Effects of insulin on Schistosomatium douthitti

Sexual Preferences in Nutrient Utilization Regulate Oxygen Consumption and Reactive Oxygen Species Generation in Schistosoma mansoni: Potential Implications for Parasite Redox Biology

Schistosoma mansoni, one of the causative agents of human schistosomiasis, has a unique antioxidant network that is key to parasite survival and a valuable chemotherapeutic target. The ability to detoxify and tolerate reactive oxygen species increases along S. mansoni development in the vertebrate host, suggesting that adult parasites are more exposed to redox challenges than young stages. Indeed, adult parasites are exposed to multiple redox insults generated from blood digestion, activated immune cells, and, potentially, from their own parasitic aerobic metabolism. However, it remains unknown how reactive oxygen species are produced by S. mansoni metabolism, as well as their biological effects on adult worms. Here, we assessed the contribution of nutrients and parasite gender to oxygen utilization pathways, and reactive oxygen species generation in whole unpaired adult S. mansoni worms. We also determined the susceptibilities of both parasite sexes to a pro-oxidant challenge. We observed that glutamine and serum importantly contribute to both respiratory and non-respiratory oxygen utilization in adult worms, but with different proportions among parasite sexes. Analyses of oxygen utilization pathways revealed that respiratory rates were high in male worms, which contrast with high non-respiratory rates in females, regardless nutritional sources. Interestingly, mitochondrial complex I-III activity was higher than complex IV specifically in females. We also observed sexual preferences in substrate utilization to sustain hydrogen peroxide production towards glucose in females, and glutamine in male worms. Despite strikingly high oxidant levels and hydrogen peroxide production rates, female worms were more resistant to a pro-oxidant challenge than male parasites. The data presented here indicate that sexual preferences in nutrient metabolism in adult S. mansoni worms regulate oxygen utilization and reactive oxygen species production, which may differently contribute to redox biology among parasite sexes.

Schistosoma genomics: new perspectives on schistosome biology and host-parasite interaction

Schistosomiasis, caused mainly by Schistosoma japonicum, S. mansoni, and S. hematobium, remains one of the most prevalent and serious parasitic diseases worldwide. The blood flukes have a complex life cycle requiring adaptation for survival in fresh water as free-living forms and as parasites in snail intermediate and vertebrate definitive hosts. Functional genomics analyses, including transcriptomic and proteomic approaches, have been performed on schistosomes, in particular S. mansoni and S. japonicum, using powerful high-throughput methodologies. These investigations have not only chartered gene expression profiles across genders and developmental stages within mammalian and snail hosts, but have also characterized the features of the surface tegument, the eggshell and excretory-secretory proteomes of schistosomes. The integration of the genomic, transcriptomic, and proteomic information, together with genetic manipulation on individual genes, will provide a global insight into the molecular architecture of the biology, pathogenesis, and host-parasite interactions of the human blood flukes. Importantly, these functional genomics analyses lay a foundation on which to develop new antischistosome vaccines as well as drug targets and diagnostic markers for treatment and control of schistosomiasis.

In vivo response of Mesocestoides vogae to human insulin

SUMMARY

Successful host invasion by parasitic helminths involves detection and appropriate response to a range of host-derived signals. Insulin signal response pathways are ancient and highly-conserved throughout the metazoans. However, very little is known about helminth insulin signalling and the potential role it may play in host-parasite interactions. The response of Mesocestoides vogae (Cestoda: Cyclophyllidea) larvae to human insulin was investigated, focusing on tyrosine-phosphorylation status, glucose content, survival and asexual reproduction rate. Parasite larvae were challenged with different levels of insulin for variable periods. The parameters tested were influenced by human insulin, and suggested a host-parasite molecular dialogue.

Diversification of the insulin receptor family in the helminth parasite Schistosoma mansoni

Insulin signalling is a very ancient and well conserved pathway in metazoan cells, dependent on insulin receptors (IR) which are transmembrane proteins with tyrosine kinase activity. A unique IR is usually present in invertebrates whereas two IR members are found with different functions in vertebrates. This work demonstrates the existence of two distinct IR homologs (SmIR-1 and SmIR-2) in the parasite trematode Schistosoma mansoni. These two receptors display differences in several structural motifs essential for signalling and are differentially expressed in parasite tissues, suggesting that they could have distinct functions. The gene organization of SmIR-1 and SmIR-2 is similar to that of the human IR and to that of the IR homolog from Echinococcus multilocularis (EmIR), another parasitic platyhelminth. SmIR-1 and SmIR-2 were shown to interact with human pro-insulin but not with pro-insulin-like growth factor-1 in two-hybrid assays. Phylogenetic results indicated that SmIR-2 and EmIR might be functional orthologs whereas SmIR-1 would have emerged to fulfil specific functions in schistosomes.

