The transition from an aerobic to an anaerobic energy metabolism in transforming Schistosoma mansoni cercariae occurs exclusively in the head

The unusual kinetics of lactate dehydrogenase of Schistosoma mansoni and their role in the rapid metabolic switch after penetration of the mammalian host

Lactate dehydrogenase (LDH) from Schistosoma mansoni has peculiar properties for a eukaryotic LDH. Schistosomal LDH (SmLDH) isolated from schistosomes, and the recombinantly expressed protein, are strongly inhibited by ATP, which is neutralized by fructose-1,6-bisphosphate (FBP). In the conserved FBP/anion binding site we identified two residues in SmLDH (Val187 and Tyr190) that differ from the conserved residues in LDHs of other eukaryotes, but are identical to conserved residues in FBP-sensitive prokaryotic LDHs. Three-dimensional (3D) models were generated to compare the structure of SmLDH with other LDHs. These models indicated that residues Val187, and especially Tyr190, play a crucial role in the interaction of FBP with the anion pocket of SmLDH. These 3D models of SmLDH are also consistent with a competitive model of SmLDH inhibition in which ATP (inhibitor) and FBP (activator) compete for binding in a well-defined anion pocket. The model of bound ATP predicts a distortion of the nearby key catalytic residue His195, resulting in enzyme inhibition. To investigate a possible physiological role of this allosteric regulation of LDH in schistosomes we made a kinetic model in which the allosteric regulation of the glycolytic enzymes can be varied. The model showed that inhibition of LDH by ATP prevents fermentation to lactate in the free-living stages in water and ensures complete oxidation via the Krebs cycle of the endogenous glycogen reserves. This mechanism of allosteric inhibition by ATP prevents the untimely depletion of these glycogen reserves, the only fuel of the free-living cercariae. Neutralization by FBP of this ATP inhibition of LDH prevents accumulation of glycolytic intermediates when S. mansoni schistosomula are confronted with the sudden large increase in glucose availability upon penetration of the final host. It appears that the LDH of S. mansoni is special and well suited to deal with the variations in glucose availability the parasite encounters during its life cycle.

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.

The metabolic control of schistosome egg production

Schistosomiasis is a neglected tropical disease caused by infection with trematode parasites of the genus Schistosoma. Despite ongoing treatment programmes, the prevalence of schistosomiasis has failed to decline and the disease remains a cause of severe morbidity in millions of people. Understanding the biology of egg production by schistosomes is critical since eggs allow transmission of the infection, and when trapped in host tissues induce the immune responses that are responsible for the pathologic changes that underlie disease development. Unusually among trematodes, adult schistosomes exhibit sexual dimorphism and display a fascinating codependency in that the female is dependent on the male to grow and sexually mature. Thus, virgin females are developmentally stunted compared with females from mixed-sex infections and are unable to lay eggs. Moreover, fecund female schistosomes rapidly lose the ability to produce eggs when placed in tissue culture. Here we discuss the metabolic regulation of egg production in schistosomes, and in particular the critical role played by fatty acid oxidation in this process.

The tegument of the human parasitic worm Schistosoma mansoni as an excretory organ: the surface aquaporin SmAQP is a lactate transporter

Adult schistosomes are intravascular parasites that metabolize imported glucose largely via glycolysis. How the parasites get rid of the large amounts of lactic acid this generates is unknown at the molecular level. Here, we report that worms whose aquaporin gene (SmAQP) has been suppressed using RNAi fail to rapidly acidify their culture medium and excrete less lactate compared to controls. Functional expression of SmAQP in Xenopus oocytes demonstrates that this protein can transport lactate following Michaelis-Menten kinetics with low apparent affinity (Km = 41+/-5. 8 mM) and with a low energy of activation (E(a) = 7.18+/-0.7 kcal/mol). Phloretin, a known inhibitor of lactate release from schistosomes, also inhibits lactate movement in SmAQP-expressing oocytes. In keeping with the substrate promiscuity of other aquaporins, SmAQP is shown here to be also capable of transporting water, mannitol, fructose and alanine but not glucose. Using immunofluorescent and immuno-EM, we confirm that SmAQP is localized in the tegument of adult worms. These findings extend the proposed functions of the schistosome tegument beyond its known capacity as an organ of nutrient uptake to include a role in metabolic waste excretion.

