Secretory pathway of trypanosomatid parasites

All in one: Leishmania major STT3 proteins substitute for the whole oligosaccharyltransferase complex in Saccharomyces cerevisiae

The transfer of lipid-linked oligosaccharide to asparagine residues of polypeptide chains is catalyzed by oligosaccharyltransferase (OTase). In most eukaryotes, OTase is a hetero-oligomeric complex composed of eight different proteins, in which the STT3 component is believed to be the catalytic subunit. In the parasitic protozoa Leishmania major, four STT3 paralogues, but no homologues to the other OTase components seem to be encoded in the genome. We expressed each of the four L. major STT3 proteins individually in Saccharomyces cerevisiae and found that three of them, LmSTT3A, LmSTT3B, and LmSTT3D, were able to complement a deletion of the yeast STT3 locus. Furthermore, LmSTT3D expression suppressed the lethal phenotype of single and double deletions in genes encoding other essential OTase subunits. LmSTT3 proteins did not incorporate into the yeast OTase complex but formed a homodimeric enzyme, capable of replacing the endogenous, multimeric enzyme of the yeast cell. Therefore, these protozoan OTases resemble the prokaryotic enzymes with respect to their architecture, but they used substrates typical for eukaryotic cells: N-X-S/T sequons in proteins and dolicholpyrophosphate-linked high mannose oligosaccharides.

Leishmania lysosomal targeting signal is recognized by yeast and not by mammalian cells

Leishmaniasis, caused by Leishmania parasites, is an important public health problem worldwide. Leishmania, like other trypanosomatids, present unique biological features as compared to higher eukaryotes that can be exploited with the intent of finding new chemotherapeutical/vaccine candidates. Mechanisms of cellular sorting in Leishmania can be viewed as such potential targets. We have previously demonstrated a role for the pro-domain of a Leishmania cysteine proteinase in lysosomal targeting. In this paper, we show that this signal is not recognized by mammalian cells and is recognized by yeast; we also discuss here the implications of these findings related to evolution and further characterization of the Leishmania trafficking machinery.

The synthesis of UDP-N-acetylglucosamine is essential for bloodstream form trypanosoma brucei in vitro and in vivo and UDP-N-acetylglucosamine starvation reveals a hierarchy in parasite protein glycosylation

A gene encoding Trypanosoma brucei UDP-N-acetylglucosamine pyrophosphorylase was identified, and the recombinant protein was shown to have enzymatic activity. The parasite enzyme is unusual in having a strict substrate specificity for N-acetylglucosamine 1-phosphate and in being located inside a peroxisome-like microbody, the glycosome. A bloodstream form T. brucei conditional null mutant was constructed and shown to be unable to sustain growth in vitro or in vivo under nonpermissive conditions, demonstrating that there are no alternative metabolic or nutritional routes to UDP-N-acetylglucosamine and providing a genetic validation for the enzyme as a potential drug target. The conditional null mutant was also used to investigate the effects of N-acetylglucosamine starvation in the parasite. After 48 h under nonpermissive conditions, about 24 h before cell lysis, the status of parasite glycoprotein glycosylation was assessed. Under these conditions, UDP-N-acetylglucosamine levels were less than 5% of wild type. Lectin blotting and fluorescence microscopy with tomato lectin revealed that poly-N-acetyllactosamine structures were greatly reduced in the parasite. The principal parasite surface coat component, the variant surface glycoprotein, was also analyzed. Endoglycosidase digestions and mass spectrometry showed that, under UDP-N-acetylglucosamine starvation, the variant surface glycoprotein was specifically underglycosylated at its C-terminal Asn-428 N-glycosylation site. The significance of this finding, with respect to the hierarchy of site-specific N-glycosylation in T. brucei, is discussed.

Leishmania requires Rab7-mediated degradation of endocytosed hemoglobin for their growth

Leishmania is unable to synthesize heme and must acquire it from exogenous source, the mechanism of which is not known. We have shown that Leishmania endocytoses hemoglobin (Hb) and subsequently degrade it probably to generate heme. To understand how internalized Hb is degraded, we have cloned and expressed Rab7 homolog from Leishmania donovani. Interestingly, Rab7 in Leishmania is found to be localized both on early and late endocytic compartment and regulates both uptake and degradation of endocytosed Hb demonstrating that Rab7 in Leishmania play a very unique role connecting both early and late events of Hb endocytosis. Our data also indicate that overexpression of Rab7:WT in Leishmania induces transport of Hb to lysosomes and rapidly degrade internalized Hb. Whereas Hb transport to lysosomes and its degradation is significantly inhibited in cells overexpressing Rab7:T21N, a GDP locked mutant of Rab7. Moreover, cells overexpressing Rab7:T21N grow at a slower rate (<50%) compared with control Leishmania. Addition of exogenous hemin recovers the growth of Rab7:T21N mutant cells almost to the control level, suggesting that intracellular heme generated by Rab7-mediated Hb degradation is required for optimal growth of the parasites. Thus, our results identify a potential target which might be exploited to suppress the growth of Leishmania.

