METABOLISM OF HOOKWORMS. II. GLUCOSE METABOLISM AND GLYCOGEN SYNTHESIS IN ADULT FEMALE ANCYLOSTOMA CANINUM

Metabolomic profiling of the excretory-secretory products of hookworm and whipworm

INTRODUCTION

Soil-transmitted helminths infect billions of people, livestock and companion animals worldwide, and chronic infections with these nematodes represent a major health burden in many developing countries. On the other hand, complete elimination of parasitic helminths and other infectious pathogens has been implicated with rising rates of autoimmune and allergic disorders in developed countries. Given the enormous health impact of these parasites, it is surprising how little is known about the non-protein small metabolites of the excretory-secretory products (ESP), including their composition and pharmacological properties.

OBJECTIVES

We sought proof-of-concept that Nippostrongylus brasiliensis and Trichuris muris, rodent models of two of the most important human soil-transmitted helminths, secrete small metabolites and that some of these metabolites may have specific pharmacological functions.

METHODS

N. brasiliensis and T. muris ESP were collected from adult worms and filtered using a 10 kDa cut-off membrane to produce excretory-secretory metabolites (ESM). The ESM were analysed using targeted gas chromatography-mass spectrometry and liquid chromatography-mass spectrometry for polar and non-polar small metabolites.

RESULTS

ESM from both N. brasiliensis and T. muris contained small molecules. A total of 54 small molecules (38 polar metabolites and 16 fatty acids) were identified, 36 known polar metabolites from N. brasiliensis and 35 from T. muris. A literature review of the identified compounds revealed that 17 of them have various demonstrated pharmacological activities.

CONCLUSION

N. brasiliensis and T. muris secrete polar and non-polar small molecules with as many as 17 metabolites known to exhibit various pharmacological activities.

Hookworm-Derived Metabolites Suppress Pathology in a Mouse Model of Colitis and Inhibit Secretion of Key Inflammatory Cytokines in Primary Human Leukocytes

Iatrogenic hookworm therapy shows promise for treating disorders that result from a dysregulated immune system, including inflammatory bowel disease (IBD). Using a murine model of trinitrobenzenesulfonic acid-induced colitis and human peripheral blood mononuclear cells, we demonstrated that low-molecular-weight metabolites derived from both somatic extracts (LMWM-SE) and excretory-secretory products (LMWM-ESP) of the hookworm, Ancylostoma caninum, display anti-inflammatory properties. Administration to mice of LMWM-ESP as well as sequentially extracted fractions of LMWM-SE using both methanol (SE-MeOH) and hexane-dichloromethane-acetonitrile (SE-HDA) resulted in significant protection against T cell-mediated immunopathology, clinical signs of colitis, and impaired histological colon architecture. To assess bioactivity in human cells, we stimulated primary human leukocytes with lipopolysaccharide in the presence of hookworm extracts and showed that SE-HDA suppressed ex vivo production of inflammatory cytokines. Gas chromatography-mass spectrometry (MS) and liquid chromatography-MS analyses revealed the presence of 46 polar metabolites, 22 fatty acids, and five short-chain fatty acids (SCFAs) in the LMWM-SE fraction and 29 polar metabolites, 13 fatty acids, and six SCFAs in the LMWM-ESP fraction. Several of these small metabolites, notably the SCFAs, have been previously reported to have anti-inflammatory properties in various disease settings, including IBD. This is the first report showing that hookworms secrete small molecules with both ex vivo and in vivo anti-inflammatory bioactivity, and this warrants further exploration as a novel approach to the development of anti-inflammatory drugs inspired by coevolution of gut-dwelling hookworms with their vertebrate hosts.

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.

