Respiration and energy conservation in the filarial worm Litomosoides carinii

The Effects of Ivermectin on Brugia malayi Females In Vitro: A Transcriptomic Approach

BACKGROUND

Lymphatic filariasis and onchocerciasis are disabling and disfiguring neglected tropical diseases of major importance in developing countries. Ivermectin is the drug of choice for mass drug administration programs for the control of onchocerciasis and lymphatic filariasis in areas where the diseases are co-endemic. Although ivermectin paralyzes somatic and pharyngeal muscles in many nematodes, these actions are poorly characterized in adult filariae. We hypothesize that paralysis of pharyngeal pumping by ivermectin in filariae could result in deprivation of essential nutrients, especially iron, inducing a wide range of responses evidenced by altered gene expression, changes in metabolic pathways, and altered developmental states in embryos. Previous studies have shown that ivermectin treatment significantly reduces microfilariae release from females within four days of exposure in vivo, while not markedly affecting adult worms. However, the mechanisms responsible for reduced production of microfilariae are poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

We analyzed transcriptomic profiles from Brugia malayi adult females, an important model for other filariae, using RNAseq technology after exposure in culture to ivermectin at various concentrations (100 nM, 300 nM and 1 μM) and time points (24, 48, 72 h, and 5 days). Our analysis revealed drug-related changes in expression of genes involved in meiosis, as well as oxidative phosphorylation, which were significantly down-regulated as early as 24 h post-exposure. RNA interference phenotypes of the orthologs of these down-regulated genes in C. elegans include "maternal sterile", "embryonic lethal", "larval arrest", "larval lethal" and "sick".

CONCLUSION/SIGNIFICANCE

These changes provide insight into the mechanisms involved in ivermectin-induced reduction in microfilaria output and impaired fertility, embryogenesis, and larval development.

Advances in metabolic profiling of experimental nematode and trematode infections

Metabonomics, which is the combination of metabolic profiling of biological samples using spectroscopic methods, together with multivariate data analysis, is a powerful approach for biomarker recovery. Moreover, metabonomics holds promise to enhance our understanding of host-parasite interactions at the metabolic level, and therefore provides a framework for discovery of novel targets for diagnostics, drugs and vaccines. In this review, we summarise progress made to date with metabolic profiling strategies applied to different host-parasite models in the laboratory. First, we emphasise the application of two parasitic worm infections that are particularly relevant for Southeast Asia and the People's Republic of China, namely the trematode Schistosoma japonicum causing schistosomiasis, and the nematode Necator americanus causing hookworm disease. Next, we review metabolic profiling studies on the liver fluke Fasciola hepatica in the rat model, the intestinal fluke Echinostoma caproni harboured in mice and characterise the metabolic responses in the hamster to a S. japonicum-N. americanus co-infection. We extract parasite-specific biomarkers and distinguish them from a more general response to an infection at the biochemical level. For example, suppression of tricarboxylic acid cycle metabolites is only noted for a Schistosoma spp. infection, whereas alterations in metabolites derived from the gut microbiota are common for all the parasitic infections investigated thus far. Finally, we explore how the insight gained with experimental infections could be transferred to human populations and conclude with a section on research needs with regard to molecular diagnostics in parasitology.

Onchocerca fasciata: histochemical demonstration of succinate and NADH dehydrogenase

The activities of selected enzymes of the respiratory chain system in Onchocerca fasciata (Filarioidea: Onchocercidae) have been investigated histochemically. Thus, the localization and distributions of NADH dehydrogenase (EC 1.6.99.3), succinate dehydrogenase (SDH) (EC 1.3.99.1) and cytochrome oxidase (EC 1.9.3.1) were investigated in various tissues of the adult female worm by employing MTT, Nitro BT (dehydrogenases) and DAB (cytochrome oxidase). Different tissues varied considerably in their enzymatic activities. The hypodermis and reproductive tissues showed strong and identical localization of NADH and SDH dehydrogenase activities reflecting high metabolic rates. Little or no dehydrogenase activities were observed in the intestine and cuticle. In contrast to the two dehyrogenases, no activity was observed for cytochrome oxidase in any of the tissues in adult or embryonic stages of the worm. The significance of these results with respect to the energy metabolism of the worm is discussed. It is suggested that O. fasciata lacks a classical, mammalian-type respiratory pathway and that oxidative phosphorylation is of no importance as an energy generating pathway in this essentially anaerobic parasite.