Proteomic analysis of the secretome of Leishmania donovani

Analysis of Leishmania-conditioned medium resulted in the identification of 151 proteins apparently secreted by the parasitic protozoan Leishmania donovani and suggested a vesicle-based secretion system.

Background

Leishmania and other intracellular pathogens have evolved strategies that support invasion and persistence within host target cells. In some cases the underlying mechanisms involve the export of virulence factors into the host cell cytosol. Previous work from our laboratory identified one such candidate leishmania effector, namely elongation factor-1α, to be present in conditioned medium of infectious leishmania as well as within macrophage cytosol after infection. To investigate secretion of potential effectors more broadly, we used quantitative mass spectrometry to analyze the protein content of conditioned medium collected from cultures of stationary-phase promastigotes of Leishmania donovani, an agent of visceral leishmaniasis.

Results

Analysis of leishmania conditioned medium resulted in the identification of 151 proteins apparently secreted by L. donovani. Ratios reflecting the relative amounts of each leishmania protein secreted, as compared to that remaining cell associated, revealed a hierarchy of protein secretion, with some proteins secreted to a greater extent than others. Comparison with an in silico approach defining proteins potentially exported along the classic eukaryotic secretion pathway suggested that few leishmania proteins are targeted for export using a classic eukaryotic amino-terminal secretion signal peptide. Unexpectedly, a large majority of known eukaryotic exosomal proteins was detected in leishmania conditioned medium, suggesting a vesicle-based secretion system.

Conclusion

This analysis shows that protein secretion by L. donovani is a heterogeneous process that is unlikely to be determined by a classical amino-terminal secretion signal. As an alternative, L. donovani appears to use multiple nonclassical secretion pathways, including the release of exosome-like microvesicles.

A high-resolution solution structure of a trypanosomatid FYVE domain

FYVE domain proteins play key roles in regulating membrane traffic in eukaryotic cells. The FYVE domain displays a remarkable specificity for the head group of the target lipid, phosphatidylinositol 3-phosphate (PtdIns[3]P). We have identified five putative FYVE domain proteins in the genome of the protozoan parasite Leishmania major, three of which are predicted to contain a functional PtdIns(3)P-binding site. The FYVE domain of one of these proteins, LmFYVE-1, bound PtdIns(3)P in liposome-binding assays and targeted GFP to acidified late endosomes/lysosomes in mammalian cells. The high-resolution solution structure of its N-terminal FYVE domain (LmFYVE-1[1-79]) was solved by nuclear magnetic resonance. Functionally significant clusters of residues of the LmFYVE-1 domain involved in PtdIns(3)P binding and dependence on low pH for tight binding were identified. This structure is the first trypanosomatid membrane trafficking protein to be determined and has been refined to high precision and accuracy using residual dipolar couplings.

Living in a phagolysosome; metabolism of Leishmania amastigotes

Leishmania amastigotes primarily proliferate within macrophages in the mammalian host. Genome-based metabolic reconstructions, combined with biochemical, reverse genetic and mRNA or protein profiling studies are providing new insights into the metabolism of this intracellular stage. We propose that the complex nutritional requirements of amastigotes have contributed to the tropism of these parasites for the amino acid-rich phagolysosome of macrophages. Amastigote metabolism in this compartment is robust because many metabolic mutants are capable of either growing normally or persisting long term in susceptible animals. New approaches for measuring amastigote metabolism in vivo are discussed.

Comparisons of mutants lacking the Golgi UDP-galactose or GDP-mannose transporters establish that phosphoglycans are important for promastigote but not amastigote virulence in Leishmania major