Growth factor receptors in helminth parasites: Signalling and host-parasite relationships

Parasitic helminths remain major pathogens of both humans and animals throughout the world. The success of helminth infections depends on the capacity of the parasite to counteract host immune responses but also to exploit host-derived signal molecules for its development. Recent progress has been made in the characterization of growth factor receptors of various nematode and flatworm parasites with the demonstration that transforming growth factor beta (TGF-beta), epidermal growth factor (EGF) and insulin receptor signalling pathways are conserved in helminth parasites and potentially implicated in the host-parasite molecular dialogue and parasite development.

Schistosome transcriptomes: new insights into the parasite and schistosomiasis

Schistosomiasis is one of the most serious parasitic diseases. More than 250 million people are infected with schistosomes in the tropics or subtropics. The parasitic flukes have some unique biological features: dioecism, complex life cycles, mechanisms to avoid host immune responses, and an apparent reliance on host endocrine and immune signals to complete their development, maturation and egg production. Recently, a large dataset of expressed sequence tags (ESTs) were generated from Schistosoma japonicum and Schistosoma mansoni, from which numerous novel genes were identified. The transcriptome analyses provide the basis for a comprehensive understanding of the molecular mechanisms involved in schistosome nutrition and metabolism, host-dependent development and maturation, immune evasion and invertebrate evolution. In addition, new potential vaccine candidates and drug targets have been predicted.

An unusual receptor tyrosine kinase of Schistosoma mansoni contains a Venus Flytrap module

Previous studies have suggested that successful development of the parasitic helminth Schistosoma mansoni must be dependent on an adaptative molecular dialogue with its hosts and on the existence of receptors for growth factors and hormones. Attempts to identify a homolog of the insulin receptor (IR) have led us to characterize a new receptor tyrosine kinase (RTK) molecule in S. mansoni. SmRTK-1 is an integral membrane protein with a single membrane-spanning sequence separating an extracellular ligand-binding domain and a cytoplasmic TK domain. Structural and phylogenetic analyses of the kinase domain of SmRTK-1 confirmed its similarity to IR catalytic domains. However, sequence analysis of the extracellular domain of SmRTK-1 revealed similarity with various proteins (such as drug receptors) that share a structure known as the Venus Flytrap (VFT) module. Alignment with other VFT modules for which the structure has been solved was used to generate a 3D model of the putative VFT module of SmRTK-1. Phylogenetic analysis indicated that the SmRTK-1 VFT module was closer to that of the GABA(B) receptor. Numerous RTK genes recently discovered in vertebrate and invertebrate species code for large families of modular proteins with diverse structures and ligand-binding specificities. SmRTK-1 probably represents a new class of RTK whose function remains to be determined. RTKs are present in all metazoans and associated with the control of metabolism, growth and development. The preferential localization of SmRTK-1 in sporocyst germinal cells and ovocytes could be in favor of its function in schistosome growth and differentiation.

Hormones and nuclear receptors in schistosome development

A substantial but disparate body of evidence suggests that hormones affect the development of schistosomes within their definitive hosts. Here, Raymond Pierce and colleagues review such evidence for host steroid and thyroid hormones, and for ecdysteroids, and link this to the expanding knowledge of the nuclear receptors for these hormones. Phylogenetic analysis of the nuclear receptor superfamily and the characterization of the first schistosome nuclear receptors suggest that steroids and thyroid hormone probably act indirectly, or by pathways not involving the control of gene transcription. However, the probability that schistosome nuclear receptors exist for a variety of unique ligands opens up exciting possibilities for targeted drug development.

Schistosoma mansoni: insulin independence

Several authors have reported that schistosomes are affected by host hormones, including insulin. We found that insulin in a 10,000-fold range of concentrations failed to affect glucose consumption of males, females, or pairs of Schistosoma mansoni incubated for periods up to 24 hr. Insulin, vasopressin, or tetradecanoyl phorbol acetate (an insulin mimic) did not affect the uptake and incorporation of [14C]glucose and [14C]deoxyglucose over the active transport range. Competitive binding studies using 125I-insulin and 125I-insulin-like growth factor II, with varying concentrations of unlabeled hormones, demonstrated only nonspecific binding to intact worms or subcellular fractions. This generalized nonspecific uptake of label was also shown by autoradiography, suggesting that specific insulin receptors are absent from S. mansoni. We conclude that plasma glucose levels are sufficient to supply the metabolic requirements of schistosomes by diffusion, and that no insulin-dependent mechanisms have evolved in these parasites.