Adult schistosomes depend for growth and development on hormonal signals from the mammalian host, which may include the insulin signalling pathway. In this project, I firstly used microarray analysis to demonstrate that human insulin can be utilised by adult S. japonicum in culture, resulting in the modulation of distinct metabolic effects as reflected in transcriptional levels of parasite genes. The addition of insulin resulted in the differential expression of 1,101 genes with many related to functions corresponding to the biological and metabolic effects of insulin reported for mammalian cells. Those identified genes in male or female S. japonicum worms that were up or down regulated after exposure to insulin were predominantly involved in growth and development, with significant sex-specific responses evident. Insulin appeared to play a similar role in male parasites as those seen in classical mammalian systems including an increase in protein synthesis though gene transcription and the stimulation of mRNA translation and control protein degradation via the ubiquitin proteasome pathway. Microarray analysis indicated that insulin not only leads to increased gene expression of the PI3-K pathway, which enhances parasite growth, but may also play a role in the sexual differentiation and fecundity of female worms by activating the MAPK pathway. As the insulin target proteins, two types of insulin receptors from Schistosoma japonicum were isolated, S. japonicum insulin receptors 1 (SjIR-1) and 2 (SjIR-2), with features similar to insulin receptors from other taxa. The sequences share 70% and 74% identity to S. mansoni insulin receptor 1 and 2 (SmIR-1 and SmIR-2), respectively. SjIR-1 and SjIR-2 are highly conserved in their tyrosine kinase domain to other IRs from Homo, Mus musculus and Drosophila melanogaster. SjIR-2 is located in the parenchyma in males and in the vitelline glands of female worms, which occupy most of male or female tissue and play an important role in growth or fecundity. In contrast, SjIR-1 was located in the tegument and intestinal epithelium of adult worms, representing much smaller cellular regions compared with the voluminous vitelline tissue or parenchyma. This observation was further confirmed by real time PCR showing that SjIR-2 was more abundantly expressed in S. japonicum adult worm than SjIR-1. Phylogenetic analysis showed that SjIR-2 and SmIR-2 are closer to EmIR than to SjIR-1 and SmIR-1, indicating that SjIR-1 and SmIR-1 might perform specific functions in schistosomes, while SjIR-2, SmIR-2 and EmIR might share similar roles in parasite growth and development in the three parasitic flatworms. Structure modelling recovered the conserved structure between the SjIRs and Homo sapiens IR (HIR) implying a common predicted binding mechanism in the ligand domain and the same downstream signal transduction processing in the tyrosine kinase domain as in HIR. Two-hybrid analysis was used to confirm that the ligand domains of SjIR-1 and SjIR-2 contain the insulin binding site. Incubation of adult worms in vitro, both with a specific insulin receptor inhibitor and anti-SjIRs antibodies, resulted in a significant decrease in worm glucose levels, suggesting again the same function for SjIRs in regulating glucose uptake as described for mammalian cells. Adult worms of S. japonicum possess insulin receptors that can specifically bind to insulin, indicating that the parasite can utilize host insulin for development and growth by sharing the same pathway as mammalian cells in regulating glucose uptake. In vaccination/challenge trials, there was no significant reduction in adult worm burdens with either of the SjLD vaccines. However, there were significant reductions in mean lengths of adult worms ranging from 22-25% in the SjLD1 vaccinated group to 37-42% in the SjLD2 vaccinated groups, significant reductions in faecal eggs in both the SjLD1 (66%) and SjLD2 (68%) vaccinated groups, and a reduction in liver egg numbers in the SjLD1(33%) vaccinated group. These results show that although the SjLDs vaccines were unable to reduce adult worm numbers by clearing them from the vaccinated mice, nevertheless, they significantly depressed the growth of male and female adult worms and affected female egg production. The protective efficacy obtained in terms of the substantial decrease in faecal eggs exceeded that of many of the recently available schistosome antigens and prototype vaccine formulations, which, at best, elicit 40–50% protection in animals using the standard readouts of reduced worm burden or egg production and viability. Overall, disruption of this insulin pathway leading to parasite starvation through the prevention of glucose uptake thereby affecting parasite growth, development and female fecundity, provides a new intervention target and transmission blocking approach to combat schistosomiasis and may be applicable for the control of other debilitating parasitic infections as well.
| Identifer | oai:union.ndltd.org:ADTP/285427 |
| Creators | Hong You |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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