Abundant surface Leishmania phosphoglycans (PGs) containing [Gal(beta1,4)Man(alpha1-PO(4))]-derived repeating units are important at several points in the infectious cycle of this protozoan parasite. PG synthesis requires transport of activated nucleotide-sugar precursors from the cytoplasm to the Golgi apparatus. Correspondingly, null mutants of the L. major GDP-mannose transporter LPG2 lack PGs and are severely compromised in macrophage survival and induction of acute pathology in susceptible mice, yet they are able to persist indefinitely and induce protective immunity. However, lpg2(-) L. mexicana amastigotes similarly lacking PGs but otherwise normal in known glycoconjugates remain able to induce acute pathology. To explore this further, we tested the infectivity of a new PG-null L. major mutant, which is inactivated in the two UDP-galactose transporter genes LPG5A and LPG5B. Surprisingly this mutant did not recapitulate the phenotype of L. major lpg2(-), instead resembling the L. major lipophosphoglycan-deficient lpg1(-) mutant. Metacyclic lpg5A(-)/lpg5B(-) promastigotes showed strong defects in the initial steps of macrophage infection and survival. However, after a modest delay, the lpg5A(-)/lpg5B(-) mutant induced lesion pathology in infected mice, which thereafter progressed normally. Amastigotes recovered from these lesions were fully infective in mice and in macrophages despite the continued absence of PGs. This suggests that another LPG2-dependent metabolite is responsible for the L. major amastigote virulence defect, although further studies ruled out cytoplasmic mannans. These data thus resolve the distinct phenotypes seen among lpg2(-) Leishmania species by emphasizing the role of glycoconjugates other than PGs in amastigote virulence, while providing further support for the role of PGs in metacyclic promastigote virulence.

Leishmania mexicana: Molecular cloning and characterization of enolase

The gene of Leishmania mexicana enolase was cloned and overexpressed in Escherichia coli as an active enzyme; the protein was biochemically analyzed. This enolase shares with enolases from other trypanosomatids the presence of three atypical residues, each with a reactive side group, near the active site, already described for the enzyme from Trypanosoma brucei. The natural enzyme was purified, using a three-step procedure, from a cytosolic fraction of L. mexicana promastigotes. The kinetic properties of the purified recombinant enzyme were similar to those of the natural enzyme. Both the recombinant and natural enzyme were inhibited by inorganic pyrophosphate. Subcellular localization analysis after differential centrifugation showed that the enzyme activity is only associated with the cytosolic fraction. However, an apparently inactive form of enolase was detected by Western blots in the microsomal fraction. Digitonin treatment of parasites and immunofluorescence studies with permeabilized and non-permeabilized parasites showed that enolase is also associated with membranes and it was found at the external face of the plasma membrane.

Sugar nucleotide pools of Trypanosoma brucei, Trypanosoma cruzi, and Leishmania major

The cell surface glycoconjugates of trypanosomatid parasites are intimately involved in parasite survival, infectivity, and virulence in their insect vectors and mammalian hosts. Although there is a considerable body of work describing their structure, biosynthesis, and function, little is known about the sugar nucleotide pools that fuel their biosynthesis. In order to identify and quantify parasite sugar nucleotides, we developed an analytical method based on liquid chromatography-electrospray ionization-tandem mass spectrometry using multiple reaction monitoring. This method was applied to the bloodstream and procyclic forms of Trypanosoma brucei, the epimastigote form of T. cruzi, and the promastigote form of Leishmania major. Five sugar nucleotides, GDP-alpha-d-mannose, UDP-alpha-d-N-acetylglucosamine, UDP-alpha-d-glucose, UDP-alpha-galactopyranose, and GDP-beta-l-fucose, were common to all three species; one, UDP-alpha-d-galactofuranose, was common to T. cruzi and L. major; three, UDP-beta-l-rhamnopyranose, UDP-alpha-d-xylose, and UDP-alpha-d-glucuronic acid, were found only in T. cruzi; and one, GDP-alpha-d-arabinopyranose, was found only in L. major. The estimated demands for each monosaccharide suggest that sugar nucleotide pools are turned over at very different rates, from seconds to hours. The sugar nucleotide survey, together with a review of the literature, was used to define the routes to these important metabolites and to annotate relevant genes in the trypanosomatid genomes.

Two functionally divergent UDP-Gal nucleotide sugar transporters participate in phosphoglycan synthesis in Leishmania major

In the protozoan parasite Leishmania, abundant surface and secreted molecules, such as lipophosphoglycan (LPG) and proteophosphoglycans (PPGs), contain extensive galactose in the form of phosphoglycans (PGs) based on (Gal-Man-PO(4)) repeating units. PGs are synthesized in the parasite Golgi apparatus and require transport of cytoplasmic nucleotide sugar precursors to the Golgi lumen by nucleotide sugar transporters (NSTs). GDP-Man transport is mediated by the LPG2 gene product, and here we focused on transporters for UDP-Gal. Data base mining revealed 12 candidate NST genes in the L. major genome, including LPG2 as well as a candidate endoplasmic reticulum UDP-glucose transporter (HUT1L) and several pseudogenes. Gene knock-out studies established that two genes (LPG5A and LPG5B) encoded UDP-Gal NSTs. Although the single lpg5A(-) and lpg5B(-) mutants produced PGs, an lpg5A(-)/5B(-) double mutant was completely deficient. PG synthesis was restored in the lpg5A(-)/5B(-) mutant by heterologous expression of the human UDP-Gal transporter, and heterologous expression of LPG5A and LPG5B rescued the glycosylation defects of the mammalian Lec8 mutant, which is deficient in UDP-Gal uptake. Interestingly, the LPG5A and LPG5B functions overlap but are not equivalent, since the lpg5A(-) mutant showed a partial defect in LPG but not PPG phosphoglycosylation, whereas the lpg5B(-) mutant showed a partial defect in PPG but not LPG phosphoglycosylation. Identification of these key NSTs in Leishmania will facilitate the dissection of glycoconjugate synthesis and its role(s) in the parasite life cycle and further our understanding of NSTs generally.

