Schistosoma mansoni: sex-specific modulation of parasite growth by host immune signals

Development of female schistosomes from infectious cercariae to mature egg-producing adults requires both male schistosomes and an intact adaptive immune system. By examining single sex infections in immunodeficient mice, we provide evidence that female schistosome development is not directly influenced by the adaptive immune system, whereas male development is. Our data are consistent with a sequential model of schistosome development, where the adaptive immune system signals development of mature males, which subsequently stimulate development of mature females. The male schistosome therefore appears to play a central role both in transducing signals from the adaptive immune system and in facilitating female development.     

Tegumental glucose permeability in male and female Schistosoma mansoni

Tegumental hexose transporters have been kinetically characterized in mated and separated male and female Schistosoma mansoni 8-12 wk postinfection. Significant gender-specific differences in Km and Vmax were observed. In mated males, the estimated constants (mean +/- SE) were: Km = 0.63 +/- 0.31 mM, Vmax = 0.93 +/- 0.44 nmol/mg worm water/min, and the Kd = 0.25 +/- 0.09 microliter/mg worm water/min. In mated females the kinetics were: Km = 0.99 +/- 0.40 mM, Vmax = 1.22 +/- 0.42 nmol/mg worm water/min, and Kd = 0.60 +/- 0.14 microliter/mg worm water/min. The influx of 2-deoxy-D-glucose and 3-O-methylglucose has been similarly characterized; these analogs share the same glucose transporter in male and female schistosomes. 2-Deoxy-D-glucose has a higher affinity, and 3-O-methylglucose a lower affinity, than does glucose. Because mated male schistosomes supply glucose to female partners, similarities between the free glucose concentration of the male and the affinity of the transporter determined for mated female schistosomes suggest that male-to-female transfer may be a potentially rate-limiting step in glucose utilization by the female. Permeability x surface are (PS) products and Vmax/Km ratios were significantly elevated in mated schistosomes, suggesting that the transporter is primarily localized to the dorsal surface of the male. Gender- and mating-specific analyses of PS products indicate that tegumental permeability to glucose is significantly increased in mated schistosomes, and compares very favorably to that of the host liver.     

Extended survival and reproductive potential of single-sex male and female Schistosoma japonicum within definitive hosts

Schistosomiasis is caused by dioecious helminths of the genus Schistosoma. Recent work indicated that unpaired female and male schistosomes can survive within their definitive host for at least 1 year, although the viability or fertility of these worms after subsequent pairing remained untested. We performed two experiments on laboratory mice, one with female Schistosoma japonicum exposure first and male schistosomes second and another vice versa. After surviving as single-sex unpaired forms for up to 1 year, 58.5% of male and 70% of female schistosomes were able to mate and produce viable eggs. This highlights an additional biological challenge in achieving elimination of schistosomiasis.     

Failure of in vitro-cultured schistosomes to produce eggs: how does the parasite meet its needs for host-derived cytokines such as TGF-β?

When adult schistosome worm pairs are transferred from experimental hosts to in vitro culture they cease producing viable eggs within a few days. Female worms in unisexual infections fail to mature, and when mature adult females are separated from male partners they regress sexually. Worms cultured from the larval stage are also permanently reproductively defective. The cytokine transforming growth factor beta derived from the mammalian host is considered important in stimulating schistosome female worm maturation and maintenance of fecundity. The means by which schistosomes acquire TGF-β have not been elucidated, but direct uptake in vivo seems unlikely as the concentration of free, biologically active cytokine in host blood is very low. Here we review the complexities of schistosome development and male–female interactions, and we speculate about two possibilities on how worms obtain the TGF-β they are assumed to need: (i) worms may have mechanisms to free active cytokine from the latency-inducing complex of proteins in which it is associated, and/or (ii) they may obtain the cytokine from alpha 2-macroglobulin, a blood-borne protease inhibitor to which TGF-β can bind. These ideas are experimentally testable.     