Toxicity of cadmium and zinc mixtures to the decaudized cercarial life span ofDiplostomum spathaceum

The effects of cadmium and zinc mixtures at concentrations ranging from 0.1 to 10,000 μg l−1on the life-span of decaudized cercarial bodies (cercariae that have shed their tails) ofDiplostomum spathaceum(Trematoda: Diplostomatidae) was investigated. Cercariae were exposed to metal mixtures of equal and unequal concentrations, and a low-dose pre-treatment followed by a high-dose exposure mixtures. Metal mixtures demonstrated variable effects on decaudized cercariae either by increasing or reducing their life-span compared to single metal exposures dependent on concentration and the type of mixed metal treatment. Prolonged exposure to equal metal mixtures at low concentrations (0.1–100 μg l−1) resulted in a reduction in the life-span of decaudized cercariae at 0.1 and 100 μg l−1in those individuals decaudized during the initial 24 h exposure period compared with those decaudized during the final 24 h period of cercarial survival, whilst in controls there was no significant life-span change between the two time periods. Decaudized cercariae which were exposed to low concentrations (0.1–100 μg l−1) of equal metal mixtures were also evaluated for their role as an indicator of larval ‘fitness’ for migrating through the tissues of their target fish host for those individuals decaudized during the initial 24 h exposure period, and demonstrated only a limited change in their life-span compared to control and single metal exposures. The importance of metal mixtures in parasite establishment in the fish host is discussed.

Energy generation in parasitic helminths

Although parasitic helminths are a very heterogeneous group of organisms, they share many interesting properties in their energy metabolism. In certain stages of their life cycle, they all have a large capacity for anaerobic functioning. In other stages, an aerobic energy metabolism prevails. Parasites have to adapt to different environments in which the availability of oxygen and food varies widely. These variations in their external conditions strongly influence their energy metabolism. Here, Louis Tielens presents an introduction to the current ideas on the bioenergetics of parasitic helminths, focusing on the differences in energy metabolism between various stages (free-living and parasitic), and paying special attention to the mechanisms involved in the transitions between the different methods of energy generation.

Acetyl:succinate CoA-transferase in procyclic Trypanosoma brucei. Gene identification and role in carbohydrate metabolism

Acetyl:succinate CoA-transferase (ASCT) is an acetate-producing enzyme shared by hydrogenosomes, mitochondria of trypanosomatids, and anaerobically functioning mitochondria. The gene encoding ASCT in the protozoan parasite Trypanosoma brucei was identified as a new member of the CoA transferase family. Its assignment to ASCT activity was confirmed by 1) a quantitative correlation of protein expression and activity upon RNA interference-mediated repression, 2) the absence of activity in homozygous Deltaasct/Deltaasct knock out cells, 3) mitochondrial colocalization of protein and activity, 4) increased activity and acetate excretion upon transgenic overexpression, and 5) depletion of ASCT activity from lysates upon immunoprecipitation. Genetic ablation of ASCT produced a severe growth phenotype, increased glucose consumption, and excretion of beta-hydroxybutyrate and pyruvate, indicating accumulation of acetyl-CoA. Analysis of the excreted end products of (13)C-enriched and (14)C-labeled glucose metabolism showed that acetate excretion was only slightly reduced. Adaptation to ASCT deficiency, however, was an infrequent event at the population level, indicating the importance of this enzyme. These studies show that ASCT is indeed involved in acetate production, but is not essential, as apparently it is not the only enzyme that produces acetate in T. brucei.

