SummaryToxoplasma gondii belongs to the phylum Apicomplexa and is an important cause of congenital disease and infection in immunocompromised patients. Like most apicomplexans, T. gondii possesses several plant-like features, such as the chloroplast-like organelle, the apicoplast. We describe and characterize a novel organelle in T. gondii tachyzoites, which is visible by light microscopy and possesses a broad similarity to the plant vacuole. Electron tomography shows the interaction of this vacuole with other organelles. The presence of a plant-like vacuolar proton pyrophosphatase (TgVP1), a vacuolar proton ATPase, a cathepsin L-like protease (TgCPL), an aquaporin (TgAQP1), as well as Ca 2+ /H + and Na + /H + exchange activities, supports similarity to the plant vacuole. Biochemical characterization of TgVP1 in enriched fractions shows a functional similarity to the respective plant enzyme. The organelle is a Ca 2+ store and appears to have protective effects against salt stress potentially linked to its sodium transport activity. In intracellular parasites, the organelle fragments, with some markers colocalizing with the late endosomal marker, Rab7, suggesting its involvement with the endocytic pathway. Studies on the characterization of this novel organelle will be relevant to the identification of novel targets for chemotherapy against T. gondii and other apicomplexan parasites as well.

Sterols are constituents of the cellular membranes that are essential for their normal structure and function. In mammalian cells, cholesterol is the main sterol found in the various membranes. However, other sterols predominate in eukaryotic microorganisms such as fungi and protozoa. It is now well established that an important metabolic pathway in fungi and in members of the Trypanosomatidae family is one that produces a special class of sterols, including ergosterol, and other 24-methyl sterols, which are required for parasitic growth and viability, but are absent from mammalian host cells. Currently, there are several drugs that interfere with sterol biosynthesis (SB) that are in use to treat diseases such as high cholesterol in humans and fungal infections. In this review, we analyze the effects of drugs such as (a) statins, which act on the mevalonate pathway by inhibiting HMG-CoA reductase, (b) bisphosphonates, which interfere with the isoprenoid pathway in the step catalyzed by farnesyl diphosphate synthase, (c) zaragozic acids and quinuclidines, inhibitors of squalene synthase (SQS), which catalyzes the first committed step in sterol biosynthesis, (d) allylamines, inhibitors of squalene epoxidase, (e) azoles, which inhibit C14α-demethylase, and (f) azasterols, which inhibit Δ24(25)-sterol methyltransferase (SMT). Inhibition of this last step appears to have high selectivity for fungi and trypanosomatids, since this enzyme is not found in mammalian cells. We review here the IC50 values of these various inhibitors, their effects on the growth of trypanosomatids (both in axenic cultures and in cell cultures), and their effects on protozoan structural organization (as evaluted by light and electron microscopy) and lipid composition. The results show that the mitochondrial membrane as well as the membrane lining the protozoan cell body and flagellum are the main targets. Probably as a consequence of these primary effects, other important changes take place in the organization of the kinetoplast DNA network and on the protozoan cell cycle. In addition, apoptosis-like and autophagic processes induced by several of the inhibitors tested led to parasite death.

We report on the antiproliferative effects and the ultrastructural and biochemical alterations induced in vitro by 22,26-azasterol, a sterol ⌬ 24(25) -methyltransferase (24-SMT) inhibitor, on Leishmania amazonensis. When promastigotes and amastigotes were exposed to 100 nM 22,26-azasterol, complete growth arrest and cell lysis ensued after 72 (promastigotes) or 120 (amastigotes) h. Exposure of parasites to this azasterol led to the complete depletion of parasite endogenous sterols (episterol and 5-dehydroepisterol) and their replacement by 24-desalkyl sterols (zymosterol, cholesta-5,7,24-trien-3␤-ol, and cholesta-7,24-dien-3␤-ol), while 14-methylzymosterol and 4,14-dimethyl-zymosterol accumulated as a result of simultaneous incubation of the parasites with 22,26-azasterol and ketoconazole, a known inhibitor of the parasite's sterol C14-demethylase. These results confirmed that 24-SMT is the primary site of action of the azasterol. Profound changes were also observed in the phospholipid compositions of treated cells, in which a twofold reduction in the content of phosphatidylserine was observed; this was accompanied by a concomitant increase in the content of phosphatidylinositol. Transmission electron microscopy showed that 22,26-azasterol induced marked morphological changes, including mitochondrial swelling, invaginations of the inner mitochondrial membrane, and the appearance of large bodies containing concentric membranes. Other modifications included increases in the numbers of acidocalcisomes, megasomes, and lipid inclusions and the appearance of typical autophagic structures and cell body protrusions toward the flagellar pocket. We conclude that the dramatic alteration of the lipid composition of the parasite's membranes induced by the drug underlies the ultrastructural alterations that lead to the loss of cell viability and that 24-SMT inhibitors could be useful as selective antileishmanial agents.

This paper describes the synthesis of some novel azasterols based on (20R,22xi)-5alpha-pregnan-20-(piperidin-2-yl)-3beta,20-diol. These compounds are potential inhibitors of the enzyme sterol 24-methyltransferase (24-SMT), which is a vital enzyme in the biosynthesis of ergosterol and related 24-alkyl sterols. Structure-activity studies were undertaken to understand the important features for activity against the enzyme, with the aim of increasing activity and selectivity. The compounds were evaluated for inhibition of recombinant Leishmania major 24-SMT and the effect of compounds on sterol composition and parasite proliferation. Essentially, compounds which showed good activity against the recombinant enzyme had a significant effect on the sterol composition and growth of parasites. The activity of compounds was found to be related to the basicity and stereochemical location of the nitrogen. Also, presence of an unprotected 3beta-OH seemed to be important for activity. However, some azasterols which were not good inhibitors of 24-SMT also showed antiproliferative activity, suggesting that there may be other modes of actions of these compounds.