Activities of glycogen phosphorylase, alanine aminotransferase and aspartate aminotransferase in adult worms of Litomosoides carinii recovered from pyridoxine deficient cotton rats (Sigmodon hispidus)

This paper demonstrates that the activities of glycogen phosphorylase (GP), alanine aminotransferase (ALT) and aspartate aminotransferase (AST) are reduced in adult worms of the filarial nematode Litomosoides carinii recovered from pyridoxine-deficient cotton rats when compared to worms recovered from pyridoxine-sufficient controls. GP, ALT and AST activities were determined in adult worms L. carinii recovered from cotton rat hosts over a 20-week experimental period. Activities of GP, ALT and AST in the parasite showed a direct correlation with the dietary pyridoxine intake of their host. Throughout the experiment, enzyme activities were significantly lower (P < 0.001) in worms from rats fed a pyridoxine-free diet ad libitum that in worms from rats fed either a stock colony diet, a pyridoxine-free diet ad libitum with daily supplementation of 100 micrograms pyridoxine or limited amounts of pyridoxine-free diet with daily supplementation of 100 micrograms pyridoxine. The lower than normal activity of GP, ALT, AST and other enzymes dependent on the biologically active derivative of pyridoxine, the coenzyme pyridoxal-5-phosphate (PLP), interferes with the protein, carbohydrate and lipid metabolism of L. carinii and may in part cause the reduced establishment, development and growth of the parasite in pyridoxine-deficient hosts.

Comparative study and cDNA cloning of the flavoprotein subunit of mitochondrial complex II (succinate-ubiquinone oxidoreductase: fumarate reductase) from the dog heartworm, Dirofilaria immitis

Mitochondrial complex II functions as a fumarate reductase (FRD), the reverse reaction of succinate dehydrogenase (SDH), and plays an important role in the anaerobic respiratory chain of parasitic helminths. In this study, complex II from the dog heartworm, Dirofilaria immitis adult, which is thought to act as a homolactatic fermenter, was examined in terms of its enzymatic features and primary structure in order to investigate the possible role of mitochondria in this filaria. Mitochondria from D. immitis adult showed high FRD activity when the enzymatic assay was performed using methylviologen as an artificial electron donor. The ratio of SDH to FRD in D. immitis was comparable to that in Ascaris suum adult, which is known to have an anaerobic mitochondrial respiratory chain with a high FRD activity of complex II. The FRD activity of D. immitis mitochondria was inhibited by the sulfhydryl reagent N-ethylmaleimide (NEM), while that of A. suum complex II was resistant to this inhibitor. The presence of the flavoprotein (Fp) subunit, which contains the substrate binding active site, was confirmed in D. immitis mitochondria by immunoblotting using a monoclonal antibody against the A. suum Fp subunit. By homology probing with the polymerase chain reaction, the entire cDNA for the D. immitis adult Fp was cloned and sequenced. The deduced amino acid sequence showed significant homology to that of A. suum and other mitochondrial Fps, in contrast to much less similarity to bacterial FRD, even though the D. immitis complex II showed high FRD activity. These results are the first indication of the presence of a functional complex II in D. immitis mitochondria.

Oxidation and reduction of cytochrome c by mitochondrial enzymes of Setaria cervi

A mitochondria-rich fraction isolated from the cuticle-hypodermis-muscle system of Setaria cervi, a bovine filarial parasite, possessed substrate-coupled cytochrome c reductases and cytochrome c oxidase in appreciable activities. All these activities were located predominantly in the membranes. NADH-coupled cytochrome c reductase was more prominent than NADPH- and succinate-coupled reductases. All the three reductases exhibited marked sensitivity to rotenone and antimycin A. Salicylhydroxamic acid strongly inhibited succinate requiring reductase and cytochrome c oxidase, but the other two reductases only mildly. Sodium azide activated the reductases but substantially inhibited the oxidase activity. Potassium cyanide activated the succinate requiring reductase but did not cause any noticeable change in the activities of pyridine nucleotide linked reductases. Anthelmintics also influenced these activities but no definite correlation could be drawn regarding their mode of action.

