein products of the gland, to add to the two we previously described, which may potentially interact with the incoming host cells to determine their fate. Finally, we have shown the antibodies from infected hosts can recognise and bind to the esophageal gland secretions. This raises the possibility that gland constituents may serve as a new source of candidates for both a therapeutic and prophylactic vaccine.

Schistosomiasis chemotherapy

After malaria, schistosomiasis (or bilharzia) is the second most prevalent disease in Africa, and is occurring in over 70 countries in tropical and subtropical regions. It is estimated that 600 million people are at risk of infection, 200 million people are infected, and at least 200,000 deaths per year are associated with the disease. All schistosome species are transmitted through contact with fresh water that is infested with free-swimming forms of the parasite, which is known as cercariae and produced by snails. When located in the blood vessels of the host, larval and adult schistosomes digest red cells to acquire amino acids for growth and development. Vaccine candidates have been unsuccessful up to now. Against such devastating parasitic disease, the antischistosomal arsenal is currently limited to a single drug, praziquantel, which has been used for more than 35 years. Because the question of the reduction of the activity of praziquantel was raised recently, it is thus urgent to create new and safe antischistosomal drugs that should be combined with praziquantel to develop efficient bitherapies.

Beyond heme detoxification: a role for hemozoin in iron transport in S. japonicum

Hemozoin (Hz) is considered a disposal product during the digestion of red blood cells by some blood-feeding parasites, such as Plasmodium, Schistosome, and Rhodnius. The only function of Hz that has been reported is to detoxify the free heme (Fe((III))-protoporphyrin-IX) in worms. Here we report a new role for Hz in iron transport in Schistosoma japonicum. Using transmission electron microscopy (TEM), we observed that S. japonicum hemozoin (sjHz) granules were a group of electron-dense, globe-, and comma-shaped granules. At the anterior end of female worm gut, these dark brown granules were found to be mixed with biconcave disc-shaped erythrocytes, in the middle portion of the gut these granules attached to destroyed erythrocytes and in the posterior portion of the gut no intact erythrocytes were observed except free sjHz granules. By energy dispersive spectroscopy (EDS) and Prussian blue iron staining, we found that these iron-containing sjHz granules are degraded near the microvilli adjacent to vitelline glands, resulting in the accumulation of a large amount of iron in the vitelline cells and eggs of developed S. japonicum. The accumulation of iron in vitelline glands was synchronized with the increase of sjHz granules in the gut. When S. japonicum just contained a little amount of sjHz granules in gut, hardly any accumulation of iron was detected in vitelline glands. However, when the lumen of gut filled full with sjHz granules, large amounts of iron was detected in vitelline glands. Solexa sequencing revealed that expression of iron store protein, ferritin-1 (CAX77379.1), is just significantly up-regulated in worms that contained a large amount of sjHz in gut. In contrast to the idea that sjHz granules are simply by-product of heme detoxification, we found that formation and degradation of sjHz granules in vivo likely serve for the iron transport. Our findings provide new insights into the biological significance of Hz formation.

Falcipain-2 inhibition by suramin and suramin analogues

Falcipain-2 is a cysteine protease of the malaria parasite Plasmodium falciparum that plays a key role in the hydrolysis of hemoglobin, a process that is required by intraerythrocytic parasites to obtain amino acids. In this work we show that the polysulfonated napthylurea suramin is capable of binding to falcipain-2, inhibiting its catalytic activity at nanomolar concentrations against both synthetic substrates and the natural substrate hemoglobin. Kinetic measurements suggest that the inhibition occurs through an noncompetitive allosteric mechanism, eliciting substrate inhibition. Smaller suramin analogues and those with substituted methyl groups also showed inhibition within the nanomolar range. Our results identify the suramin family as a potential starting point for the design of falcipain-2 inhibitor antimalarials that act through a novel inhibition mechanism.

Effect of ketoconazole, a cytochrome P450 inhibitor, on the efficacy of quinine and halofantrine against Schistosoma mansoni in mice