Procyclic Trypanosoma brucei do not use Krebs cycle activity for energy generation

The importance of a functional Krebs cycle for energy generation in the procyclic stage of Trypanosoma brucei was investigated under physiological conditions during logarithmic phase growth of a pleomorphic parasite strain. Wild type procyclic cells and mutants with targeted deletion of the gene coding for aconitase were derived by synchronous in vitro differentiation from wild type and mutant (Delta aco::NEO/Delta aco::HYG) bloodstream stage parasites, respectively, where aconitase is not expressed and is dispensable. No differences in intracellular levels of glycolytic and Krebs cycle intermediates were found in procyclic wild type and mutant cells, except for citrate that accumulated up to 90-fold in the mutants, confirming the absence of aconitase activity. Surprisingly, deletion of aconitase did not change differentiation nor the growth rate or the intracellular ATP/ADP ratio in those cells. Metabolic studies using radioactively labeled substrates and NMR analysis demonstrated that glucose and proline were not degraded via the Krebs cycle to CO(2). Instead, glucose was degraded to acetate, succinate, and alanine, whereas proline was degraded to succinate. Importantly, there was absolutely no difference in the metabolic products released by wild type and aconitase knockout parasites, and both were for survival strictly dependent on respiration via the mitochondrial electron transport chain. Hence, although the Krebs cycle enzymes are present, procyclic T. brucei do not use Krebs cycle activity for energy generation, but the mitochondrial respiratory chain is essential for survival and growth. We therefore propose a revised model of the energy metabolism of procyclic T. brucei.

Vampires, Pasteur and reactive oxygen species. Is the switch from aerobic to anaerobic metabolism a preventive antioxidant defence in blood-feeding parasites?

Several species of parasites show a reduction of their respiratory activity along their developmental cycles after they start to feed on vertebrate blood, relying on anaerobic degradation of carbohydrates to achieve their energy requirements. Usually, these parasites choose not to breathe despite of living in an environment of high oxygen availability such as vertebrate blood. Absence of the 'Pasteur effect' in most of these parasites has been well documented. Interestingly, together with the switch from aerobic to anaerobic metabolism in these parasites, there is clear evidence pointing to an increase in their antioxidant defences. As the respiratory chain in mitochondria is a major site of production of reactive oxygen species (ROS), we propose here that the arrest of respiration constitutes an adaptation to avoid the toxic effects of ROS. This situation would be especially critical for blood-feeding parasites because ROS produced in mitochondria would interact with pro-oxidant products of blood digestion, such as haem and/or iron, and increase the oxidative damage to the parasite's cells.

Induction cues for tegument formation during the transformation of Schistosoma mansoni cercariae

Adult schistosomes are parasitic blood flukes that have a continuous double lipid bilayered membrane surrounding the entire worm. This tegumental membrane is synthesised during invasion of the vertebrate host by free-swimming infectious forms called cercariae. As cercariae invade their final hosts they lose their tails and encounter a changing environment that includes altered temperature, sugar concentration and osmolarity. We have identified a glucose transporter protein designated SGTP4 that is found exclusively in the outer adult tegument and on membranous vesicles within the tegumental cytoplasm. By using immunofluorescence analysis to monitor the appearance and distribution of SGTP4 we can track the process of new tegumental membrane formation and examine the cues that trigger this developmental pathway. Cercariae in water do not transform their tegument while those incubated in rich medium do so rapidly. We have examined which of the many constituents of rich medium are responsible for triggering this transformation. Incubation in a solution of moderate osmolarity (120 mOsM PBS) is sufficient by itself to trigger tegument transformation, albeit at a slower rate relative to incubation in rich medium. Adding either glucose (to 100 mM) to the solution or increasing the temperature of incubation (from 22 degrees C to 37 degrees C) further increased the rate of tegument biogenesis. The introduction of glucose together with an increase in the incubation temperature further accelerated the process, suggesting that these factors act synergistically to promote transformation rates. The critical nature of osmolarity in inducing the process is highlighted by the fact that transformation proceeds as efficiently in 360 mOsM alone as it does in rich medium. While the fatty acids linolenic acid (cis-9, cis-12, cis-15-octadecatrienoic acid at 1 mM) and capric acid (Decanoic acid, at 0.1 mM) have both been proposed to stimulate tegumental transformation, we show that neither promotes the morphogenesis of a normal schistosomulum tegument. The schistosomicide praziquantel (to 1 mM) has no detectable effect on new tegument formation.