Antioxidant system of Litomosoides carinii and Setaria cervi: effect of a macrofilaricidal agent

Filarial parasites, Litomosoides carinii and Setaria cervi, showed great susceptibility to the oxidants generated in vitro by the xanthine/xanthine-oxidase system. In order to counteract such injurious effects, both the filariids possessed an active antioxidant enzymes system. Superoxide dismutase, catalase and glutathione peroxidase were detected in appreciable amounts but glutathione reductase and glucose-6-phosphate dehydrogenase in very low quantities. The former three enzymes were also found to be released by the parasites into the ambient medium. The released enzymes may be responsible for scavenging the host-generated oxidants present in the immediate surroundings of the parasites and thereby enabling them to live comfortably in the host. This Institute-based antifilarial agent namely Compound 82/437 which is 2,2'-dicarbomethoxylamino-5,5'-dibenzimidazolylketone, markedly inhibited catalase and glutathione peroxidase of both L. carinii and S. cervi. The compound, therefore, appears to render the filariids prone to H2O2 toxicity leading to penultimate damage.

In vitro cultivation of adult Litomosoides carinii: evaluation of basic culture media, gas phases and supplements

Adult Litomosoides carinii, recovered from cotton rats (Sigmodon hispidus) 4-5 months post-infection (p.i.), were cultivated in vitro with emphasis on investigations into the development of intra-uterine embryonic stages. Baseline values for the embryonic status of female worms were established immediately after recovery from the hosts. In such females, on average, 16% of the intra-uterine stages were fully formed microfilariae while the remainder belonged to the early embryonic classes that were characterized. For the evaluation of culture success, apart from survival of worms in vitro, the rate of microfilariae development (mf rate) served as a major parameter. Of the five basic culture media RPMI 1640, F12, L15, NCTC 135, and IMDM, tested without supplementation, RPMI 1640 yielded by far the best results (survival = 14 days; mf rate = 41%), and was therefore chosen as the routine medium. In comparison with 5% CO2 in nitrogen, a gas phase of 5% CO2 in air was superior, although the resulting oxygen tension of 138 mmHg in the medium was not physiological. Addition of 10% newborn or foetal bovine serum to the medium in some cases distinctly influenced results. Effects of different batches of sera varied from 'filaricidal' to 'very promoting'. Co-cultivation of worms and Sigmodon cells, or rhesus monkey LLCMK2 cells, only marginally improved results. Of the serum substitutes Ultroser G, BMS, and Clex, the latter had a moderately promoting effect. The protein supplements bovine serum albumin, transferrin and haemoglobin significantly improved results, and could replace certain batches of serum. Supplementation with the haemin moiety alone was less effective than with haemoglobin. The anti-oxidants glutathione plus ascorbic acid proved beneficial in combination with a serum supplement only. Results from other experiments with multiple supplementation also suggest that various supplements may act only in a synergistic manner. The longest average time that adult L. carinii survived in vitro was 3-4 weeks. The highest mf rate was 78%, which indicated that all normal embryonic stages present in the uteri of a female at the start of culture completed their development to microfilariae, however, oogenesis ceased in vitro. The parameters for embryonic development employed proved to be highly sensitive for the judgment of various culture conditions.

On the pathogenesis of anaemia and leukopenia in filarial (Litomosoides carinii) infection of Mastomys natalensis

Mastomys natalensis infected with the filarial parasite Litomosoides carinii show anaemia and leukopenia. Alterations start with the onset of microfilaraemia. Anaemia is temporally macrocytic (up to 80 days after infection), subsequently normocytic and hypochromic, accompanied by reticulocytosis. Increased intravascular haemolysis (i) and functional disorders of the haemopoetic system (ii) are involved in the pathogenesis. i: Circulating erythrocytes showed increased osmofragility. Hypoglycaemia demonstrated in parasitaemic animals may be one reason. ii: Histological and electron microscopical investigations of the bone marrow revealed markedly enhanced haemopoiesis in infected animals. However, a high proportion of cells was found pathologically altered already beginning in the late prepatency and increasing in the further course of infection. Thus, dyshaemopoiesis may result in the production of morphologically and functionally aberrant cells which are rapidly eliminated by the MPS which is highly activated in L. carinii infected M. natalensis.

The pathways of energy generation in filarial parasites

It has been commonly accepted that most adult filarial parasites use the glycolytic breakdown of carbohydrates to lactate as a preferred route to supply their energy requirements. Their ability to catabolize glucose by oxygen-dependent pathways is rather limited. An exception to this is the rodent filarial species Litomosoides carinii, which requires a unique type of aerobic glucose metabolism to maintain motor activity and survival. However, the prominent role of carbohydrates as energy substrates for filariids may no longer be tenable. Recent studies have shown that glutamine is a major energy source in filarial worms and that a fully oxidative mitochondrial metabolism can be employed for the utilization of this substrate.

