Major Human Parasite Research Articles

The Brugia malayi genome project: expressed sequence tags and gene discovery

To advance and facilitate molecular studies of Brugia malayi, one of the causative agents of human lymphatic filariasis, an expressed sequence tag (EST)-based gene discovery programme has been carried out. Over 22 000 ESTs have been produced and deposited in the public databases by a consortium of laboratories from endemic and non-endemic countries. The ESTs have been analysed using custom informatic tools to reveal patterns of individual gene expression that may point to potential targets for future research on anti-filarial drugs and vaccines. Many genes first discovered as ESTs are now being analysed by researchers for immunodiagnostic, vaccine and drug target potential. Building on the success of the B. malayi EST programme, significant EST datasets are being generated for a number of other major parasites of humans and domesticated animals, and model parasitic species.

Current progress in the development and use of artemether for chemoprophylaxis of major human schistosome parasites.

Human schistosomiasis, a chronic and debilitating parasitic disease of the tropics, is ranked second after malaria in terms of public health importance. At present, there is no vaccine available, and chemotherapy is the cornerstone of schistosomiasis control. Praziquantel is the drug of choice. Oxamniquine has become difficult to obtain and metrifonate has recently been withdrawn from the market. Rapid re-infection following treatment and concern about praziquantel resistance called for the search of novel drugs for prevention and cure of schistosomiasis. Significant progress has been made with artemether, the methyl ether of dihydroartemisinin, already widely used for the treatment of malaria. The present article reviews the literature that led to the development of artemether for chemoprophylaxis in schistosomiasis, and it summarises the experiences so far obtained with its use to control schistosomiasis in different endemic settings. Topics covered include an overview of the global burden of schistosomiasis and approaches for its control; the nature and features of artemisinin and related derivatives, initially discovered as antimalarials, other bioactivities, and their recent discovery of antischistosomal properties; a historic account disclosing the antischistosomal activity of artemether; in vivo assessment of drug susceptibility of different developmental stages of schistosome parasites; artemether-induced pathology evidenced by scanning and transmission electron microscopy; the possible mechanism of action; in vivo studies with combination therapy of artemether and praziquantel; results of randomised controlled clinical trials of oral artemether for the prevention of patent infection and morbidity; and, ultimately the translation of this knowledge into public health action in different endemic settings towards a more integrated approach of schistosomiasis control.

Particle selection by snail hosts of human schistosomes in relation to their survival strategies and possible control by ‘environmental antibodies’

Summary Experiments were undertaken with Biomphalaria glabrata to (i) elucidate the mechanisms used for selecting, regulating and utilizing sand particles and (ii) evaluate particulate controlled‐release formulations (CRF), or ‘environmental antibodies’, to control target snails. The snails selected sand particles by size, mass and density. The numbers and mass of sand particles in the guts increased with snail size, and fed snails had significantly more sand particles in their guts than deprived snails. Snails benefited from ingesting sand particles. Increasing the weights of 63–125‐µm sand particles from 8·5 mg to 150–170 mg resulted in progressive, significant, increases in the numbers of sand particles in the various gut compartments and also enhanced snail growth. Snails did not discriminate between sand and glass particles of the same size. When offered unwashed acidic sand there was a lag of 5–6 h before ingestion, but this could be reduced by preconditioning in media rich in base cations or chemical factors of snail origin. Under equilibrium conditions ingested sand constituted approximately 2·5% of the total snail weight but this amount was exceeded when snails were presented with large quantities of sand. The parts of the gut involved in sorting and regulating the turnover of sand particles and food were described. The gizzard, caecal and digestive gland strings traversed the gut as far as the rectum in approximately 2·8 h, 6–8 h and 8 h, respectively. Coprophagy by the snails facilitated the recycling of sand and its retention in the gut. Optimally sized, 63–250‐µm, protein microcapsules containing butanoate and maltose were ingested by snails in significantly greater amounts than untreated control microcapsules. The addition of copper caused the capsules to become repellent. These results will advance the design of effective, ecologically acceptable, environmental antibodies in the form of controlled‐release microcapsules to control schistosome snails. This work illustrates how a combination of detailed study of snail anatomy, feeding experiments and ecological understanding can contribute to the control of the intermediate hosts of a major human parasite.

