Glycogen metabolism in Schistosoma mansoni worms after their isolation from the host

Schistosoma mansoni does not and cannot oxidise fatty acids, but these are used for biosynthetic purposes instead

Adult schistosomes, parasitic flatworms that cause the tropical disease schistosomiasis, have always been considered to be homolactic fermenters and, in their energy metabolism, strictly dependent on carbohydrates. However, more recent studies suggested that fatty acid β-oxidation is essential for egg production by adult female Schistosoma mansoni. To address this conundrum, we performed a comprehensive study on the lipid metabolism of S. mansoni. Incubations with [¹⁴C]-labelled fatty acids demonstrated that adults, eggs and miracidia of S. mansoni did not oxidise fatty acids, as no ¹⁴CO₂ production could be detected. We then re-examined the S. mansoni genome using the genes known to be involved in fatty acid oxidation in six eukaryotic model reference species. This showed that the earlier automatically annotated genes for fatty acid oxidation were in fact incorrectly annotated. In a further analysis we could not detect any genes encoding β-oxidation enzymes, which demonstrates that S. mansoni cannot use this pathway in any of its lifecycle stages. The same was true for Schistosoma japonicum and all other schistosome species that have been sequenced. Absence of β-oxidation, however, does not imply that fatty acids from the host are not metabolised by schistosomes. Adult schistosomes can use and modify fatty acids from their host for biosynthetic purposes and incorporate those in phospholipids and neutral lipids. Female worms deposit large amounts of these lipids in the eggs they produce, which explains why interference with the lipid metabolism in females will disturb egg formation, even though fatty acid β-oxidation does not occur in schistosomes. Our analyses of S. mansoni further revealed that during the development and maturation of the miracidium inside the egg, changes in lipid composition occur which indicate that fatty acids deposited in the egg by the female worm are used for phospholipid biosynthesis required for membrane formation in the developing miracidium.

Transcriptional profiles of adult male and female Schistosoma japonicum in response to insulin reveal increased expression of genes involved in growth and development

Microarray analysis was used to investigate differential gene regulation in adult male and female Schistosoma japonicum cultured in the presence or absence of insulin in vitro. A total of 1,101 genes were up- or down-regulated in response to insulin, the majority of differential expression occurring 24h after the addition of insulin to the cultures. Genes differentially expressed in male or female worms were predominantly involved in growth and development, with significant sex-specific differences in transcriptional profiles evident. Insulin appeared to promote protein synthesis and control protein degradation more prominently in male parasites. The study also indicated that insulin plays a more pronounced role in the uptake of glucose in unpaired female parasites, as reflected in the increased stimulation of gene expression of the phosphatidylinositol 3-kinase sub-pathway of insulin signalling. Insulin may also impact on the sexual differentiation and fecundity of female schistosomes by activation of the mitogenic-activated protein kinase sub-pathway.

The incorporation, modification and turnover of fatty acids in adult Schistosoma mansoni

Schistosoma mansoni is known to be unable to synthesize fatty acids and sterols de novo, but the parasite is capable of synthesizing phospholipids and triacylglycerols from precursors obtained from the host. The present study focuses on the dynamics of the incorporation of fatty acids in adult parasites. This study showed that fatty acids were rapidly metabolized into complex lipids and that oleate (18:1) was efficiently converted to eicosenoate (20:1) by chain elongation, whereas palmitate was not elongated at an appreciable rate. This chain elongation mainly involved fatty acids that were previously esterified to complex lipids. Furthermore it was shown that in adult parasites triacylglycerols do not serve as fatty-acyl donors in phospholipid synthesis as had been suggested to be the case in schistosomula, because: (1) Immediately after pulse-labelling the specific activity of fatty acids in phospholipids was higher than in triacylglycerols; and (2) the specific activity of eicosenoate, which had been formed by chain elongation of incorporated oleate. was higher in phospholipids than in triacylglycerols. Fatty acids that were esterified to phospholipids had a high turnover, in contrast to fatty acids esterified to triacylglycerols, which persisted for extended periods of time in this lipid class (days rather than hours).