Glycoproteomics of Trypanosoma cruzi trypomastigotes using subcellular fractionation, lectin affinity, and stable isotope labeling

Herein we detail the first glycoproteomic analysis of a human pathogen. We describe an approach that enables the identification of organelle and cell surface N-linked glycoproteins from Trypanosoma cruzi, the causative agent of Chagas' disease. This approach is based on a subcellular fractionation protocol to produce fractions enriched in either organelle or plasma membrane/cytoplasmic proteins. Through lectin affinity capture of the glycopeptides from each subcellular fraction and stable isotope labeling of the glycan attachment sites with H(2)18O, we unambiguously identified 36 glycosylation sites on 35 glycopeptides which mapped to 29 glycoproteins. We also present the first expression evidence for 11 T. cruzi specific glycoproteins and provide experimental data indicating that the mucin associated surface protein family (MASP) and dispersed gene family (DGF-1) are post-translationally modified by N-linked glycans.

Sorting signals required for trafficking of the cysteine-rich acidic repetitive transmembrane protein in Trypanosoma brucei

In trypanosomatids, endocytosis and exocytosis are restricted to the flagellar pocket (FP). The cysteine-rich acidic repetitive transmembrane (CRAM) protein is located at the FP of Trypanosoma brucei and potentially functions as a receptor or an essential component for lipoprotein uptake. We characterized sorting determinants involved in efficient trafficking of CRAM to and from the FP of T. brucei. Previous studies indicated the presence of signals in the CRAM C terminus, specific for its localization to the FP and for efficient endocytosis (H. Yang, D. G. Russell, B. Zeng, M. Eiki, and M.G.-S. Lee, Mol. Cell. Biol. 20:5149-5163, 2000.) To delineate functional domains of putative sorting signals, we performed a mutagenesis series of the CRAM C terminus. Subcellular localization of CRAM mutants demonstrated that the amino acid sequence between -5 and -14 (referred to as a transport signal) is essential for exporting CRAM from the endoplasmic reticulum to the FP, and mutations of amino acids at -12 (V), -10 (V), or -5 (D) led to retention of CRAM in the endoplasmic reticulum. Comparison of the endocytosis efficiency of CRAM mutants demonstrated that the sequence from amino acid -5 to -23 (referred to as a putative endocytosis signal) is required for efficient endocytosis and overlaps with the transport signal. Apparently the CRAM-derived sorting signal can efficiently interact with the T. brucei micro1 adaptin, and mutations at amino acids essential for the function of the transport signal abolished the interaction of the signal with T. brucei micro1, strengthening the hypothesis of the involvement of the clathrin- and adaptor-dependent pathway in trafficking of CRAM via the FP.

Translation of open reading frame in kinetoplast DNA minicircles of clinical isolates of L. donovani

Till today, it remains an enigma whether the open reading frames said to be transcribed in minicircle sequences are indeed translated into protein products or not. We establish a protein-coding gene in minicircle variable region of kinetoplast DNA from clinical isolates of Leishmania donovani. The protein was expressed as an N-tagged green fluorescent protein (GFP) fusion protein in leishmanial expression system. Fluorescence microscopy of the transfectants carrying recombinant GFP construct showed the protein to be localized on the plasmalemma of the parasite. This shows that the minicircle transcript is indeed translated into a protein product in the parasite cell and further points toward probable biological function of minicircles in kinetoplastids.

GPI-anchored proteins and free GPI glycolipids of procyclic form Trypanosoma brucei are nonessential for growth, are required for colonization of the tsetse fly, and are not the only components of the surface coat

