Digenetic trematodes: quinone tanning system in eggshells

Isolation, enrichment and primary characterisation of vitelline cells from Schistosoma mansoni obtained by the organ isolation method

In the emerging era of post-genomic research on schistosomes, new methods are required to functionally analyse genes of interest in more detail. Among other tools, schistosome cell lines are needed to overcome present research constraints. Based on a recently established organ isolation protocol for adult Schistosoma mansoni, we report here on the successful enrichment of vitellarium tissue and isolation of vitelline cells. Morphological analyses performed by bright field, fluorescence, scanning and transmission electron microscopy showed typical features of S1 to S4 stage vitelline cells. In addition, molecular analyses using reverse transcription-PCR confirmed the identity of vitelline cells. Cytological and physiological studies included staining experiments with viability dyes and a neutral lipid stain, as well as calcium (Ca2+) imaging. Together they demonstrated cell viability, the possibility to define the differentiation stage of individual vitelline cells, and the suitability to investigate Ca(2+)-associated processes herein. Finally, fluorescence-activated cell sorting was shown to be a convenient way to separate and enrich S1 to S4 stage vitelline cells. In summary, these results demonstrate the expedience of the organ isolation protocol to obtain vitellarium tissue. Importantly, the protocol allows vitelline cells representing defined differentiation stages to be purified, which can be cultured in vitro and used to investigate diverse aspects of schistosome reproductive biology in the post-genomic era.

Histochemical Study of the Progenetic Trematode Alloglossidium renale

A histochemical study of the progenetic trematode Alloglossidium renale has demonstrated the absence of any secreted material between the adult worm and the host (freshwater shrimp) antennal gland tubules. Host tissue is affected only by the compression, abrasion, and ingestion by the parasite, and host tubule cells near the worm have the same staining patterns as those distant from the parasite. The trematode sometimes dies within the host, leaving a necrotic mass whose histochemical staining differs significantly from the living organism. In the necrotic mass, the only recognizable features were the ova and the vitellarium, which atrophied and resulted in tyrosine-positive staining within the mass. A melanin reaction was not observed in the host using a specialized ferro-ferricyanide stain. The only apparent host response to infection was a layer of damaged squamous host cells adhering to the necrotic worm. The results confirm benign host-parasite effects and a highly evolved relationship between the host and parasite, perhaps bordering on commensalism.

siRNA-mediated knockdown of two tyrosinase genes from Schistosoma japonicum cultured in vitro

The cross-linking process of eggshell proteins in helminths is dependent on the activities of tyrosinases (TYRs), which can be inhibited by phenol oxidase inhibitors. Two genes encoding TYRs, SjTYR1 and SjTYR2, have been identified in Schistosoma japonicum. In this study, siRNA-mediated RNA interference (RNAi) was performed to silence these two SjTYR genes to evaluate their roles in eggshell formation. The effects of individual or double knockdown of the SjTYR genes were compared by determining SjTYR1/SjTYR2 transcript levels, enzyme activities, and by observing the morphology and amounts of intrauterine eggs. Results showed that SjTYR transcript levels were significantly reduced on the 3rd day post-RNAi. Significant reductions in TYR enzyme activities, as well as obvious changes in morphology and the number of intrauterine eggs followed the reductions in SjTYR transcript levels. On the 8th day after simultaneous knockdown of both SjTYR genes, which effected a 40% reduction in SjTYR1 transcript level and a 59% reduction in SjTYR2 transcript level, we observed an 80% reduction in diphenol oxidase (DPO) activity of TYRs, and a 74% reduction in the number of normal eggs in female uteri. Knockdown of both SjTYR genes has a greater effect than single knockdown of the SjTYR genes. These results demonstrate that both SjTYRs play an important role in eggshell sclerotization of S. japonicum, and that their enzyme activities depend on the transcript levels of two SjTYR genes.

Studies on the enzyme phenol oxidase in a freshwater monogenetic trematode

Cytochemical localization of the enzyme phenol oxidase inNeomurraytrema tengrahas been studied. Results reveal that the enzyme reacts with substrates such as catechol, hydroquinone, pyrogallol, dopa, doparmine, epinephrine and tyramine, but not with tyrosine and protocatechuic acid. Thus it shows activity with a wide range of phenols, aminated, mono and diphenols and also with deaminated and decarboxylated, di- and polyphenols. The maximum activity of the enzyme occurs between 40°C and 50°C with a pH optimum of 6–6.

Expression and characterization of two tyrosinases from the trematode Schistosoma japonicum

Tyrosinase (TYR) was thought to play a critical role during trematode egg production. In this study, we analyzed two genes (SjTYR1 and SjTYR2), derived from Schistosoma japonicum genome databases, which encode proteins with significant homologies to mammalian TYR. They exhibited the typical TYR topology, including two copper-binding domains and a highly conserved cysteine-rich domain. Semi-quantitative reverse transcription polymerase chain reaction showed that two SjTYR genes were mainly expressed in the female adult worm. A complementary DNA coding the putative common copper center domain of each SjTYR was cloned and inserted into a pET-28a-c(+) prokaryotic expression vector. After purification, the recombinant proteins expressed in Escherichia coli were used to produce their specific antibodies. The native active SjTYRs enzyme appeared to function as a homodimer, the subunits of which were linked to each other via covalent disulfide bonds. Both female and male worms possessed monophenol oxidase and diphenol oxidase activities of TYR. The relative enzymatic activities were 0.165 min(-1) mg(-1) and 0.0805 min(-1) mg(-1), which were inhibited by a copper-chelating agent (allyl thiourea) and correlated with disruption of female egg production. Our results revealed that SjTYRs might play a significant role during eggshell formation.