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

It has been shown that in intact cercariae of Schistosoma mansoni in water, both head and tail had an identical, aerobic energy metabolism. As long as the environment was water, glucose was mainly degraded to carbon dioxide by both head and tail whether or not these two were still connected to each other. Transfer of intact cercariae into a simple salt medium supplemented with glucose resulted in a very rapid transition towards a more anaerobic energy metabolism: the production of lactate and pyruvate increased, whereas the production of carbon dioxide remained more or less constant. A concomitant rise in temperature to 37 degrees C was not essential for this biochemical transition, but made it more pronounced. Experiments on isolated cercarial bodies and tails in a transforming medium demonstrated that the tail oxidized glucose to carbon dioxide, whereas bodies produced mainly pyruvate and lactate. The results showed that the metabolic transition towards a more anaerobic energy metabolism occurred only in the head and not in the tail of the cercariae. Loss of the tail was shown not to be a pre-requisite for this transition, nor did it by itself trigger a metabolic switch in the resulting cercarial body.

Schistosoma mansoni: biochemical characteristics of the antischistosomal effects of Ro 15-5458

We evaluated a variety of biochemical parameters in Schistosoma mansoni isolated from mice up to 4 days after dosing with 15 mg/kg Ro 15-5458. While no drug effect could be demonstrated in the utilization of media glucose, glycogen content, gut pigment, or ATP levels of the parasites, a significant reduction (P less than or equal to 0.05) in parasite weight and protein content was observed. Possible drug actions that may contribute to the loss in parasite protein and perhaps ultimately result in parasite death have been investigated. We noted significant reduction in the incorporation of leucine and thymidine into acid-insoluble fractions of the parasites. The reduction in the incorporation of leucine into parasite proteins was nonspecific and preceded the effect of the drug on the uptake of the amino acid. Parasite and host liver RNA isolated after dosing were translated in vitro in a rabbit reticulocyte system. Drug-treated parasite mRNA, but not that of the host, was less effective than control mRNA in directing the incorporation of [35S]methionine. We propose a hypothesis that attributes the loss in protein content to a defect in the biosynthesis of parasite proteins as a result of a drug-induced reduction in the quantity of mRNA in the parasites This effect of Ro 15-5458 on the parasite may provide the basis for its schistosomicidal action.

Continuous synthesis of glycogen by individual worm pairs of Schistosoma mansoni inside the veins of the final host

Hamsters infected with Schistosoma mansoni were operated upon to install a permanent canula into their blood stream. After recovery of the hamster, this canula was used for the injection of radioactively labelled glucose. In this way the glycogen metabolism of S. mansoni could be studied while the parasites remained undisturbed in their natural habitat. The consecutive injection of [U-14C]glucose and [1-3H]glucose permitted an analysis of possible changes in the glycogen synthesis of individual worm pairs with time. The results showed that the synthesis of glycogen by each worm pair was fairly constant with time. Furthermore, all individual worm pairs synthesised glycogen continuously; not even 2 min passed without its formation. Only small differences in glycogen synthesis were observed between parasites isolated from different locations in the veins of the hamster. These results exclude the possibility that the worm pairs had alternating periods of glycogen synthesis and degradation, and they also disprove the idea that synthesis and degradation occur at two different sites in the bloodstream of the hamster. The experiments further showed that glycogen synthesis was proportional to the amount of glycogen already present, which in turn was shown to be proportional to the size of the parasite. From this study it can be concluded that the replenishment of the endogenous glycogen reserves of S. mansoni is not induced by a marked decrease in the glycogen levels, but occurs slowly and continuously.