The procyclic form of Trypanosoma brucei exists in the midgut of the tsetse fly. The current model of its surface glycocalyx is an array of rod-like procyclin glycoproteins with glycosylphosphatidylinositol (GPI) anchors carrying sialylated poly-N-acetyllactosamine side chains interspersed with smaller sialylated poly-N-acetyllactosamine-containing free GPI glycolipids. Mutants for TbGPI12, deficient in the second step of GPI biosynthesis, were devoid of cell surface procyclins and poly-N-acetyllactosamine-containing free GPI glycolipids. This major disruption to their surface architecture severely impaired their ability to colonize tsetse fly midguts but, surprisingly, had no effect on their morphology and growth characteristics in vitro. Transmission electron microscopy showed that the mutants retained a cell surface glycocalyx. This structure, and the viability of the mutants in vitro, prompted us to look for non-GPI-anchored parasite molecules and/or the adsorption of serum components. Neither were apparent from cell surface biotinylation experiments but [3H]glucosamine biosynthetic labeling revealed a group of previously unidentified high apparent molecular weight glycoconjugates that might contribute to the surface coat. While characterizing GlcNAc-PI that accumulates in the TbGPI12 mutant, we observed inositolphosphoceramides for the first time in this organism.

Characterization of a Trypanosoma cruzi antigen with homology to intracellular mammalian lectins

Two cDNAs, isolated from a Trypanosoma cruzi amastigote library immunoscreened with sera from patients with Chagas disease, encode proteins with sequence homology to eukaryotic components of the cellular sorting and recycling machinery. These proteins, denominated TcAGL, present an N-terminal lectin domain and a C-terminal region containing repetitive amino acids and a poly-glutamine tract. They are products of polymorphic alleles of a single copy gene constitutively expressed during the parasite life cycle. Polyclonal antibodies obtained from mice immunized with the recombinant antigen recognize proteins with apparent molecular weight ranging from 95 to 120 kDa in cell lysates from all three life stages and in various strains of the parasite. Sera from Chagas disease patients recognize the recombinant antigen in ELISA and immunoprecipitation assays but not in Western blot assays under denaturing conditions. Consistent with its proposed role in the glycoprotein secreting pathway, immunofluorescence analyses and expression of a green fluorescent protein-tagged TcAGL protein indicate a sub-cellular localization in the vicinity of the flagellar pocket membrane and the Golgi complex of the parasite.

Recent developments in the molecular, biochemical and functional characterization of GPI8 and the GPI-anchoring mechanism [review]

Glycoconjugates are utilized by eukaryotic organisms ranging from yeast to humans for the cell surface expression of a wide variety of proteins and lipids. These glycoconjugates are expressed as enzymes or receptors and serve a diversity of functions, including cell signaling and cell survival. In parasitic protozoans, glycoconjugates play roles in infectivity, survival, virulence and immune evasion. Among the alternate glycoconjugate structures that have been identified, glycosylphosphatidylinositols (GPIs) represent a universal structure for the anchorage of proteins, lipids, and phosphosaccharides to cellular membranes. Biosynthesis of the GPI is a multi-step process that culminates in the attachment of the assembled GPI to a precursor protein. This final step in the transfer of the GPI to a protein is catalyzed by GPI8 of the putative transamidase complex (TAM). GPI8 functions dually to perform the proteolytic cleavage of the C-terminal signal sequence of the precursor protein, followed by the formation of an amide bond between the protein and the ethanolamine phosphate of the GPI. This review summarizes the current aggregate of biochemical, gene-disruption and active site mutagenesis studies, which provide evidence that GPI8 is responsible for the protein-GPI anchoring reaction. We describe recently published studies that have identified other potential components of the TAM complex and that have elucidated their likely role in protein-GPI attachment. Further, we discuss the biochemical, molecular and functional differences between protozoan and mammalian GPI8 and the protein-GPI anchoring machinery. Finally, we will present the implications of these studies for the development of anti-parasite drug therapies.

Regulated cleavage of intracellular glycosylphosphatidylinositol in a trypanosome. Peroxisome-to-endoplasmic reticulum translocation of a phospholipase C

Cell exposure to hypo-osmolarity and alkalinity triggers a spectrum of responses including activation of phospholipases. Glycosylphosphatidylinositol-specific phospholipase C (GPI-PLC) is expressed in Trypanosoma brucei, a protozoan parasite that causes human African trypanosomiasis. We examined possible contributions of GPI-PLC to the response of T. brucei to hypo-osmotic or mildly alkaline conditions. GPIs were detected at the endoplasmic reticulum (ER). They were cleaved after exposure of T. brucei to hypo-osmolarity or mild alkalinity, which also, strikingly, caused translocation of GPI-PLC from glycosomes (peroxisomes) to the ER. A catalytically inactive Gln81Leu mutant of GPI-PLC failed to cleave GPIs despite being transported from glycosomes to the ER after hypo-osmotic or mild alkaline treatment of the parasites. In contrast, a Cys347Ser mutant of the enzyme could not exit glycosomes after treatment of cells expressing the protein with mild base or hypo-osmotic buffer. We conclude that: (a) GPI-PLC contributes to loss of GPIs in T. brucei treated with hypo-osmotic or mildly alkaline buffer; (b) access of GPI-PLC to its substrate in vivo can be regulated post-translationally; (c) translocation of GPI-PLC from glycosomes to the ER is important for in vivo cleavage of GPIs; (d) Cys347 is part of a peptide motif required for post-translational targeting of GPI-PLC to the ER. Glycosome-to-ER movement of GPI-PLC reveals a novel pathway for intracellular protein traffic. The physiological significance of GPI digestion in cells exposed to mildly alkalinity or hypo-osmolarity is discussed.

