Novel mannose carrier in the trypanosomatid Crithidia fasciculata behaving as a short alpha-saturated polyprenyl phosphate

Isoprenoid biosynthesis in the erythrocytic stages of Plasmodium falciparum

The development of new drugs is one strategy for malaria control. Biochemical pathways localised in the apicoplast of the parasite, such as the synthesis of isoprenic precursors, are excellent targets because they are different or absent in the human host. Isoprenoids are a large and highly diverse group of natural products with many functions and their synthesis is essential for the parasite's survival. During the last few years, the genes, enzymes, intermediates and mechanisms of this biosynthetic route have been elucidated. In this review, we comment on some aspects of the methylerythritol phosphate pathway and discuss the presence of diverse isoprenic products such as dolichol, ubiquinone, carotenoids, menaquinone and isoprenylated proteins, which are biosynthesised during the intraerythrocytic stages of Plasmodium falciparum.

Antileishmanial activity of the terpene nerolidol

The activity of nerolidol, a sesquiterpene used as a food-flavoring agent and currently under testing as a skin penetration enhancer for the transdermal delivery of therapeutic drugs, was evaluated against Leishmania species. Nerolidol inhibited the growth of Leishmania amazonensis, L. braziliensis, and L. chagasi promastigotes and L. amazonensis amastigotes with in vitro 50% inhibitory concentrations of 85, 74, 75, and 67 microM, respectively. The treatment of L. amazonensis-infected macrophages with 100 microM nerolidol resulted in 95% reduction in infection rates. Inhibition of isoprenoid biosynthesis, as shown by reduced incorporation of [2-(14)C]mevalonic acid (MVA) or [1-(14)C]acetic acid precursors into dolichol, ergosterol, and ubiquinone, was observed in nerolidol-treated promastigotes. This drug effect can be attributed to the blockage of an early step in the mevalonate pathway, since incorporation of the precursor [1(n)-(3)H]farnesyl pyrophosphate in polyisoprenoids is not inhibited by nerolidol. L. amazonensis-infected BALB/c mice were treated with intraperitoneal doses of 100 mg/kg/day for 12 days or topically with 5 or 10% ointments for 4 weeks. Significant reduction of lesion sizes in nerolidol treated mice was observed for both treatment routes. However, long-term follow up indicated that the disease was not cured in this highly susceptible animal model. Nonetheless, the in vitro activity of nerolidol against these parasites may prove a useful tool for the development of new drugs for the treatment of leishmaniasis. In addition, biosynthesis of dolichols with 11 and 12 isoprene units was identified in Leishmania, as described for other trypanosomatids and Apicomplexa.

The ins(ide) and out(side) of dolichyl phosphate biosynthesis and recycling in the endoplasmic reticulum

The precursor oligosaccharide donor for protein N-glycosylation in eukaryotes, Glc3Man9GlcNAc(2)-P-P-dolichol, is synthesized in two stages on both leaflets of the rough endoplasmic reticulum (ER). There is good evidence that the level of dolichyl monophosphate (Dol-P) is one rate-controlling factor in the first stage of the assembly process. In the current topological model it is proposed that ER proteins (flippases) then mediate the transbilayer movement of Man-P-Dol, Glc-P-Dol, and Man5GlcNAc(2)-P-P-Dol from the cytoplasmic leaflet to the lumenal leaflet. The rate of flipping of the three intermediates could plausibly influence the conversion of Man5GlcNAc(2)-P-P-Dol to Glc3Man(9)GlcNAc(2)-P-P-Dol in the second stage on the lumenal side of the rough ER. This article reviews the current understanding of the enzymes involved in the de novo biosynthesis of Dol-P and other polyisoprenoid glycosyl carrier lipids and speculates about the role of membrane proteins and enzymes that could be involved in the transbilayer movement of the lipid intermediates and the recycling of Dol-P and Dol-P-P discharged during glycosylphosphatidylinositol anchor biosynthesis, N-glycosylation, and O- and C-mannosylation reactions on the lumenal surface of the rough ER.

Active isoprenoid pathway in the intra-erythrocytic stages of Plasmodium falciparum: presence of dolichols of 11 and 12 isoprene units

N-glycosylation of proteins is required for the intra-erythrocytic schizogony of Plasmodium falciparum. In eukaryotic cells, this process involves the transfer of oligosaccharides from a dolichyl pyrophosphate derivative to asparagine residues. We have identified dolichol, dolichyl phosphate and dolichyl pyrophosphate species of 11 and 12 isoprenoid residues by metabolic labelling with [(3)H]farnesyl pyrophosphate, [(3)H]geranylgeranyl pyrophosphate and [(14)C]acetate in the different intra-erythrocytic stages of P. falciparum. This is the first demonstration of short-chain dolichols in the phylum Apicomplexa. The results demonstrate the presence of an active isoprenoid pathway in the intra-erythrocytic stages of P. falciparum. Parasites treated with mevastatin, a 3-hydroxy-3-methylglutaryl-CoA reductase inhibitor, show depressed biosynthesis of dolichol, dolichyl phosphate and isoprenoid pyrophosphate. This effect is observed in all intra-erythrocytic stages of the parasite life cycle, but is most pronounced in the ring stage. N-linked glycosylation of proteins was inhibited in the ring and young-trophozoite stages after mevastatin treatment of parasite cultures. Therefore the isoprenoid pathway may represent a different approach to the development of new anti-malarial drugs.