Building on foundations

edited by G.D. Schmidt and J. Janovy, Jr,

The range and biological activity of FMRFamide-related peptides and classical neurotransmitters in nematodes

Nematodes include both major parasites of humans, livestock and plants in addition to free-living species such as Caenorhabditis elegans. The nematode nervous system (especially in C. elegans) is exceptionally well defined in terms of the number, location and projections of the small number of neurons in the nervous system and their integration into circuits involved in regulatory behaviours vital to their survival. This review will summarize what is known about the biological activity of neurotransmitters in nematodes: the biosynthetic pathways and genes involved, their receptors, inactivation mechanisms and secondary messenger signalling systems. It will cover the 'classical' transmitters, such as acetylcholine (ACh), GABA, glutamate, serotonin, dopamine, octopamine, noradrenaline and nitric oxide. The localization of peptides throughout the nematode nervous system is summarized, in addition to the isolation of nematode neuropeptides by both traditional biochemical techniques and more modern genetic means. The major contribution of the completion of the C. elegans genome-sequencing program is highlighted throughout. Efforts to unravel neurotransmitter action in various physiological actions such as locomotion, feeding and reproduction are detailed as well as the various inactivation mechanisms for the current complement of nematode transmitters.

The initiation and function of Th2 responses during infection with Schistosoma mansoni.

Through experiments in which pro-inflammatory mediators have been neutralized in schistosomeinfected WT and IL-4 -/- mice we have been investigating the nature of the immune response that is required to allow survival during the period of acute disease that accompanies the onset of egg production by the parasitic worms. The developing picture is that of an early pro-inflammatory type-1 like response (characterized by the production of IFN-gamma and TNF-alpha and NO) which in the short term is able to control the deleterious effects on the liver associated with the arrival of the eggs, followed rapidly by a potent egg-induced Th2 response which simultaneously subsumes the role of protecting the liver and downregulates the production of the inflammatory mediators, thereby averting the serious consequence associated with the continued high level production of NO. Current research is directed towards: 1) understanding how pro-inflammatory mediators are protective during schistosomiasis, and 2) elucidating the underlying mechanisms through which the Th response is biased in a Th2 direction following exposure to schistosome eggs.

Proteins with Glycosylphosphatidylinositol (GPI) Signal Sequences Have Divergent Fates during a GPI Deficiency: GPIs ARE ESSENTIAL FOR NUCLEAR DIVISION IN TRYPANOSOMA CRUZI

Glycosylphosphatidylinositols (GPIs) are membrane anchors for cell surface proteins of several major protozoan parasites of humans, including Trypanosoma cruzi, the causative agent of Chagas' disease. To investigate the general role of GPIs in T. cruzi, we generated GPI-deficient parasites by heterologous expression of T. brucei GPI-phospholipase C. Putative protein-GPI intermediates were depleted, causing the biochemical equivalent of a dominant-negative loss of function mutation in the GPI pathway. Cell surface expression of major GPI-anchored proteins was diminished in GPI-deficient T. cruzi. Four proteins that are normally GPI-anchored in T. cruzi exhibited different fates during the GPI shortage; Ssp-4 and p75 were secreted prematurely, while protease gp50/55 and p60 were degraded intracellularly. These observations demonstrate that secretion and intracellular degradation of GPI-anchored proteins may occur in the same genetic background during a GPI deficiency. We postulate that the interaction between a protein-GPI transamidase and the COOH-terminal GPI signal sequence plays a pivotal role in determining the fate of these proteins. At a nonpermissive GPI deficiency, T. cruzi amastigotes inside mammalian cells replicated their single kinetoplast but failed at mitosis. Hence, in these protozoans, GPIs appear to be essential for nuclear division, but not for mitochondrial duplication.

Complete sequence of the Schistosoma mansoni glyceraldehyde-3-phosphate dehydrogenase gene encoding a major surface antigen

Schistosomiasis, which is caused by the intravascular helminth Schistosoma mansoni, is a major human parasite infection affecting a number of subtropical countries [1]. The interface between host and parasite has been extensively studied with the aim of identifying targets for vaccine-induced immunological attacks. Three reports have identified major targets of immunity on the surface of the parasite as enzymes, namely a glutathione Stransferase (GST) [2], a triose phosphate isomerase (TPI) [3,4] and a glyceraldehyde-3phosphate dehydrogenase (GAPDH), a 37-kDa antigen the cDNA of which has been cloned [5]. S. rnansoni GAPDH has been characterized as a major antigen for antibodies in the sera of the most resistant subjects to schistosomiasis [6] and has been detected on intact living schistosomula by immunofluorescence [5].