Effects of dibucaine on the endocytic/exocytic pathways in Trypanosoma cruzi

Although local anesthetics (LA) are considered primarily Na+-channel blockers in the past decade, an alternative action of LA as inhibitors of fusion among compartments of the endocytic/exocytic pathways was described. In epimastigote forms of Trypanosoma cruzi, we observed that 50 mM dibucaine reduced the rates of uptake of bovine serum albumin (BSA) and immunoglobulin to 60% of control values in addition to the delay of exocytosis of cysteine proteases. Fusion among endocytic compartments was not inhibited in the presence of dibucaine because previously labeled reservosomes was loaded with a second label in sequential pulse-chase experiments. However, dibucaine reduced the degradation of BSA-gold complex in the reservosomes, which was not caused either by an inhibition of the whole proteolytic activity of the parasite or by a reduction on the expression levels of cruzipain. The immunocytochemical analysis suggested that the inhibition of the degradation of gold-labeled BSA in reservosomes could be due to a subversion of the regular traffic of proteases toward the reservosomes in dibucaine-treated cells.

Molecular cloning of the Leishmania major UDP-glucose pyrophosphorylase, functional characterization, and ligand binding analyses using NMR spectroscopy

The dense glycocalyx surrounding the protozoan parasite Leishmania is an essential virulence factor. It protects the parasite from hostile environments in the sandfly vector and mammalian host and supports steps of development and invasion. Therefore, new therapeutic concepts concentrate on disturbing glycocalyx biosynthesis. Deletion of genes involved in the metabolism of galactose and mannose have been shown to drastically reduce Leishmania virulence. Here we report the identification of Leishmania major UDP-glucose pyrophosphorylase (UGP). UGP catalyzes the formation of UDP-glucose from glucose 1-phosphate and UTP. This activation step enables glucose to enter metabolic pathways and is crucial for the activation of galactose. UDP-galactose is made from UDP-glucose by nucleotide-donor transfer to galactose 1-phosphate or by epimerization of the glucose moiety. Isolated in a complementation cloning approach, the activity of L. major UGP was proven in vitro. Moreover, purified protein was used to investigate enzyme kinetics, quaternary organization, and binding of ligands. Whereas sequestration by oligomerization is a known regulatory mechanism for eukaryotic UGPs, the recombinant as well as native L. major UGP migrated as monomer in size exclusion chromatography and in accord with this showed simple Michaelis-Menten kinetics toward all substrates. In saturation transfer difference (STD)-NMR studies, we clearly demonstrated that the molecular geometry at position 4 of glucose is responsible for substrate specificity. Furthermore, the gamma-phosphate group of UTP is essential for binding and for induction of the open conformation, which then allows entry of glucose 1-phosphate. Our data provide the first direct proof for the ordered bi-bi mechanism suggested in earlier studies.

Virulence of Leishmania major in macrophages and mice requires the gluconeogenic enzyme fructose-1,6-bisphosphatase

Leishmania are protozoan parasites that replicate within mature phagolysosomes of mammalian macrophages. To define the biochemical composition of the phagosome and carbon source requirements of intracellular stages of L. major, we investigated the role and requirement for the gluconeogenic enzyme fructose-1,6-bisphosphatase (FBP). L. major FBP was constitutively expressed in both extracellular and intracellular stages and was primarily targeted to glycosomes, modified peroxisomes that also contain glycolytic enzymes. A L. major FBP-null mutant was unable to grow in the absence of hexose, and suspension in glycerol-containing medium resulted in rapid depletion of internal carbohydrate reserves. L. major Deltafbp promastigotes were internalized by macrophages and differentiated into amastigotes but were unable to replicate in the macrophage phagolysosome. Similarly, the mutant persisted in mice but failed to generate normal lesions. The data suggest that Leishmania amastigotes reside in a glucose-poor phagosome and depend heavily on nonglucose carbon sources. Feeding experiments with [(13)C]fatty acids showed that fatty acids are poor gluconeogenic substrates, indicating that amino acids are the major carbon source in vivo. The need for amino acids may have forced Leishmania spp. to adapt to life in the mature phagolysosome.

