The chemical nature of the egg-shell of helminths. I. Absence of quinone tanning in the egg-shell of the liver fluke, Fasciola hepatica

Vitelline gland histochemistry in the commensal temnocephalidParacaridinicola platei(Fernando, 1952) Baer, 1953, together with some notes on the egg

Histochemical tests indicate the presence of tanning precursors namely protein, phenols and phenolase in the vitelline glands of the temnocephalidParacaridinicola platei. This suggests the occurrence of a tanned egg shell in this organism, a feature it shares with many other parasitic and free living platyhelminths. The large eggs are laid singly and are attached to the gill lamellae of the host.

Stabilization of egg-shell inParaplerurus sauridae(Digenea: Hemiuridae)

The newly formed egg-shell from eggs in the proximal part of the uterus ofParaplerurus sauridaeis colourless and transparent, whereas the shell from eggs in the distal uterus is golden yellow in colour. Histochemical tests reveal the presence of phenol, protein and phenolase in the freshly formed egg-shell. The egg-shell from eggs in the distal uterus shows the property of sclerotin. Histochemical reactions, chromatography and spectrum analysis reveal the presence of a dityrosyl linkage in the egg-shell. The nature of the protein component in the egg-shell is discussed.

The identification of a Clonorchis sinensis gene encoding an antigenic egg protein

The cDNA library of Clonorchis sinensis was screened for genes encoding antigenic proteins by using sera from clonorchiasis patients. A gene of 888 bp encoding a 28-kDa protein (Cs28) was cloned and found to contain a high percentage of glycine (20%), tyrosine (11%), and lysine (11%). The amino acid sequence of Cs28 showed 60% homology with the vitelline B precursor protein of Opisthorchis viverrini and of 33% homology with the vitelline B1 and B2 proteins of Fasciola hepatica. A strong positive reaction was observed in the intrauterine eggs of adult C. sinensis by immunohistochemical analysis using specific immune sera against recombinant Cs28 protein (rCs28). By immunoblot analysis, rCs28 displayed an antigenic reaction with 73% of the serum samples from 115 cases of clonorchiasis. In addition, it cross-reacted with the sera of 77.5% of 40 opisthorchiasis cases, 90% of 20 schistosomiasis cases, and 50% of 10 paragonimiasis cases. However, no cross-reactions were observed with the sera of sparganosis or cysticercosis patients. In conclusion, the Cs28 protein was identified as an egg protein of C. sinensis and as an antigen common to the trematode species examined.

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.

Amino acid metabolism in helminths

Amino acids are major constituents of biological material. Chemically they are extremely stable and combine a relatively simple molecular structure with a wide range of properties and functions. In general, amino acid metabolism in helminths has been relatively neglected and the information available is often uneven and of uncertain quality. However, the search for new target sites for anthelmintic development has led to a renewed interest in this area. The amino acid composition of helminths is similar to that of other invertebrates and no unique amino acids have been reported. With the possible addition of tyrosine, helminths seem to require the same 10 essential amino acids as mammals and, where studied in detail, the pathways of amino acid synthesis in helminths are similar to those of mammals. Although amino acids are not a significant energy source in parasites, helminths are able to catabolize amino acids by pathways which, again, appear identical to those found in mammals. Helminths have also been shown to carry out a number of oxidative reactions associated with amino acid metabolism, including cysteine dioxygenase, proline hydroxylase and tryptophan hydroxylase. There are, however, differences in detail between the pathways of amino acid metabolism in helminths and mammals, particularly in the metabolism of the sulphur amino acids and arginine and proline. These differences may be exploitable in anthelmintic design and proline analogues and proline biosynthesis inhibitors show some potential as fasciolicides (Sheers et al., 1982). Differences in metabolism between parasites and their hosts may be the result of parasitic adaptation or they may merely reflect general features of the invertebrate phyla as a whole. Thus a comparison of amino acid metabolism in parasitic helminths with that of their free-living relatives may give some insight into the biochemical basis of parasitism.

Cloning and characterization of a female genital complex cDNA from the liver fluke Fasciola hepatica

A cDNA clone whose RNA is abundant in the female genital complex of the liver fluke Fasciola hepatica has been isolated from a cDNA library in lambda gt10 by differential screening. The pattern of expression in different fluke tissues and at different stages of miracidium formation suggests that this gene is expressed in the F. hepatica vitelleria. The nucleotide sequence of the cloned cDNA was determined and the primary structure of the putative protein was deduced. The proposed protein is rich in glycine, lysine, and tyrosine and its overall amino acid composition agrees with that reported for the F. hepatica egg shell. The clone has homology with DNA from other trematodes; this homology is higher in organisms in which egg development is similar to that of F. hepatica and suggests that the protein is conserved in organisms in which miracidium formation occurs in fresh water.

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 production and functional morphology of helminth egg-shells

The high energy costs of egg-shell production in many helminths suggests that this structure plays an important role in their biology. The mechanical and chemical resistance of the egg-shell and the barrier it provides to the entry and loss of material are important in the survival of the free-living stages within the egg. The presence of egg filaments may increase the availability of infective stages and the operculum may be important in the infective process.

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.