Molecular and functional characterization of glucose transporter genes of the fox tapeworm Echinococcus multilocularis

Alveolar echinococcosis (AE) is a zoonotic parasitosis caused by larvae of the fox tapeworm, Echinococcus multilocularis. E. multilocularis is distributed widely in the Northern hemisphere, causing serious health problems in various animals and humans. E. multilocularis, like other cestodes, lacks a digestive tract and absorbs essential nutrients, including glucose, across the syncytial tegument on its external surface. Therefore, it is hypothesized that E. multilocularis uses glucose transporters on its surface similar to a closely-related species, Taenia solium. Based on this hypothesis, we cloned and characterized glucose transporter homologues from E. multilocularis. As a result, we obtained full-length sequences of 2 putative glucose transporter genes (EmGLUT1 and EmGLUT2) from E. multilocularis. In silico analysis predicted that these were classified in the solute carrier family 2 group. Functional expression analysis using Xenopus oocytes demonstrated clear uptake of 2-deoxy-D-glucose (2-DG) by EmGLUT1, but not by EmGLUT2 in this experimental system. EmGLUT1 was shown to have relatively high glucose transport activity. Further analyses using the Xenopus oocyte system revealed that 2-DG uptake of EmGLUT1 did not depend on the presence or concentration of Na⁺ nor H⁺, respectively. Immunoblot analyses using cultured metacestode, ex vivo protoscolex, and adult worm samples demonstrated that both EmGLUTs were stably expressed during each developmental stage of the parasite. Based on the above-mentioned findings, we conclude that EmGLUT1 is a simple facilitated glucose transporter and possibly plays an important role in glucose uptake by E. multilocularis throughout its life cycle.

Pathophysiology ofHymenolepis diminutainfections inTenebrio molitor:effect of parasitism on haemolymph proteins

The effects of metacestodes ofHymenolepis diminutaon haemolymph proteins ofTenebrio molitorare restricted to female hosts. In beetles aged 15 days post-emergence, and harbouring 12-day-old metacestodes, haemolymph concentration is 46·7% higher than in non-infected animals and similar effects are found in longer standing infections. Electrophoresis of haemolymph revealed the presence of 13 bands. Densitometric analysis showed that only band 2/3 was significantly elevated in infected hosts although band 7/8 also showed an increase. These bands were also present in egg homogenates and are thought to be vitellogenins. It is therefore proposed that the excess protein found in infected beetles is a female-specific protein resulting from an interaction between the parasite and the host endocrine system.

Tenebrio molitor(Coleoptera): effect of metacestodes ofHymenolepis diminuta(Cestoda) on haemolymph amino acids

Haemolymph free amino acids (HFAAs) in adultTenebrio molitorvaried in concentration with beetle age. Between 9 and 30 days post-emergence the concentration range was 34–94 mMwith a peak occurring in 15-day-old insects. Much of this age-related variation was due to changes in the concentration of a single amino acid, proline. An analysis of concentrations of individual HFAAs in 15-day-old beetles, following infection with metacestodes ofHymenolepis diminuta, revealed that changes in female insects are more marked than those occurring in males. In the former, changes occurred in the concentration of 9 amino acids (isoleucine, leucine, arginine, serine, threonine, tyrosine, phenylalanine, proline and alanine/citrulline) whereas in the latter only 4 amino acids were affected (threonine, glycine, histidine and arginine). It is suggested thatH. diminutamay interact with the endocrine system of its beetle host.

Manipulation of host food availability and use of multiple exposures to assess the crowding effect onHymenolepis diminutainTribolium confusum

We studied the ‘crowding effect’ inTribolium confusuminfected withHymenolepis diminuta. Manipulations included age and number of parasites, and diet, sex, age and number of exposures of hosts. Volume per parasite was unaffected until an intensity of at least 5–10 parasites per host, then declined approximately inversely as intensities increased. Parasite size was affected by host sex but not age or reproductive status. Host diet affected parasite size and the impact of crowding. Daily gain in parasite volume peaked partway through the developmental period and preceded the first evidence of a crowding effect. Parasites that established during a second exposure had a transient developmental delay but eventually grew as large or larger than parasites from a single exposure with the same total intensity. Parasites responded to crowding by differential allocation of resources. Cercomer volume decreased even with slight crowding, the capsule surrounding the scolex was not reduced until crowding became more severe, and scolex width was reduced only in the most extreme conditions. The data support the hypothesis that the crowding effect in this system is driven primarily by nutrient, rather than space limitations.