Effect of Natural l- to d-Amino Acid Conversion on the Organization, Membrane Binding, and Biological Function of the Antimicrobial Peptides Bombinins H

Antimicrobial peptides (AMPs) are evolutionarily old components of innate immunity found in all living pluricellular organisms. Interestingly, some organisms express families of AMPs with only a slight variation among their members, possibly to increase their spectrum of activity. Despite the growing body of knowledge about their biological activity and mode of action on bacteria, only a few of them have been tested on Leishmania, a worldwide spread protozoan pathogen, and the parameters contributing to this activity are yet to be determined. We report on the anti-Leishmania activity and mode of action of bombinins H2 and H4 isolated from the skin secretion of the frog Bombina variegata. H4, the most active, is the first natural AMP of animal origin with a single L- to D-amino acid isomerization. Membrane depolarization and membrane permeation assays, as well as electron microscopy, suggest that the lethal mechanism involves plasma membrane permeation and/or disruption. To better understand the enhanced activity of H4, we determined the peptide's structure in membranes mimicking those of mammals, bacteria, and Leishmania by using ATR-FTIR and CD spectroscopies and assessed their membrane binding by using surface plasmon resonance. The data reveal that (i) H2 but not H4 partially aggregates in membranes mimicking those of Leishmania, (ii) H2 is slightly more helical than H4 in all membranes, and (iii) H4 binds the Leishmania model membrane approximately 5-fold better than H2. This study highlights the importance of a single alpha-amino acid epimerization as a tool used by nature to modulate the activity of AMPs. In addition, our findings suggest bombinins H as potential templates for the development of new drugs with a new mode of action against Leishmania.

Clathrin-independent endocytosis: new insights into caveolae and non-caveolar lipid raft carriers

A number of recent studies have provided new insights into the complexity of the endocytic pathways originating at the plasma membrane of mammalian cells. Many of the molecules involved in clathrin coated pit internalization are now well understood but other pathways are less well defined. Caveolae appear to represent a low capacity but highly regulated pathway in a restricted set of tissues in vivo. A third pathway, which is both clathrin- and caveolae-independent, may constitute a specialized high capacity endocytic pathway for lipids and fluid. The relationship of this pathway, if any, to macropinocytosis or to the endocytic pathways of lower eukaryotes remains an interesting open question. Our understanding of the regulatory mechanisms and molecular components involved in this pathway are at a relatively primitive stage. In this review, we will consider some of the characteristics of different endocytic pathways in high and lower eukaryotes and consider some of the common themes in endocytosis. One theme which becomes apparent from comparison of these pathways is that apparently different pathways can share common molecular machinery and that pathways considered to be distinct actually represent similar basic pathways to which additional levels of regulatory complexity have been added.

Bloodstream form Trypanosoma brucei depend upon multiple metacaspases associated with RAB11-positive endosomes

Trypanosoma brucei possesses five metacaspase genes. Of these, MCA2 and MCA3 are expressed only in the mammalian bloodstream form of the parasite, whereas MCA5 is expressed also in the insect procyclic form. Triple RNAi analysis showed MCA2, MCA3 and MCA5 to be essential in the bloodstream form, with parasites accumulating pre-cytokinesis. Nevertheless, triple null mutants (deltamca2/3deltamca5) could be isolated after sequential gene deletion. Thereafter, deltamca2/3deltamca5 mutants were found to grow well both in vitro in culture and in vivo in mice. We hypothesise that metacaspases are essential for bloodstream form parasites, but they have overlapping functions and their progressive loss can be compensated for by activation of alternative biochemical pathways. Analysis of deltamca2/3deltamca5 revealed no greater or lesser susceptibility to stresses reported to initiate programmed cell death, such as treatment with prostaglandin D2. The metacaspases were found to colocalise with RAB11, a marker for recycling endosomes. However, variant surface glycoprotein (VSG) recycling processes and the degradation of internalised anti-VSG antibody were found to occur similarly in wild type, deltamca2/3deltamca5 and triple RNAi induced parasites. Thus, the data provide no support for the direct involvement of T. brucei metacaspases in programmed cell death and suggest that the proteins have a function associated with RAB11 vesicles that is independent of known recycling processes of RAB11-positive endosomes.

Clathrin-independent endocytosis: new insights into caveolae and non-caveolar lipid raft carriers

A number of recent studies have provided new insights into the complexity of the endocytic pathways originating at the plasma membrane of mammalian cells. Many of the molecules involved in clathrin coated pit internalization are now well understood but other pathways are less well defined. Caveolae appear to represent a low capacity but highly regulated pathway in a restricted set of tissues in vivo. A third pathway, which is both clathrin- and caveolae-independent, may constitute a specialized high capacity endocytic pathway for lipids and fluid. The relationship of this pathway, if any, to macropinocytosis or to the endocytic pathways of lower eukaryotes remains an interesting open question. Our understanding of the regulatory mechanisms and molecular components involved in this pathway are at a relatively primitive stage. In this review, we will consider some of the characteristics of different endocytic pathways in high and lower eukaryotes and consider some of the common themes in endocytosis. One theme which becomes apparent from comparison of these pathways is that apparently different pathways can share common molecular machinery and that pathways considered to be distinct actually represent similar basic pathways to which additional levels of regulatory complexity have been added.