The fear that schistosomes will become resistant to praziquantel (PZQ) motivates the search for alternatives to treat schistosomiasis. The antimalarials quinine (QN) and halofantrine (HF) possess moderate antischistosomal properties. The major metabolic pathway of QN and HF is through cytochrome P450 (CYP) 3A4. Accordingly, this study investigates the effects of CYP3A4 inhibitor, ketoconazole (KTZ), on the antischistosomal potential of these quinolines against Schistosoma mansoni infection by evaluating parasitological, histopathological, and biochemical parameters. Mice were classified into 7 groups: uninfected untreated (I), infected untreated (II), infected treated orally with PZQ (1,000 mg/kg) (III), QN (400 mg/kg) (IV), KTZ (10 mg/kg)+QN as group IV (V), HF (400 mg/kg) (VI), and KTZ (as group V)+HF (as group VI) (VII). KTZ plus QN or HF produced more inhibition (P<0.05) in hepatic CYP450 (85.7% and 83.8%) and CYT b5 (75.5% and 73.5%) activities, respectively, than in groups treated with QN or HF alone. This was accompanied with more reduction in female (89.0% and 79.3%), total worms (81.4% and 70.3%), and eggs burden (hepatic; 83.8%, 66.0% and intestinal; 68%, 64.5%), respectively, and encountering the granulomatous reaction to parasite eggs trapped in the liver. QN and HF significantly (P<0.05) elevated malondialdehyde levels when used alone or with KTZ. Meanwhile, KTZ plus QN or HF restored serum levels of ALT, albumin, and reduced hepatic glutathione (KTZ+HF) to their control values. KTZ enhanced the therapeutic antischistosomal potential of QN and HF over each drug alone. Moreover, the effect of KTZ+QN was more evident than KTZ+HF.

First insight into the effect of single oral dose therapy with artemisinin-naphthoquine phosphate combination in a mouse model of Schistosoma mansoni infection

Praziquantel is the current drug of choice against schistosomiasis. The dependency on praziquantel exclusively is problematic, given the spread of the disease and the threat of drug resistance. This study investigates an alternative antischistosomal drug using the compound naphthoquine phosphate tablet, which is a novel single oral dose antimalarial drug, containing a combination of naphthoquine phosphate and artemisinin. In the present study, the therapeutic efficacies of different artemisinin-naphthoquine phosphate combination-dosing protocols were evaluated in experimentally infected mice harbouring juvenile or adult stages of Schistosoma mansoni (Egyptian strain). The study shows that the oral administration of artemisinin-naphthoquine phosphate combination in a single dose of 400 mg/kg on day 7 p.i. resulted in a significant worm burden reduction of 95.07%. When used at a dose of 600 mg/kg on day 21 p.i., all female worms were killed before depositing eggs, resulting in complete absence of eggs in hepatic and intestinal tissues. The same dose given on day 42 p.i. reduced total and female worm burdens by 93.36% and 94.17%, respectively. In addition, artemisinin-naphthoquine phosphate combination induced significant reductions of 80.18% and 76.73% in the hepatic and intestinal tissue egg loads, respectively. Artemisinin-naphthoquine phosphate combination also induced significant alterations in the oogram pattern with elevated levels of dead eggs. Antipathological activities were evident in the amelioration of hepatic granulomata. Our findings hold promise for the development of a novel antischistosomal drug using an artemisinin-naphthoquine phosphate combination. Further in vitro and in vivo studies should be launched to elucidate the possible mechanism/s of action and to study the effect of artemisinin-naphthoquine phosphate combination on other human schistosomes.

Schistosoma mansoni hemozoin modulates alternative activation of macrophages via specific suppression of Retnla expression and secretion

The trematode Schistosoma mansoni is one of the etiological agents of schistosomiasis, a key neglected tropical disease responsible for an estimated annual loss of 70 million disability-adjusted life years. Hematophagy represents the primary nutrient acquisition pathway of this parasite, but digestion of hemoglobin also liberates toxic heme. Schistosomes detoxify heme via crystallization into hemozoin, which is subsequently regurgitated into the host's circulation. Here we demonstrate that during experimental schistosomiasis, hemozoin accumulating in the mouse liver is taken up by phagocytes at a time coincident with the development of the egg-induced T-helper 2 (Th2) granulomatous immune response. Furthermore, the uptake of hemozoin also coincides with the hepatic expression of markers of alternative macrophage activation. Alternatively activated macrophages are a key effector cell population associated with protection against schistosomiasis, making hemozoin well placed to play an important immunomodulatory role in this disease. To systematically explore this hypothesis, S. mansoni hemozoin was purified and added to in vitro bone marrow-derived macrophage cultures concurrently exposed to cytokines chosen to reflect the shifting state of macrophage activation in vivo. Macrophages undergoing interleukin-4 (IL-4)-induced alternative activation in the presence of hemozoin developed a phenotype specifically lacking in Retnla, a characteristic alternatively activated macrophage product associated with regulation of Th2 inflammatory responses. As such, in addition to its important detoxification role during hematophagy, we propose that schistosome hemozoin also provides a potent immunomodulatory function in the coevolved network of host-parasite relationships during schistosomiasis.