The role of amino acids in the energy generating pathways of Litomosoides carinii

The filarial parasite Litomosoides carinii was able to survive for longer than 15 h in basic filarial medium (BFM) containing either glutamine or alanine as a sole substrate. The filariids were more motile in BFM containing glucose, but even higher motility was recorded in media containing both glucose and glutamine. Incubations under aerobic conditions showed that radiolabelled glutamine was metabolised primarily to CO2. In addition, small amounts of lactate and acetate were excreted in almost equimolar quantities. Incubations where both glutamine and glucose were present demonstrated that the glutamine carbon utilised by the parasite could be completely recovered in the above three end products. The glutamine nitrogen could be recovered in the additional excretory products, alanine and ammonia. The glutamine-dependent viability of L. carinii was affected by known inhibitors of the mitochondrial respiratory chain. Glucose utilisation, and the production of CO2 from this substrate, were greatly stimulated by the presence of glutamine in the external medium. Various carbon balance studies, in conjunction with enzymatic analyses, suggest that in L. carinii, glutamine provides an input of carbon into the tricarboxylic acid (TCA) cycle, probably at the level of alpha-ketoglutarate. This increased availability of Krebs cycle intermediates will stimulate the rate of pyruvate oxidation via acetyl-CoA and the TCA cycle, and thus increase the rate of carbon flux through glycolysis. The energetic advantage associated with the utilisation of the glucose/glutamine substrate couple may explain the worm's enhanced motor activity compared to incubations with glucose as the sole energy source. Alanine was found to be degraded by the filariid to equivalent amounts of lactate, acetate and CO2, indicating a relatively low energetic efficiency. There was no detectable uptake of glutamate. A variety of other amino acids tested were neither metabolised nor able to maintain worm viability in vitro.

Litomosoides carinii: mode of action in vitro of benzothiazole and amoscanate derivatives with antifilarial activity

It is suggested that the recently developed benzothiazole and amoscanate derivatives with antifilarial activity exert their action in vitro by an inhibition of mitochondrial-derived respiration. It was confirmed that the drugs CGP 20376, 21835, 20308, 21306, and 6140 cause a rapid immobilization in vitro of the adult filarial worm, Litomosoides carinii, the time required being similar to rotenone at the same concentration. The other drugs investigated, CGPs 20309, 21833, 24589, 23518, and 13231, were also effective; however, they required much longer incubation times. Submitochondrial particles (SMP) were prepared from Ascaris muscle and rat liver. The concentration of drug causing 50% inhibition of respiration (IC50) was calculated. It was found that the drugs most rapidly inhibiting respiration have IC50s for NADH oxidase of less than 25 microM in both Ascaris and rat liver SMP. This effect on SMP respiration could be overcome by using succinate as a substrate, indicating the site of inhibition to be within complex I of the mitochondrial respiratory chain. Further experiments showed that whereas the respiratory chain's NADH:ferricyanide reductase was unaffected by these drugs, there were pronounced effects on both Ascaris and rat liver NADH:quinone reductase activity. This suggests that the inhibition within complex I occurs after the flavoprotein dehydrogenase, but before the site of the quinone reduction. The other compounds examined, which had a slower effect on motility, also showed inhibition of the NADH oxidase, but not to as great an extent as the aforementioned compounds. The compounds most active against motility were also most effective at inhibiting respiration in intact adult L. carinii. Analysis of the aerobic end products produced by L. carinii showed that acetate production was greatly reduced even in the presence of low concentrations of the drugs. There was also a slight decrease in lactate production. However, a direct effect on the glycolytic pathway was ruled out by two observations. One, that the production of lactate from cell-free extracts of L. carinii is unaffected by the presence of the drugs, and secondly, that a protozoan, Giardia lamblia, reliant on glycolysis for energy production, can survive for long periods of time in the presence of high concentrations of the drugs. A correlation can be observed between the time for immobilization of the filarial worm and the strength of inhibition of mitochondrial respiration. Therefore, it is suggested that, at least in vitro, the mechanism of toxicity of these antifilarials in L. carinii is due to the blocking of the respiratory chain at a site similar to that of rotenone.