Trypanosoma cruzi calreticulin is a lectin that binds monoglucosylated oligosaccharides but not protein moieties of glycoproteins

Trypanosoma cruzi is a protozoan parasite that belongs to an early branch in evolution. Although it lacks several features of the pathway of protein N-glycosylation and oligosaccharide processing present in the endoplasmic reticulum of higher eukaryotes, it displays UDP-Glc:glycoprotein glucosyltransferase and glucosidase II activities. It is herewith reported that this protozoan also expresses a calreticulin-like molecule, the third component of the quality control of glycoprotein folding. No calnexin-encoding gene was detected. Recombinant T. cruzi calreticulin specifically recognized free monoglucosylated high-mannose-type oligosaccharides. Addition of anti-calreticulin serum to extracts obtained from cells pulse-chased with [35S]Met plus [35S]Cys immunoprecipitated two proteins that were identified as calreticulin and the lysosomal proteinase cruzipain (a major soluble glycoprotein). The latter but not the former protein disappeared from immunoprecipitates upon chasing cells. Contrary to what happens in mammalian cells, addition of the glucosidase II inhibitor 1-deoxynojirimycin promoted calreticulin-cruzipain interaction. This result is consistent with the known pathway of protein N-glycosylation and oligosaccharide processing occurring in T. cruzi. A treatment of the calreticulin-cruzipain complexes with endo-beta-N-acetylglucosaminidase H either before or after addition of anti-calreticulin serum completely disrupted calreticulin-cruzipain interaction. In addition, mature monoglucosylated but not unglucosylated cruzipain isolated from lysosomes was found to interact with recombinant calreticulin. It was concluded that the quality control of glycoprotein folding appeared early in evolution, and that T. cruzi calreticulin binds monoglucosylated oligosaccharides but not the protein moiety of cruzipain. Furthermore, evidence is presented indicating that glucosyltransferase glucosylated cruzipain at its last folding stages.

Characterisation of protein isoprenylation in procyclic form Trypanosoma brucei

Protein modification by isoprenylation is essential in mammals and other eukaryotes, but has not been demonstrated in the parasitic protozoa of the order kinetoplastida. A key regulatory enzyme of the mevalonate pathway, hydroxymethylglutaryl-coenzyme A reductase (HMG-R), and end products of the path, including dolichols, are present in Trypanosoma brucei. By metabolical labelling of procyclic form trypanosomes in the presence of compactin, an efficient inhibitor of HMG-R, followed by one-dimensional gel electrophoresis, we demonstrate that protein isoprenylation indeed takes place in this organism and at least 14 polypeptides bear the modification. Further characterization of labelled isoprenyl groups by methyl iodide cleavage and high pressure liquid chromatography identified both the farnesyl and geranylgeranyl moieties found covalently attached to proteins in other eukaryotes. The latter moiety was more abundant, as found in mammalian systems. Prolonged incubation with compactin grossly affected cell morphology and altered a number of subcellular structures as seen by electron microscopy. High concentrations of compactin were toxic, whilst lower concentrations were cytostatic. The primary morphological lesion is distinct from that of synvinolin, another inhibitor of HMG-R. The morphological changes correlated with a complete inhibition of HMG-R activity by compactin. Surprisingly there was a complete lack of HMG-R activity in procyclic cells grown for 1 or several days in 100 microM compactin, suggesting that degradation of the enzyme had occurred and compensatory upregulation mechanisms could not be successfully exploited by the parasite to overcome HMG-R inhibition. Subsequent alterations to the overall cell shape are seen after 3 days of compactin exposure. Overall these data indicate that T. brucei has an essential protein isoprenylation pathway that is conserved with the higher eukaryotes. Additionally, products of the MVA pathway are implicated in maintenance of cell architecture.

Biosynthesis of protein-linked oligosaccharides in trypanosomatid flagellates

In this article, Armando Parodi presents a summary of the knowledge of the structure and biosynthesis of mammalian Asn-linked (N-linked) oligosacchorides and compares this with what is known in trypanosomatids.

Isolation and partial purification of lipid-linked oligosaccharides from calf pancreas

Lipid-linked oligosaccharides containing at least five mannose residues (0.7 nmol/g of tissue) were isolated from calf pancreas by chloroform-methanol-water extraction and purification on DEAE-cellulose and Sephadex LH-20, and affinity chromatography on concanavalin A-Sepharose in the presence of nonionic detergent. The addition, prior to the chromatographic steps, of 14C-labeled lipid-linked oligosaccharides (synthesized in vitro by calf pancreas microsomes in the presence of GDP-D-[14C]mannose and UDP-D-[14C]glucose, respectively) as internal standards, indicated a final yield ranging from 38 to 50%. Analysis of the oligosaccharide residues by liquid chromatography of the lipid-free preparation, monitored by u.v. absorbance and radioactivity measurement of the tritiated compounds, indicated a heterogeneous mixture of oligosaccharides. Its components, ranging from Man5(GlcNAc)2 to Glc3Man9(GlcNAc)2, cochromatographed with the 14C-labeled derivatives from in vitro synthesis. Calf pancreas contains lipid intermediates bearing at least six mannose residues, such as Man9(GlcNAc)2-P-P-lipid, in almost equal or even higher amounts than Glc3Man9(GlcNAc)2-P-P-dolichol.