Ontogenetic changes in helminth membrane function

During their life-cycle many parasites experience a wide range of environments including free living and those provided by a variety of intermediate and final hosts. The nutritional requirements of parasites are met by physiological processes adapted to exploit the physicochemical characteristics provided by different hosts. In helminth parasites these adaptations are frequently expressed on the tegumentary surface. As an example of adaptations within the Trematoda, the control of monosaccharide transport in Proterometra sp. is described. Environmental sodium, although not directly involved in the uptake process, nevertheless regulates the expression of transport capabilities. In the Cestoda, the uptake of monosaccharides and amino acids is described for Hymenolepis diminuta. The metacestode of this tapeworm inhabits the blood system of an arthropod, and the adult the gut of a mammal. There are quantitative and qualitative differences in the amino acids and monosaccharides in these two environments and these are reflected in the transport mechanisms exhibited by the two forms of the life-cycle. In Echinococcus granulosus the transfer of amino acids, sugars and macromolecules across the membranes of hydatid cysts and protoscoleces is described. The major difference between these two stages in the life-cycle relates to the ability of hydatid cysts to absorb macromolecules, whereas protoscoleces are impermeable to these compounds. The potential for future work is emphasized.

Experimental infection of Tribolium confusum (Coleoptera) by Hymenolepis diminuta (Cestoda): host fecundity during infection

Some effects of Hymenolepis diminuta on the fecundity of Tribolium confusum are described. Host fecundity is observed to be reduced exponentially with increasing parasite burden/host, although there are differences in the ability of individual hosts to respond to parasitism. Of particular interest is the finding that host fecundity is greatly reduced in young beetles on or by day 14 post-infection (p.i.). This age-related reduction in host fecundity is discussed in relation to the population dynamics of this host-parasite relationship.

Interactions of the cysticercoids of Hymenolepis diminuta and raillietina cesticillus in their intermediate host, Tribolium confusum

This study was based on the experimental exposure of beetles to homologous and heterologous infections of Hymenolepis diminuta and Raillietina cesticillus. The results demonstrated that, for both species over the range of parasite densities employed, the presence of a primary infection had no effect on the establishment success of an homologous challenge infection. The establishment success of R. cesticillus cysticercoids was not affected by the presence of an H. diminuta infection. The existence of an R. cesticillus infection, however, severely reduced the establishment success of an H. diminuta infection. The reduction in H. diminuta establishment was greatest in R. cesticillus infections less than 10 days old. The plausibility of some of the mechanisms that are potentially responsible for the reduction in H. diminuta establishment success is discussed, as is the possible ecological significance of these results.

The effect of worm age, weight and number in the infection on the absorption of glucose by Hymenolepis diminuta

In Hymenolepis diminuta the in vitro rate of absorption of glucose/unit dry weight of worm falls with increasing worm age, with increasing worm weight and as the number of worms in an infection is increased. In a 6 mM solution of glucose, a 5 mg (dry weight) worm from a 7 or 8 worm infection absorbed 80 µmoles/g dry weight/5 min whereas a 60 mg worm, also from a 7 or 8 worm infection, absorbed only 35µmoles/g dry weight/5 min. This change in the rate of absorption is, at least partly, thought to be due to changes in the relative surface area: weight ratio during growth of the worm.

The kinetic parameter, Kt glucose, increased from 1.1 mM for a 5 mg (dry weight) worm from a 7 or 8 worm infection to 2 mM for a 60 mg worm. This change in the functioning of the glucose transport system may indicate that there are two components of the glucose transport system – or two separate systems – one with a low Kt and one with a high Kt, the ratios of which change during worm growth.

The smaller the number of worms in an infection the greater the rate of glucose absorption. Using 8–day–old worms in a 6 mM glucose solution, 1 worm from a single worm infection absorbs 111 μmoles/g dry weight/5 min, 1 worm from a 7 or 8 worm infection absorbs 88 μmoles/g dry weight/5 min and 1 worm from a 45–50 worm infection absorbs 77 μmoles/g dry weight/5min. The significance of this is discussed with reference to the ‘crowding effect’ in tapeworms.