Glycoconjugate structures of parasitic protozoa

Glycoconjugates are abundant and ubiquitious on the surface of many protozoan parasites. Their tremendous diversity has implicated their critical importance in the life cycle of these organisms. This review highlights our current knowledge of the major glycoconjugates, with particular emphasis on their structures, of representative protozoan parasites, including Leishmania, Trypanosoma, Giardia, Plasmodia, and others.     

Cell fractionation of parasitic protozoa: a review

Cell fractionation, a methodological strategy for obtaining purified organelle preparations, has been applied successfully to parasitic protozoa by a number of investigators. Here we present and discuss the work of several groups that have obtained highly purified subcellular fractions from trypanosomatids, Apicomplexa and trichomonads, and whose work have added substantially to our knowledge of the cell biology of these parasites.     

Accelerating vaccine development and deployment: report of a Royal Society satellite meeting

The Royal Society convened a meeting on the 17th and 18th November 2010 to review the current ways in which vaccines are developed and deployed, and to make recommendations as to how each of these processes might be accelerated. The meeting brought together academics, industry representatives, research sponsors, regulators, government advisors and representatives of international public health agencies from a broad geographical background. Discussions were held under Chatham House rules. High-throughput screening of new vaccine antigens and candidates was seen as a driving force for vaccine discovery. Multi-stakeholder, small-scale manufacturing facilities capable of rapid production of clinical grade vaccines are currently too few and need to be expanded. In both the human and veterinary areas, there is a need for tiered regulatory standards, differentially tailored for experimental and commercial vaccines, to allow accelerated vaccine efficacy testing. Improved cross-fertilization of knowledge between industry and academia, and between human and veterinary vaccine developers, could lead to more rapid application of promising approaches and technologies to new product development. Identification of best-practices and development of checklists for product development plans and implementation programmes were seen as low-cost opportunities to shorten the timeline for vaccine progression from the laboratory bench to the people who need it.     

Interactions between the parasitic protozoa of small mammals

In the wild, small mammals are frequently infected with more than one parasite. Laboratory studies have revealed complex interactions between parasites and also between parasitic protozoa and viruses or bacteria. In general, infection with many parasites is accompanied by a period of immunodepression during which superimposed infections are favoured, giving rise to more intense and prolonged secondary infections while the original infection is unaffected. On the other hand, organisms that activate macrophages may protect die host against a subsequent infection. These kinds of interactions have been investigated in the laboratory using Trypanosoma musculi, T. lewisi, Giardia muris, Spironucleus muris, Babesia microti and Heligmosomoides polygyrus , all of which occur in British small mammals, suggesting that such interactions occur in the field, are worth investigating and should be considered in epidemiological studies.     

Clan CD cysteine peptidases of parasitic protozoa

Parasitic protozoa contain an abundance of cysteine peptidases that are crucial for a range of important biological processes. The most studied cysteine peptidases of parasitic protozoa belong to the group of papain-like enzymes known as clan CA. However, several more recently identified cysteine peptidases differ fundamentally from the clan CA enzymes and have been included together in clan CD. Enzymes of this clan have now been identified in parasitic protozoa. Many have important roles and also differ significantly from known mammalian counterparts. The main characteristics of clan CD enzymes are outlined here, in particular glycosylphosphatidylinositol (GPI):protein transamidase, metacaspase and separase, and their differences from the clan CA enzymes are described.     

Cystatin-like cysteine proteinase inhibitors of parasitic protozoa

A new method has been developed for detecting cystatins and other cysteine proteinase inhibitors. The method, which involves protein separation by SDS-PAGE followed by a cysteine proteinase overlay step, is more sensitive than previously reported techniques: as little as 1 ng of recombinant human cystatin C can be detected and cysteine proteinase inhibitors could also be detected in complex protein mixtures such as bovine foetal serum. The method has been used to show, for the first time, cysteine proteinase inhibitors in lysates of a range of parasitic protozoa (Trypanosoma brucei, Leishmania mexicana mexicana, Toxoplasma gondii and Tritrichomonas foetus) and to confirm that one occurs in the free-living ciliate Tetrahymena pyriformis. Cystatin-like inhibitory activity was also demonstrated in boiled lysates of L. mexicana mexicana using conventional assays methods.     