Differential adhesive properties of the surface of human blood flukes Schistosoma mansoni. Sexual, regional, and developmental differences in interfacial free energies and structure of glycocalyx

While infecting a vertebrate host, blood flukes (Schistosoma mansoni) must continually resist adhesions by immune effector cells. However, the male and female schistosomes must adhere to one another in order to establish and maintain the sexual pairing process after 4 wk postinfection. Using a contact angle method, the relative adhesiveness of male and female parasites were determined. Results indicate that schistosomes restrict effector cell adhesion through developmental, sexual, and regional differences in adhesive properties.     

Differential adhesive properties of the surface of human blood flukesSchistosoma mansoni

While infecting a vertebrate host, blood flukes (Schistosoma mansoni) must continually resist adhesions by immune effector cells. However, the male and female schistosomes must adhere to one another in order to establish and maintain the sexual pairing process after 4 wk postinfection. Using a contact angle method, the relative adhesiveness of male and female parasites were determined. Results indicate that schistosomes restrict effector cell adhesion through developmental, sexual, and regional differences in adhesive properties.     

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.     

Herbimycin A suppresses mitotic activity and egg production of female Schistosoma mansoni

The eggs of the endoparasite Schistosoma are the causative agent of schistosomiasis, an important disease of humans, which is endemic in (sub-) tropical regions. The absence of a vaccine with sufficient protective qualities and increasing resistance to approved and established drugs like praziquantel, justify the exploration of novel ways to fight schistosomes. Our strategy is based on interference with the sexual maturation of the female. Prerequisites for gonad development in adult females are a continuous pairing contact with the male and significantly increased mitotic activity. In this study we show that the male governs sexual maturation of the female, as the separation of couples causes a clear reduction of female mitotic activity and, consequently, egg production. We demonstrate that treatment of schistosomes with Herbimycin A, an inhibitor of protein tyrosine kinases (PTKs), mimics the separation of couples as the drug blocks mitotic activity and egg production of paired females. However, the synthesis of the eggshell precursor protein p14 is elevated. Furthermore, we show for the first time in invertebrates that Herbimycin A decreases tyrosine phosphorylation and PTK stability in schistosomes. Summarised, our data provide evidence that PTKs have key functions in regulating gonad development, eggshell gene expression and, consequently, egg production. Therefore, we suggest envisaging schistosome PTKs as novel targets for strategies to combat schistosomiasis.     

MicroRNAs Are Involved in the Regulation of Ovary Development in the Pathogenic Blood Fluke Schistosoma japonicum

Schistosomes, blood flukes, are an important global public health concern. Paired adult female schistosomes produce large numbers of eggs that are primarily responsible for the disease pathology and critical for dissemination. Consequently, understanding schistosome sexual maturation and egg production may open novel perspectives for intervening with these processes to prevent clinical symptoms and to interrupt the life-cycle of these blood-flukes. microRNAs (miRNAs) are key regulators of many biological processes including development, cell proliferation, metabolism, and signal transduction. Here, we report on the identification of Schistosoma japonicum miRNAs using small RNA deep sequencing in the key stages of male-female pairing, gametogenesis, and egg production. We identified 38 miRNAs, including 10 previously unknown miRNAs. Eighteen of the miRNAs were differentially expressed between male and female schistosomes and during different stages of sexual maturation. We identified 30 potential target genes for 16 of the S. japonicum miRNAs using antibody-based pull-down assays and bioinformatic analyses. We further validated some of these target genes using either in vitro luciferase assays or in vivo miRNA suppression experiments. Notably, suppression of the female enriched miRNAs bantam and miR-31 led to morphological alteration of ovaries in female schistosomes. These findings uncover key roles for specific miRNAs in schistosome sexual maturation and egg production.     

The sex lives of parasites: Investigating the mating system and mechanisms of sexual selection of the human pathogen Schistosoma mansoni

The mating systems of internal parasites are inherently difficult to investigate although they have important implications for the evolutionary biology of the species, disease epidemiology, and are important considerations for control measures. Using parentage analyses, three topics concerning the mating biology of Schistosoma mansoni were investigated: the number of mates per adult male and female, variance in reproductive success among individuals, and the potential role for sexual selection on male body size and also mate choice for genetically dissimilar individuals. Results indicated that schistosomes were mostly monogamous, and evidence of only one mate change occurred over a period of 5-6 weeks. One male was polygynous and contained two females in its gynecophoral canal although offspring were only detected for one of the females. Even though they were primarily monogamous and the sex ratio near even, reproductive success was highly variable, indicating a potential role for sexual selection. Male body size was positively related to reproductive success, consistent with sexual selection via male-male competition and female choice for large males. However, relatedness of pairs was not associated with their reproductive success. Finally, genetically identical individuals differed significantly in their reproductive output and identical males in their body size, indicating important partner and environmental effects on these traits.     