Recognition of Plasmodium falciparum asexual stage antigens by antibodies in sera from people exposed to Plasmodium vivax.

An analysis of Plasmodium falciparum antigen-specific antibodies was performed on 44 serum samples from Guatemala, a region endemic for P. vivax. Most sera showed positive reactivity to P. falciparum asexual stage antigens by indirect immunofluorescent assay, enzyme-linked immunosorbent assay, and immunoprecipitation analysis using biosynthetically labeled parasites. Although several antigens were recognized by these sera, proteins with molecular weights of 195 kD, 140 kD, and in the range of 70-80 kD were strongly recognized by many of the sera studied. No such reactivity was observed for any of the surface antigens in the male and female gametes and zygotes of P. falciparum. These studies suggest that P. vivax and P. falciparum, two major human parasites, may share certain epitopes in several antigens of immunologic significance.

Plasmodium vivax malaria: parasite biology defines potential targets for vaccine development

Although progress in the development of an antimalarial vaccine has been mostly obtained through the study of P. falciparum, significant advances have recently been made in the study of P. vivax, the other major human malarial parasite. Antigens which have been shown to be important in P. falciparum have been characterized and in some cases cloned in P. vivax. Other studies have examined some of the more specific biological characteristics of P. vivax. Among these are studies on components present in caveolae--vesicle complexes of the infected erythrocyte, on the occurrence of delayed hepatic development leading to relapse, or on the Duffy erythrocyte antigen as a key receptor for parasite invasion. Although progress has been made in the short-term in vitro maintenance of P. vivax, the inability to maintain the parasite in continuous culture led to the investigation of wild parasite populations in patients; occurrence of extensive antigenic and karyotype polymorphism was detected in this way, as was a double-blocking and enhancing activity of human antibodies on parasite development in the vector. The association of monoclonal antibodies with DNA recombinant technology allowed the characterization of a number of P. vivax antigens to be made. Among these, an antigen shared between sexual and asexual stages was shown to constitute a target for transmission-blocking immunity. The cloning of an antigen involved in transmission-blocking immunity, along with that of the surface antigen of the sporozoite (CSP) and of a major surface antigen of the invasive merozoite (PV200) constitutes a significant step towards the development of a multivalent recombinant vaccine against P. vivax.

Lectin-like polypeptides of P. falciparum bind to red cell sialoglycoproteins.

Attempts to control human malaria by immunological means could be compromised by antigenic variability within and between different strains of malarial parasites1. A useful alternative approach might be to block parasite antigens which are important in the mechanisms of invasion of red cells. As the major human parasite Plasmodium falciparum is highly specific for human red cells, isolation of the proteins involved in the recognition of red cells by this parasite might be of particular value. Recent studies suggest that the major red cell sialoglycoproteins (SGPs), glycophorins A, B and possibly C, may carry the sites recognized by the parasite2-4. Furthermore, because certain carbohydrates present on SGPs such as N-acetylglucosamine are able to block invasion by the parasite5, they may be involved in the initial interaction between parasite and red cell. We have now identified parasite proteins which bind to SGP or N-acetylglucosamine on Sepharose 4B columns. Three proteins, of molecular weights (MWs) 140,000 (140K), 70K and 35K, seem to be specifically bound by N-acetylglucosamine.

Human Parasitism by the Brown Dog Tick1

Out of 643 collections of the brown dog tick, Rhipicephalus sanguineus (Latreille), from the United States only 13 were from humans and only 4 of these were biting records. A host-preference study for this species of tick showed 3.5% of the larvae tested, 2% of the nymphs, and 5% of the adults were induced to attach to humans after 24 hours. These figures are in contrast to 25% of the larvae, 57.7% of the nymphs, and 59.5% of the adults that attached to guinea pigs and 21.3% of the larvae, 61.1% of the nymphs, and 38.1% of the adults that attached to dogs over the same period of time. These observations indicate that R. sanguineus is not a major human parasite.