Characterization of dolichol monophosphate- and dolichol diphosphate-linked saccharides in trypanosomatid flagellates

Dolichol-P- and dolichol-P-P-linked saccharides were isolated from several trypanosomatid flagellates incubated with [U-14C]glucose. Formation of Glc-P-dolichol and Man-P-dolichol was observed in Herpetomonas muscarum and Leishmania adleri, whereas only the latter derivative was synthesized in Trypanosoma dionisii and Leptomonas samueli. The main and largest dolichol-P-P-linked oligosaccharide formed in Trypanosoma conorhini, T. dionisii, L. samueli, Herpetomonas samuelpessoai and H. muscarum appeared to be Man9GlcNAc2, whereas in Blastocrithidia culicis it was Man6GlcNAc2. In L. adleri there were two main oligosaccharides linked to dolichol-P-P, Man6GlcNAc2 and Man5GlcNAc2. The The structures of the oligosaccharides were identical with those of the intermediates in the formation of Glc3Man9GlcNAc2-P-P-dolichol in higher eucaryotes. It was concluded that similarly to Trypanosoma cruzi, Crithidia fasciculata and Leishmania mexicana, no glucosylated derivatives of dolichol-P-P were formed in the additional seven trypanosomatids studied here. The results obtained suggest that the defective step could be in some cases the formation of Glc-P-dolichol and in others, the transfer of glucose residues from the latter compound to dolichol-P-P-linked oligosaccharides.

Characterization of Crithidia fasciculata oligosaccharide-lipid and its elongation by bovine mammary microsomes

It was recently shown that a Man7(GlcNAc)2-lipid species serves as the precursor for the biosynthesis of asparagine-linked glycoproteins in the trypanosomatid Crithidia fasciculata. Preliminary results indicated it to be similar to the intermediate in the major pathway for the biosynthesis of lipid-linked Glc3Man9(GlcNAc)2 in animal systems. To explore the potential of this glycolipid as an acceptor for studying the biosynthesis of mammary glycoproteins, we conducted a detailed structural analysis of the labelled Man7(GlcNAc)2-lipid isolated from exponentially growing cells of C. fasciculata. The results showed its structure to be Man alpha 1----2Man alpha 1----2Man alpha 1----3(Man alpha 1----2Man alpha 1----3Man alpha 1----6)Man beta----GlcNAc beta----GlcNAc, identical to the nonasaccharide synthesized by the animal systems. Incubation of the Man7(GlcNAc)2-lipid with bovine mammary microsomes along with GDP-mannose or mannosyl-phosphodolichol elongates it to give Man9(GlcNAc)2-lipid having the same structure as the corresponding intermediate in animal systems. Inhibition of the elongation reaction by EDTA or amphomycin indicates that the intermediary formation of mannosyl-phosphodolichol is required for the incorporation of mannose residues into the nonasaccharide-lipid. Mannosyl.phosphoretinol failed to serve as a mannosyl donor in this reaction.

Glycoprotein biosynthesis in Chlamydomonas. A mannolipid intermediate with the properties of a short-chain alpha-saturated polyprenyl monophosphate

A crude membrane preparation of the unicellular green alga Chlamydomonas reinhardii was found to catalyse the incorporation of D-[14C]mannose from GDP-D-[14C]-mannose into a chloroform/methanol-soluble compound and into a trichloroacetic acid-insoluble polymer fraction. The labelled lipid revealed the chemical and chromatographic properties of a short-chain (about C55-C65) alpha-saturated polyprenyl mannosyl monophosphate. In the presence of detergent both long-chain (C85-C105) dolichol phosphate and alpha-unsaturated undecaprenyl phosphate (C55) were found to be effective as exogenous acceptors of D-mannose from GDP-D-[14C]mannose to yield their corresponding labelled polyprenyl mannosyl phosphates. Exogenous dolichyl phosphate stimulated the incorporation of mannose from GDP-D-[14C]mannose into the polymer fraction 5-7-fold, whereas the mannose moiety from undecaprenyl mannosyl phosphate was not further transferred. Authentic dolichyl phosphate [3H]mannose and partially purified mannolipid formed from GDP-[14C]mannose and exogenous dolichyl phosphate were found to function as direct mannosyl donors for the synthesis of labelled mannoproteins. These results clearly indicate the existence of dolichol-type glycolipids and their role as intermediates in transglycosylation reactions of this algal system. Both the saturation of the alpha-isoprene unit and the length of the polyprenyl chain may be regarded as evolutionary markers.