Schistosoma mansoni, S. japonicum, S. haematobium: glycogen content and glucose uptake in parasites from fasted and control hosts

The glycogen content of male and female Schistosoma mansoni has been measured in flukes from normally fed hosts and those from fasted hosts. In infections from both the mouse and the hamster, a significant reduction in schistosomal glycogen of males is seen hours after food is withdrawn from the host. Reductions in protein content of the schistosomes were only observed in hamster infections fasted at least 72 hr. The livers of infected mice not only decrease in size during fasting, but there is a concomitant reduction in glycogen per unit wet weight. Comparisons of glycogen:protein ratios of mansonian males, females, and host livers indicate that the fasting-induced loss of liver glycogen is also observed in the male schistosome, but not the female. Studies of both S. mansoni and S. haematobium pairs from fed hosts suggest that the ratio of glycogen:protein contents in the male schistosome correlates with the glycogen:protein ratio of the female partner. Measurements of glucose uptake in vitro suggest that greater uptake rates may be observed in flukes perfused from fasted hosts. In S. japonicum from infected mice, a reduction in male glycogen was also detected as early as after a 6-hr fasting period, but changes in the females were not significant. Unmated male S. japonicum also exhibit a reduction in glycogen levels after fasting, but the quantity of worm glycogen present in these males remains higher than comparable mated males. In mice entrained to a regulated pattern of available food, fluctuations in glycogen content of the male schistosomes were observed, but in the female partners fluctuations were of a smaller magnitude.(ABSTRACT TRUNCATED AT 250 WORDS)     

Correlation between lipid partition coefficients and surface permeation inSchistosoma japonicum

Summary Comparison of transintegumental membrane permeability and partition coefficients of selected nonelectrolytes was attempted to correlate the parameters of lipid solubility, hydrophilicity, and membrane permeation in male and female schistosomes (parasites of the portal venous tributaries of man). Surface permation (measured by the triple isotope technique) and octanol/water partition coefficients were determined for 17 compounds (acetamide, aminopyrine, antipyrine, benzyl alcohol, butanol, caffeine, ethanol, ethylene glycol, glycerol, inosine, mannitol, methanol, polyethylene glycol, propylene glycol, sucrose, thiourea, and urea).Linear regression analyses comparing the logarithm of the partition coefficient to transintegumental uptakes indicate a positive correlation in both sexes:R=0.76 (P<0.001) for males, andR=0.77 (P<0.001) for females. Similarly, linear regression analyses comparing hydrogen bond number with the logarithm of tissue uptake index demonstrate a high (negative) correlation in both males (R=–0.85,P<0.001) and females (R=–0.90,P<0.001). The male and female schistosomes showed no statistically significant differences in correlation of these parameters. Surface permeation was the same in male and female schistosomes, suggesting that male-female variations in integumental uptake rates previously observed may be restricted to metabolites which enter by way of a selective carrier system.     

Schistosoma mansoni: TGF-beta signaling pathways

Schistosome parasites have co-evolved an intricate relationship with their human and snail hosts as well as a novel interplay between the adult male and female parasites. We review the role of the TGF-beta signaling pathway in parasite development, host-parasite interactions and male-female interactions. The data to date support multiple roles for the TGF-beta signaling pathway throughout schistosome development, in particular, in the tegument which is at the interface with the host and between the male and female schistosome, development of vitelline cells in female worms whose genes and development are regulated by a stimulus from the male schistosome and embryogenesis of the egg. The human ligand TGF-beta1 has been demonstrated to regulate the expression of a schistosome target gene that encodes a gynecophoric canal protein in the schistosome worm itself. Studies on signaling in schistosomes opens a new era for investigation of host-parasite and male-female interactions.     