Schistosoma mansoni: females enhance [14C]-tyrosine incorporation in males maintained in vitro

Incorporation of radiolabelled tyrosine was examined in males of Schistosoma mansoni maintained in vitro. Adult worms were recovered from mice and only those that were paired were used. Males and females were separated and placed in 4 degrees C RPMI 1640 containing 10% fetal calf serum (FCS). Individual males were incubated for 1 h either alone or with a female. Other males were incubated in a medium containing 40 ng ml-1 recombinant tumour necrosis factor alpha (rTNF alpha). Subsequently, males were incubated with or without females for 1 h in a medium containing 20 muCi ml-1 [14C]-tyrosine. All incubations were carried out in RPMI 1640 containing 10% FCS at 37 degrees C under 5% CO2. Untreated single males incorporated small amounts of labelled tyrosine. Pretreatment of single males with rTNF alpha did not affect their tyrosine incorporation (P = 0.693). However, males maintained in vitro with females incorporated 221% more tyrosine than the controls (P = 0.00008), suggesting that a chemical stimulus from the female affects tyrosine incorporation in males.

Sex determination and female reproductive development in the genus Schistosoma: a review

Parasites of the genus Schistosoma were among the first metazoans to develop separate sexes, which is chromosomally determined in the fertilized egg. Despite the occurrence of specific sex chromosomes, the females of most Schistosomatidae species do not complete their somatic development and reach no sexual maturity without the presence of males. Indeed, the most controversial and at the same time most fascinating aspect about the sexual development of Schistosoma females lies on discover the nature of the stimulus produced by males that triggers and controls this process. Although the nature of the stimulus (physical or chemical) is a source of controversy, there is agreement that mating is a necessary requirement for maturation to occur and for migration of the female to a definitive final site of residence in the vascular system of the vertebrate host. It has also been proposed that the stimulus is not species-specific and, in some cases, not even genus-specific. Despite a vast literature on the subject, the process or processes underlying the meeting of males and females in the circulatory system have not been determined and as yet no consensus exists about the nature of the stimulus that triggers and maintains female development. In the studies about their role, Schistosoma males have been considered, at times pejoratively, the brother, the muscles or even the liver of females. Indeed, it still remains to be determined whether the stimulus responsible for female maturation involves the transfer of hormones, nutrients, neuromediators, mere tactile stimulation or a combination of chemotactic and thigmotactic factors.

Histological analysis of the egg capsule of the ovoviviparous polystomatid monogenean, Pseudodiplorchis americanus

Transmission of Pseudodiplorchis americanus is restricted to the brief period when its host, a desert toad, enters water to spawn. The parasite accumulates its entire annual reproductive output within an elongated uterus during the 10-11-month period of host hibernation. Embryos of P. americanus, at all stages of development, are retained within the uterus which eventually becomes packed with around 150 encapsulated infective larvae. Recently formed eggs, which comprise a fertilized ovum and 2-3 vitelline cells, are closely surrounded by a primary eggshell which stains positively for acidic proteins and keratin. Initially, during passage along the proximal uterus, the egg capsule is only 60 microns in diameter, but as it passes to the distal uterus it expands to 800 microns in diameter to accommodate the growing larva. Due to chemical alterations or complete replacement of the shell, the final (secondary) egg capsule is a large sac-like structure composed of elastin. The flexible nature of this shell maximizes the numbers of infective larvae which can be stored in utero. TEM studies have revealed this capsule to be composed of multi-laminate membranes with a specialized cytoplasmic lining involved in a unique mechanism for embryo nutrition. This is the first report of an elastin-type eggshell within the Monogenea.

Histochemical studies on eggshell formation in Paramphistomum cervi (Digenea: Paramphistomatidae)

The formation of the egg mass in the ootype, containing the oocyte, vitelline cells, numerous spermatozoa and Mehlis' gland secretion marks the beginning of eggshell formation. Mehlis' gland cells pour their lipoprotein secretion into the ootype; this secretion forms a thin template around the egg mass and upon this template the shell globules are deposited. Eggshell formation begins in the proximal uterus and is completed in the middle uterus. The eggshells are devoid of phenols and phenolase indicating that hardening is not by quinone tanning. The eggshells contain keratin and are stabilized by disulphide linkages.

The subcellular localization of labelled tyrosine in the vitelline cells of Schistosoma mansoni

Autoradiographic studies at light and electron microscope level showed that tritiated tyrosine injected into the peritoneal cavity of infected mice localised in the cells of adult Schistosoma mansoni within one hour. The amino acid was avidly taken up by vitelline cells compared to other tissues. Subcellular localization was in the granular endoplasmic reticulum and in the vitelline droplets. The label was also present in the tissues of the tegument and gut but the route of entry into the parasite was not determined.