Glucose Transport and Metabolism in Mammalian-stage Schistosomes

Adult schistosomes transport nutrients from the host bloodstream across their outer body covering or tegument. The tegument is a cytologically unusual structure; it is a syncytium bounded externally by two lipid bilayer membranes. In this review, Patrick Skelly, Louis Tielens and Chuck Shoemaker reconsider our understanding of how glucose enters schistosomes across this unusual outer covering in the light of recent papers characterizing glucose transport proteins and glucose metabolism pathways in these parasites.

Ultrastructural and biochemical detection of biotin and biotinylated polypeptides in Schistosoma mansoni

Biotinylation is proposed for the identification of surface proteins in Schistosoma mansoni using the streptavidin-HRP conjugate for the detection of labeled polypeptides. However, control samples also showed several endogenous biotinylated polypeptides. In an attempt to determine the possibility of nonspecific binding between the streptavidin-HRP conjugate and polypeptides from S. mansoni, the conjugate was blocked with biotinamidecaproate-N-hydroxysuccinimide ester (BcapNHS) before biotin-streptavidin blotting. No bands were detected on the nitrocellulose sheet, demonstrating the specific recognition of biotin by the streptavidin present in the conjugate. Whole cercariae and cercarial bodies and tails showed several endogenous biotinylated polypeptides. The biotin concentration was 13 micrograms/190,000 cercariae. Adult worms presented less endogenous biotinylated polypeptides than cercariae. These results may be due to changes in the environment from aerobic to anaerobic conditions when cercarial bodies (schistosomula) are transformed into adult worms and a decrease in CO2 production may occur. Cercariae, cercarial bodies and adult male worms were examined by transmission electron microscopy employing an avidin-colloidal gold conjugate for the detection of endogenous biotin. Gold particles were distributed mainly on the muscle fibers, but dispersed granules were observed in the tegument, mitochondria and cytosol. The discovery of endogenous biotin in S. mansoni should be investigated in order to clarify the function of this vitamin in the parasite.

Differential expression of two succinate dehydrogenase subunit-B genes and a transition in energy metabolism during the development of the parasitic nematode Haemonchus contortus

The carbohydrate metabolism of free-living and parasitic stages of the sheep nematode Haemonchus contortus was studied, and it was demonstrated that during development a switch occurred from Krebs-cycle activity towards a more fermentative metabolism. During this switch a transition might take place in complex II of the respiratory chain. In the free-living (L3) and early parasitic (XL3) stages, complex II catalyses the oxidation of succinate to fumarate via the Krebs cycle, whereas in adults complex II functions in the reverse reaction, the reduction of fumarate to succinate. L3 and XL3 were shown to already possess a large anaerobic capacity. They survived well in the absence of oxygen or in the presence of cyanide, which completely blocked respiration. Krebs-cycle activity, however, was only partially inhibited by cyanide; the XL3s in particular produced in the presence of cyanide large amounts of propanol, the production of which probably functions as an alternative electron sink. For further investigation of the observed metabolic switch, complex II of the respiratory chain, a key enzyme involved in this switch, was studied. The B subunit of complex II was cloned and sequenced. These clones all showed sequences similar to the B subunit of succinate dehydrogenase from other species, and included the amino-terminal signal sequence for importation into mitochondria. Two genes were identified, types 1 and 2, based on the DNA and amino acid sequences and on the lack of cross-reaction to each other when used as probes on Southern blots. On Northern blots, the two genes showed a different expression pattern during the development of the parasite.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of Schistosoma mansoni genes involved in anaerobic and oxidative glucose metabolism during the cercaria to adult transformation