Of dogs and hookworms: man's best friend and his parasites as a model for translational biomedical research

We present evidence that the dog hookworm (Ancylostoma caninum) is underutilised in the study of host-parasite interactions, particularly as a proxy for the human-hookworm relationship. The inability to passage hookworms through all life stages in vitro means that adult stage hookworms have to be harvested from the gut of their definitive hosts for ex vivo research. This makes study of the human-hookworm interface difficult for technical and ethical reasons. The historical association of humans, dogs and hookworms presents a unique triad of positive evolutionary pressure to drive the A. caninum-canine interaction to reflect that of the human-hookworm relationship. Here we discuss A. caninum as a proxy for human hookworm infection and situate this hookworm model within the current research agenda, including the various 'omics' applications and the search for next generation biologics to treat a plethora of human diseases. Historically, the dog hookworm has been well described on a physiological and biochemical level, with an increasing understanding of its role as a human zoonosis. With its similarity to human hookworm, the recent publications of hookworm genomes and other omics databases, as well as the ready availability of these parasites for ex vivo culture, the dog hookworm presents itself as a valuable tool for discovery and translational research.

Energy metabolism in the developing larval stages of Ancylostoma tubaeforme and Haemonchus contortus: glycolytic and tncarboxylic acid cycle enzymes

The activities of glycolytic and related enzymes and the tricarboxylic acid cycle enzymes were measured in freshly isolated 1st- (Li), 2nd- (L2) and 3rd-stage (L3) larvae of both Ancylostoma tubaeforme and Haemonchus contortus. All enzymes of the glycolytic pathway were present in all developmental stages of both strongylid nematodes although higher levels of activities were obtained in the pre-infective 1st- and 2nd-stage larvae than in the infective 3rd stage. However, the pre-infective larvae contained lower levels of pyruvate kinase (PK) than the infective larvae. Consequently, the pyruvate kinase to phosphoenolpyruvate carboxykinase (PEPCK) ratios were 0·23 and 0·26 for the L1s and L2s for A. tubaeforme and 0·36 and 0·21 for those of H. contortus respectively. High levels of activity of glucose-6-phosphate dehydrogenase obtained in the bacteriophagous pre-infective larvae were consistent with high rates of morphogenesis and substrate synthesis characteristic of the pre-infective stages. All the tricarboxylic acid cycle enzymes were present in the infective larvae of both nematodes while in the pre-infective Li and L2 stages, the enzymes at the beginning of the cycle, namely aconitate hydratase and NAD-linked isocitrate dehydrogenase, were not detected. A scheme was proposed for the energy metabolism of these developing larvae. In this scheme, the pre-infective larvae were shown to operate an anaerobic metabolic pathway involving the carboxylation of phosphoenolpyruvate (PEP) by phosphoenolpyru vate carboxykinase (PEPCK) to form oxaloacetate (OAA), whereas in the infective larvae the metabolic pathway favouring the direct dephosphorylation of PEP, as in vertebrate tissues, was followed.

Hookworm-provoked IgE-mediated Pathology: Capricious Damage or Remarkable Strategy?

The existence of the canine hookworm Ancylostoma caninum in the human gut seems to be curtailed by an effective IgE-mediated inflammatory response against the helminth's feeding secretions, which, by damaging the host, compromises the parasite's feeding opportunities. In contrast, immunity does not develop against adapted, anthropophilic hookworm species, and it is suggested this relates to modulation of the inflammatory response by hookworm secretions. Here, John Croese discusses this hypothesis and its implications for the development of drugs to suppress allergy.

Effects of host serum on feeding by Trichostrongylus colubriformis (nematoda)

The effect of host serum on in vitro feeding by Trichostrongylus colubriformis was studied by incubating adult helminths in goat serum containing the dye, Rhodamine B. The amount of dye ingested was determined by fluorometric analysis. Immune serum from goats infected with T. colubriformis suppressed helminth feeding, while normal serum from uninfected goats did not. Suppression of feeding by immune serum increased with the duration of the host's infection. Heat-inactivation (56 degrees C) of immune serum did not affect its suppressive activity. Pre-exposure of worms to immune serum decreased subsequent feeding activity. However, rigorous washing of helminths restored their feeding to levels that were similar to untreated worms. Indirect immunofluorescent studies with immune serum and FITC conjugated rabbit anti-goat IgG demonstrated binding of immunoglobulin to the cuticle, stoma and excretory pore of whole worms. Feeding inhibition of immune serum was associated with IgG1 isotype. Results of the present studies indicated that IgG was responsible for in vitro suppression of T. colubriformis feeding and may be one effector of immunity to T. colubriformis in the goat.