New drugs for the treatment of human parasitic protozoa

Whereas parasitic diseases are always a heavy burden for humanity, few are the new antiparasitic molecules marketed during the last 25 years. Thus on the 1393 new molecules marketed between 1975 and 1999, only 7 have antiprotozoan properties. This talk will detail the progress made in the treatment of the intestinal protozoa, malaria, visceral leishmaniasis and toxoplasmosis, problems with which are especially confronted the European parasitologists. The treatment of Giardia and intestinal amoebas is based on 5-nitro-imidazoles derivatives. Single-dose treatments can be used with tinidazole or secnidazole. Resistance to these compounds of Giardia were described and in these cases, treatment by quinacrine or nitazoxanide are possible alternatives. Nitazoxanide is marketed in the United States and in Australia. It seems to be a well tolerated antiparasitic agent with a broad spectrum because it is active on a lot of intestinal protozoa and helminths. It acts on the same metabolic way as the 5-nitro-imidazoles (inhibition of the ferredoxine reductase) but without synthesis of free radicals and DNA deterioration of the target cell. It is thus neither teratogenic nor mutagenic. Artemisinin derivatives allowed considerable progress in the treatment of malaria. They have short half-lifes, allowing a fast parasitic clearance and these derivatives do no provoke resistance. They are first line drugs for the treatment of malaria in areas of drug resistance. The arthemeter-lumefantrine association (Riamet, Coartem) ensures a rapid disappearance of the circulating parasites and is well tolerated. Atovaquone-proguanil (Malarone) is usable in the treatment of acute malaria but also in disease prevention with the advantage of continuing drug intake for only 7 days after having left the infected area. The treatment of leishmaniasis is always delicate and is characterized by the worrying development of antimony resistances, probably related in the European zones to the treatment of dogs. Liposomal amphotricin is an alternative of choice but remains very expensive. The heating of amphotericin to 70 degrees C during 20 minutes gives it experimental properties and efficacies comparable with that of liposomal amphotericin, but at a less cost. Miltefosine, an alkyl-phospholipide antimetabolite, is very active on visceral leishmaniasis resistant to antimonial treatment. However, its long half-life could induce the emergence of resistances. Miltefosine induces much less side effects than conventional amphotericin B. The commonly used anti-toxoplasmic drugs (sulphadiazine and pyrimethamine) were marketed some 50 years ago and are only active on the rapid forms in multiplication. No drug is really efficient on the cysts although preliminary tests with atovaquone are encouraging to treat ophthalmologic forms in immunocompetent patients. To conclude, it is important to continue to search for new antiprotozoan molecules because, for some parasites, drug resistance is an important problem. Moreover, the treatment of the pregnant women, particularly during the first trimester, is often impossible and there is a lack of galenic forms easily usable in children. A better knowledge of the metabolic pathways of protozoa (particularly the apicoplast of Apicomplexa parasites) would certainly open the posssibility to identify new drugs. To reduce and delay the appearance of resistances, mass-treatments of mass should be avoided and targeted treatments prefered as well as the use of associations of molecules having different modes of action.     

Conversion of dihydroorotate to orotate in parasitic protozoa

The conversion of dihydroorotate to orotate, one of the key reactions in the de novo pyrimidine biosynthetic pathway, has been studied in a number of parasitic protozoa. Enzyme activities capable of carrying out this reaction were detected in six members of the Kinetoplastida (Trypanosoma brucei, Trypanosoma congolense, Trypanosoma vivax, Trypanosoma lewisi, Trypanosoma cruzi, Leishmania enriettii) and three members of the genus Plasmodium (P. knowlesi, P. berghei, P. gallinaceum). The mechanism of the reaction in the two groups of protozoa were quite distinct. In the Kinetoplastida, the enzyme is an hydroxylase which occurs in the soluble fraction of the cell and probably requires tetrahydrobiopterin for activity. In contrast, in Plasmodium, the enzyme is a dehydrogenase which is particulate, probably mitochondrial, and intimately connected to the electron transport chain to which it passes electrons directly, probably at the ubiquinone level. Neither activity is regulated by fully formed pyrimidines. The enzyme in Plasmodium is similar in mechanism to the isofunctional mammalian enzyme. However, since malarial ubiquinones are apparently different from those in the mammal and since menoctone, which is active in vivo in experimental malaria, is a good inhibitor of the malarial enzyme, it could represent a useful target for chemotherapeutic attack. The enzyme in the Kinetoplastida is quite distinct from that in the mammal so that it too apparently falls into this category, though none of the currently used antitrypanosomal drugs appears to block it activity at physiological concentrations.     

Repetitive elements in genomes of parasitic protozoa.

Repetitive DNA elements have been a part of the genomic fauna of eukaryotes perhaps since their very beginnings. Millions of years of coevolution have given repeats central roles in chromosome maintenance and genetic modulation. Here we review the genomes of parasitic protozoa in the context of the current understanding of repetitive elements. Particular reference is made to repeats in five medically important species with ongoing or completed genome sequencing projects: Plasmodium falciparum, Leishmania major, Trypanosoma brucei, Trypanosoma cruzi, and Giardia lamblia. These organisms are used to illustrate five thematic classes of repeats with different structures and genomic locations. We discuss how these repeat classes may interact with parasitic life-style and also how they can be used as experimental tools. The story which emerges is one of opportunism and upheaval which have been employed to add genetic diversity and genomic flexibility.