Neutral lipids associated with haemozoin mediate efficient and rapid β-haematin formation at physiological pH, temperature and ionic composition

Background

The malaria parasite disposes of host-derived ferrihaem (iron(III)protoporphyrin IX, Fe(III)PPIX) by conversion to crystalline haemozoin in close association with neutral lipids. Lipids mediate synthetic haemozoin (β-haematin) formation very efficiently. However, the effect on reaction rates of concentrations of lipid, Fe(III)PPIX and physiologically relevant ions and biomolecules are unknown.

Methods

Lipid emulsions containing Fe(III)PPIX were prepared in aqueous medium (pH 4.8, 37°C) to mediate β-haematin formation. The reaction was quenched at various times and free Fe(III)PPIX measured colorimetrically as a pyridine complex and the kinetics and yields analysed. Products were also characterized by FTIR, TEM and electron diffraction. Autofluorescence was also used to monitor β-haematin formation by confocal microscopy.

Results

At fixed Fe(III)PPIX concentration, β-haematin yields remained constant with decreasing lipid concentration until a cut-off ratio was reached whereupon efficiency decreased dramatically. For the haemozoin-associated neutral lipid blend (NLB) and monopalmitoylglycerol (MPG), this occurred below a lipid/Fe(III)PPIX (L/H) ratio of 0.54. Rate constants were found to increase with L/H ratio above the cut-off. At 16 μM MPG, Fe(III)PPIX concentration could be raised until the L/H ratio reached the same ratio before a sudden decline in yield was observed. MPG-mediated β-haematin formation was relatively insensitive to biologically relevant cations (Na⁺, K⁺, Mg²⁺, Ca²⁺), or anions (H₂PO₄⁻, HCO₃⁻, ATP, 2,3-diphosphoglycerate, glutathione). Confocal microscopy demonstrated β-haematin formation occurs in association with the lipid particles.

Conclusions

Kinetics of β-haematin formation have shown that haemozoin-associated neutral lipids alone are capable of mediating β-haematin formation at adequate rates under physiologically realistic conditions of ion concentrations to account for haemozoin formation.

Molecular characterization of an α-N-acetylgalactosaminidase from Clonorchis sinensis

The α-N-acetylgalactosaminidase (α-NAGAL) is an exoglycosidase that selectively cleaves terminal α-linked N-acetylgalactosamines from a variety of sugar chains. A complementary DNA (cDNA) clone encoding a novel Clonorchis sinensis α-NAGAL (Cs-α-NAGAL) was identified in the expressed sequence tags database of the adult C. sinensis liver fluke. The complete coding sequence was 1,308 bp long and encoded a 436-residue protein. The selected glycosidase was manually curated as α-NAGAL (EC 3.2.1.49) based on a composite bioinformatics analysis including a search for orthologues, comparative structure modeling, and the generation of a phylogenetic tree. One orthologue of Cs-α-NAGAL was the Rattus norvegicus α-NAGAL (accession number: NP_001012120) that does not exist in C. sinensis. Cs-α-NAGAL belongs to the GH27 family and the GH-D clan. A phylogenetic analysis revealed that the GH27 family of Cs-α-NAGAL was distinct from GH31 and GH36 within the GH-D clan. The putative 3D structure of Cs-α-NAGAL was built using SWISS-MODEL with a Gallus gallus α-NAGAL template (PDB code 1ktb chain A); this model demonstrated the superimposition of a TIM barrel fold (α/β) structure and substrate binding pocket. Cs-α-NAGAL transcripts were detected in the adult worm and egg cDNA libraries of C. sinensis but not in the metacercaria. Recombinant Cs-α-NAGAL (rCs-α-NAGAL) was expressed in Escherichia coli, and the purified rCs-α-NAGAL was recognized specifically by the C. sinensis-infected human sera. This is the first report of an α-NAGAL protein in the Trematode class, suggesting that it is a potential diagnostic or vaccine candidate with strong antigenicity.

Synthesis and biological evaluation of 1,4-naphthoquinones and quinoline-5,8-diones as antimalarial and schistosomicidal agents

Improving the solubility of polysubstituted 1,4-naphthoquinone derivatives was achieved by introducing nitrogen in two different positions of the naphthoquinone core, at C-5 and at C-8 of menadione through a two-step, straightforward synthesis based on the regioselective hetero-Diels-Alder reaction. The antimalarial and the antischistosomal activities of these polysubstituted aza-1,4-naphthoquinone derivatives were evaluated and led to the selection of distinct compounds for antimalarial versus antischistosomal action. The Ag(II)-assisted oxidative radical decarboxylation of the phenyl acetic acids using AgNO(3) and ammonium peroxodisulfate was modified to generate the 3-picolinyl-menadione with improved pharmacokinetic parameters, high antimalarial effects and capacity to inhibit the formation of β-hematin.