Carbohydrate metabolism in adult schistosomes of different strains and species

In this comparative study the carbohydrate metabolism of Schistosoma japonicum, S. haematobium, S. intercalatum, S. bovis and three strains of S. mansoni was investigated. No large differences were found in the protein and glycogen contents of all species involved. In all species investigated, lactate was the main end product of carbohydrate breakdown. However, all parasites degraded part of the glucose to CO2 via the Krebs cycle. No difference was observed in the contribution of this aerobic process to energy production in the three strains of S. mansoni investigated. The differences in Krebs-cycle activity between the five schistosome species investigated were very small. Therefore, this study refutes the idea that significant differences exist in the carbohydrate metabolism of various schistosome species or strains.

Aerobic to anaerobic transition in the carbohydrate metabolism of Schistosoma mansoni cercariae during transformation in vitro

Schistosoma mansoni cercariae in water were shown to possess a largely aerobic energy metabolism, the Krebs cycle being the main terminal of carbohydrate breakdown. A metabolic transition towards a more anaerobic breakdown of carbohydrate could be achieved by incubation conditions which also stimulated biological transformation. Incubation of cercariae in a simple salt medium containing 5 mM glucose induced such a metabolic transition: beside carbon dioxide large amounts of lactate and pyruvate were excreted. The results indicate that the production of pyruvate was coupled to electron transfer in the respiratory chain. Some aspects of this unusual pyruvate production are discussed. The observed change in the end-product pattern of carbohydrate breakdown is very rapid: most of the switch occurred within 2 h. Our results show that the metabolic transition was triggered by the biological transformation itself, or by the same event that induces the biological transformation. The metabolic and the biological changes proceeded synchronously.

Synthesis and degradation of glycogen by Schistosoma mansoni worms in vitro

The glycogen stores of adult Schistosoma mansoni worms could be labelled by incubation of the worms, after an initial reduction of their glycogen content, in the presence of [6-14C]glucose. Subsequent breakdown of the labelled glycogen by the parasite revealed that glycogen was degraded to lactate and carbon dioxide. The degradation of glycogen, as compared to that of glucose, resulted in slightly different ratios of these two end-products. This indicates that glycogen breakdown did not replace glucose breakdown to the same extent in all cells and that Krebs-cycle activity was not uniformly distributed throughout the cells of this parasite. Both fructose and mannose could replace glucose as an energy source and the rate of glycogen synthesis from either of these two carbohydrates was higher than from glucose. No indications for glyconeogenesis from C3-units were found. Glycogen metabolism of S. mansoni was not influenced by hormones of the mammalian host. It is regulated by the external glucose concentration and by the level of the endogenous glycogen stores. Studies on paired and unpaired worms showed that no interaction between male and female was necessary for the synthesis of glycogen by female worms.

The energy metabolism of Schistosoma mansoni during its development in the hamster

A detailed study was made of the changes in the carbohydrate metabolism of Schistosoma mansoni occurring during both the penetration of the skin of a hamster and the subsequent development of the schistosome in the lung, liver, and mesenteric veins of the host. During infection, within a few hours a transition occurs from a fully aerobic to a largely anaerobic energy metabolism. By 5 h postinfection, about 6% of carbohydrate breakdown occurs in the aerobic reactions of the Krebs cycle, whereas the rest occurs in the anaerobic formation of lactate. The contribution of aerobic processes to carbohydrate breakdown remains at this level of 6% until 3 weeks postinfection and then gradually declines to the adult level of 2.5%. Measurement of the protein content of developing schistosomes shows that an exponential growth occurs over a 15-day period after the arrival of the schistosomes in the liver (days 11-25 postinfection). During this period the protein content of the parasites increases about 100-fold, but despite this change in size, no major changes occur in the end-product pattern of carbohydrate breakdown. We conclude that during this period the rate of oxygen diffusion into the tissues is not a limiting factor for aerobic metabolism. A limited diffusion of oxygen may play a role in the decreasing contribution of aerobic processes during the later stages of maturation of the schistosomes.