Schistosomes switch rapidly from the use of stored glycogen to a reliance on host glucose during the transformation from free-living cercariae to parasitic schistosomula. We have cloned a set of cDNAs encoding proteins involved in glucose metabolism to allow us to examine the expression of these genes during this transformation. We first obtained and characterized Schistosoma mansoni cDNA clones encoding the tricarboxylic acid cycle enzyme, mitochondrial malate dehydrogenase (SMDH) and the mitochondrial encoded electron transport protein, cytochrome oxidase subunit 1 (SCOX1). Northern blots were then prepared using mRNA isolated from whole cercariae, cercarial tails, schistosomula, adult males and adult females. The Northern blots were successively hybridized with a variety of probes including those for SMDH, SCOX, the glycolytic enzymes, hexokinase, triosephosphate isomerase and glyceraldehyde-3-phosphate dehydrogenase and several control probes. Probes were additionally hybridized to mRNA dot blots and the signals were quantified using storage phosphor technology. These studies reveal that transcripts encoding these metabolic enzymes are localized at much higher levels in cercarial tails than in whole cercariae or transformed schistosomula, and support the notion of a dominant aerobic metabolism in tails. Male and female adult worms express each of the mRNAs at roughly equal levels. Adults express the metabolic mRNAs, including those involved in oxidative glucose metabolism, at relatively high levels suggesting that adult schistosomes retain a significant capacity to produce energy through aerobic metabolism.

The facultative anaerobic energy metabolism of Schistosoma mansoni sporocysts

Schistosoma mansoni miracidia in water are known to possess an aerobic energy metabolism, the Krebs cycle being the main terminal of the breakdown of endogenous glycogen reserves. The present study demonstrated that after in vitro transformation of miracidia into sporocysts, the organisms degraded glucose to lactate and carbon dioxide in a more anaerobic ratio than do miracidia. The occurrence of a large Pasteur effect demonstrated, however, that oxidative phosphorylation was still the major process used for energy generation. After 24 h in vitro cultivation the sporocysts had consumed more external glucose and their metabolism had shifted towards lactate production. Sporocysts could cope with inhibited respiration: they had a large anaerobic capacity and survived perfectly in the presence of cyanide, producing a large amount of succinate in addition to lactate. It was demonstrated that this succinate was largely produced via phosphoenolpyruvate carboxykinase (PEPCK). This pathway, which is known to occur in most parasitic helminths, has never been demonstrated in schistosomes, not even in the miracidial stage immediately preceding the sporocysts. It was also shown that in sporocysts part of the lactate was not formed directly by glycolysis, but via a detour including fumarate and the action of PEPCK. The results demonstrated that S. mansoni sporocysts are facultative anaerobes, fully equipped to adjust their energy metabolism to the variable conditions inside their intermediate host, the snail. In the presence of oxygen, they derive most of their energy from the aerobic degradation of glucose to carbon dioxide, but under anaerobic conditions they switch towards lactate and succinate production.

The reversible effect of glucose on the energy metabolism of Schistosoma mansoni cercariae and schistosomula

This study on isolated cercarial bodies demonstrates that the biological transformation from cercaria to schistosomulum and the biochemical transition from an aerobic to an anaerobic energy metabolism are separate processes, which are not necessarily linked. The metabolic transition depends on the external glucose concentration and is fully reversible. In the presence of only a tracer amount of [6-14C]glucose, carbon dioxide was the major end product, but at higher glucose concentrations mainly lactate was formed. This effect could be demonstrated in cercarial bodies in water as well as in fully transformed schistosomula. In non-transformed cercariae a change towards a more anaerobic energy metabolism could be induced by an increase in the external glucose concentration, which demonstrated that the biochemical transition can occur in the absence of the biological transformation. Furthermore, the biological transformation can occur without a concomitant biochemical transition: in the presence of 5 mM glucose, lactate production by cercarial bodies during transformation was increased 50-fold, whereas in the presence of only a tracer amount of glucose the metabolic profile remained that of cercariae. Also, in fully transformed schistosomula, this transition to a more anaerobic energy metabolism was induced by increased glucose concentrations, but at low glucose concentrations carbon dioxide was the major end product, as in cercariae. The effect of external glucose on the metabolism was fully reversible. After a high glucose concentration had induced a more anaerobic metabolism in cercariae in water, the metabolism returned to an aerobic one upon removal of the glucose. Likewise, the metabolism in schistosomula switched back and forth between anaerobic and aerobic patterns, following successive changes in the glucose concentration.(ABSTRACT TRUNCATED AT 250 WORDS)