Chromosomes of Heterobilharzia americana (Digenea: Schistosomatidae), with ZWA sex determination, from Louisiana

Mitotic chromosomes of Heterobilharzia americana from Louisiana are described from parasite material that was dissected from snails, air-dried on slides, and stained with conventional Giemsa and C-band methods. As in other schistosomes, the female is the heterogametic sex. This Louisiana strain, however, differs from a Texas strain and other schistosome species in that the male and female have different diploid numbers of chromosomes (male, 20; female, 19), and the strain has a ZZ male/ZWA female sex-determining mechanism. The chromosomes of the male resemble those of the Texas strain in number and morphology with the Z chromosomes being metacentric and the largest elements in the karyotype. The others form a series decreasing in size to very small number 10's. Chromosomes 2,3, and 4 are subtelocentric; 5 is subtelocentric to acrocentric and is satellited; 6 is submetacentric to subtelocentric; 7 is submetacentric; 8 is subtelocentric to submetacentric; 9 is metacentric to submetacentric; and 10 is metacentric. The female complement differs from the male of this strain in having only 1 normal chromosome 5. The other number 5 and most of the original W apparently have fused tandemly to form the WA chromosome (a "neo-W").     

Reconstruction and in silico analysis of the MAPK signaling pathways in the human blood fluke, Schistosoma japonicum

At present, little is known about signal transduction mechanisms in schistosomes, which cause the disease of schistosomiasis. The mitogen-activated protein kinase (MAPK) signaling pathways, which are evolutionarily conserved from yeast to Homo sapiens, play key roles in multiple cellular processes. Here, we reconstructed the hypothetical MAPK signaling pathways in Schistosoma japonicum and compared the schistosome pathways with those of model eukaryote species. We identified 60 homologous components in the S. japonciumMAPK signaling pathways. Among these, 27 were predicted to be full-length sequences. Phylogenetic analysis of these proteins confirmed the evolutionary conservation of the MAPK signaling pathways. Remarkably, we identified S. japonicum homologues of GTP-binding protein beta and alpha-I subunits in the yeast mating pathway, which might be involved in the regulation of different life stages and female sexual maturation processes as well in schistosomes. In addition, several pathway member genes, including ERK, JNK, Sja-DSP, MRAS and RAS, were determined through quantitative PCR analysis to be expressed in a stage-specific manner, with ERK, JNK and their inhibitor Sja-DSP markedly upregulated in adult female schistosomes.     

Transintegumental uptake of metabolic substrates in male and female Schistosoma mansoni.

A new method for measuring transintegumental uptake in living schistosomes in vitro has been applied to the study of individual males and females. Uptake of a 14-C labeled test metabolite was compared to that of tritiated water (a highly diffusible reference substance). Use of the short half-life (T 1/2 = 100 min) isotope 113m-Indium, bound to EDTA (ethylene diamine tetra-acetic acid, a nondiffusible reference substance) permitted quantification of the relative amount of 14-C test substance passively adhering to the schistosoma surface. Substraction of this amount provided an estimate of net uptake. D-glucose uptake, as measured by this method, increased with time, approaching equilibrium by two min; a positive correlation between temperature and glucose uptake was also observed. Nondialyzable components in rat, human, horse and fetal calf sera did not enhance glucose uptake. In both male and female schistosomes, minimal uptakes were seen for the nonmetabolizable sugar alcohol mannitol (MW = 182). L-glucose uptake was similarly low, but high uptakes were observed in both sexes for D-glucose. In addition to confirming the stereospecificity of hexose uptake, these studies suggested our technique provides a sensitive method for measurement of both high and low uptake compounds. The uptakes of D-glucose and the L-amino acids--arginine, ornithine, lysine, histidine, phenylalanine and serine--were comparatively higher in female than male schistosomes. Slight elevations in uptake by females were observed for threonine, valine and glycine, but aspartate uptake was slightly higher in males. No dramatic male-female differences were immediately apparent for the uptakes of proline, leucine, isoleucine, tyrosine and glutamate. Schistosomal uptake of L-amino acids that are essential for vertebrates was generally higher than uptake of the nonessential amino acids.