The uptake in vitro of dyes, monosaccharides and amino acids by the filarial worm Brugia pahangi

The uptake in vitro of various substances by Brugia pahangi was investigated using infective larvae obtained from Aedes aegypti and worms removed from Meriones unguiculatus at 2, 3, 10, 20 and 90 days post-infection. Worms incubated in growth medium 199 containing 1% Trypan blue possessed demonstrable dye in the oral orifice, the anterior oesophageal lumen and the external openings of the vulva and the cloaca or anus but the dye was not found in the gut lumen even after incubation for 24 h. No uptake of ferritin particles into the intestine of the worms was found and no fluorescence could be demonstrated in the gut lumen of worms incubated in medium containing 50% (v/v) fluorescein isothiocyanate-conjugated calf serum for up to 24 h. Trypan blue uptake by the gut of Aspiculuris tetraptera was clearly observed after incubation for several hours. The uptake of D-glucose and L-leucine by B. pahangi was demonstrated using autoradiographic and scintillation counting techniques and incorporation into worm tissues was detected. Glucose was found to be readily incorporated in the apical, glycogen-rich areas of the myocytes of worms of all ages studied and in the uterine epithelium of the adult female. In contrast, a lower incorporation of D-glucose was found in the eggs, embryos and vas deferens and especially in the gut. The incorporation of L-leucine occurred throughout the tissue of the worms during a 30 min incubation. Labelling was also located over the surface of the cuticle of the worms, when incubated for a period of 15 to 60 min in L-[H]leucine. Scintillation counting techniques demonstrated that there was no uptake of 14C-labelled L-glucose or sucrose by B. pahangi. The data presented on the uptake in vitro of nutrients or other compounds by infective larvae and adult stages of B. pahangi did not demonstrate an intestinal route of uptake but indicated that the transcuticular route of uptake may be employed.

Study on the (14)C-glucose metabolism by Clonorchis sinensis: Paper Chromatographic Analyses in Combination with Autoradiography

Radioactive (14)C-glucose(U) was given to Clonorchis sinensis in Tris buffer medium, in corder to trace the metabolic fate of the labelled carbon. The labelled carbon from glucose enters into intermediary metabolites and end products of anaeroblic glycolysis, Embden-Meyerhof pathway, and of aerobic Krebs cycle. These product were identified by one or two-dimensional paper chromatography in combination with autoradoigraphy. 1)The labelled metabolites detected in this experiment corresponded to pyruvic acid, latic acid, malic acid, succinic acid and fumaric acid. 2)Amino acids, such as alanine, aspartic acid, glutamic acid, valine, theronine, and serine, derived by the degradation of (14)C- glycose were also found. 3)Labelled compounds behaving like alanine, aspartic acid and glutamic acid were observed in the chroma to gram of incubation medium. 4)The preciptation which suggests a positive reaction for protein occured when absolute ethanol was added to the incubation medium.

The effect of substrates and inhibitors on the rate of respiration of the infective larvae of Strongyloides ratti Sandground, 1925

The infective larvae of S. ratti were non-feeding and none of a wide variety of possible substrates had any effect on the respiratory rate of intact worms. Sodium succinate increased the oxygen uptake of larval homogenates.

The cytochrome chain inhibitors, sodium cyanide, sodium azide and urethane all reversibly inhibited the respiration of the infective larvae. The inhibition caused by carbon monoxide was photo-reversible and 2,4-dinitrophenol inhibited respiration rather than stimulated it. Methylene blue increased the oxygen uptake of homogenates, but had no effect on the intact larva. Iodoacetamide and sodium arsenite irreversibly inhibited respiration, whilst sodium fluoride, sodium pyrophosphate and sodium malonate were without effect on the intact larvae.

This work was carried out during the tenure of a Medical Research Council Scholarship.