Toxoplasma gondii grown in human cells uses GalNAc-containing glycosylphosphatidylinositol precursors to anchor surface antigens while the immunogenic Glc–GalNAc-containing precursors remain free at the parasite cell surface

Toxoplasma gondii is a ubiquitous parasite that infects nearly all warm-blooded animals. Developmental switching in T. gondii, from the virulent tachyzoite to the relatively quiescent bradyzoite stage, is responsible for the disease propagation after alteration of the immune status of the carrier. The redifferentiation event is characterized by an over expression of a tachyzoite specific set of glycosylphosphatidylinositol anchored surface antigens and free GPIs. T. gondii grown in animal cells uses two glycosylphosphatidylinositol precursors to anchor the parasite surface proteins. The first form has an N-acetylgalactosamine residue bound to a conserved three-mannosyl core glycan, while the second structure contains an additional terminal glucose linked to the N-acetylgalactosamine side branch. Sera from persons infected with T. gondii reacted only with the glucose-N-acetylgalactosamine-containing structure. Here we report that T. gondii cultured in human cells uses predominantly the N-acetylgalactosamine-containing structure to anchor the parasite surface antigens. On the other hand, glycosylphosphatidylinositol structures having an additional terminal glucose are found exclusively on the parasite cell surface as free glycolipids participating in the production of cytokines that are implicated in the pathogenesis of T. gondii. We also provide evidence that such free glycosylphosphatidylinositols are restricted mainly to the lipid microdomains in the parasite cell surface membrane and mostly associated with proteins involved in the parasite motility as well as invasion of the host cell.

Leishmania donovani amastigotes mobilize organic and inorganic osmolytes during regulatory volume decrease

The protozoan parasite Leishmania donovani encounters large fluctuations in osmolality as it cycles between its insect vector and human host. The flagellated promastigote exhibits regulatory volume responses involving organic and inorganic osmolytes, but little is known about volume regulation in the clinically relevant amastigote that multiplies within the parasitophorous vacuoles of mammalian host cells. Using a combination of morphological, X-ray microanalytical, and biochemical approaches we determined that non-motile amastigotes respond to hypotonic stress with (1) an amino acid and l-alanine-mediated regulatory volume decrease, and (2) a parallel release of Na+, K+, P (presumably as negatively charged phosphates), and subsequently Cl- from cytoplasm and the cell as a whole. In addition P, Zn2+, and subsequently Ca2+ increase in acidocalcisomes as Cl- content declines in this compartment. This evidence is the first to document subcellular translocation of, and thus a potential role for, zinc in volume regulatory responses. These coordinated changes in organic and inorganic osmolytes demonstrate that amastigote subcellular compartments, particularly acidocalcisomes, function in maintaining ionic homeostasis in the response of Leishmania amastigotes to hypo-osmotic stress.

Pathogenomics analysis of Leishmania spp.: flagellar gene families of putative virulence factors

The trypanosomatid flagellar apparatus contains conventional and unique features, whose roles in infectivity are still enigmatic. Although the flagellum and the flagellar pocket are critical organelles responsible for all vesicular trafficking between the cytoplasm and cell surface, still very little is known about their roles in pathogenesis and how molecules get to and from the flagellar pocket. The ongoing analysis of the genome sequences and proteome profiles of Leishmania major and L infantum, Trypanosoma cruzi, T. brucei, and T. gambiensi ( www.genedb.org ), coupled with our own work on L. chagasi (as part of the Brazilian Northeast Genome Program- www.progene.ufpe.br ), prompted us to scrutinize flagellar genes and proteins of Leishmania spp. promastigotes that could be virulence factors in leishmaniasis. We have identified some overlooked parasite factors such as the MNUDC-1 (a protein involved in nuclear development and genomic fusion) and SQS (an enzyme of sterol biosynthesis), among the described flagellar gene families. A database concerning the results of this work, as well as of other studies of Leishmania and its organelles, is available at http://nugen.lcc.uece.br/LPGate . It will serve as a convenient bioinformatics resource on genomics and pathology of the etiological agents of leishmaniasis.

Binding affinity and capacity of putative adaptor-mediated sorting of a Type I membrane protein in Leishmania mexicana

The membrane-bound acid phosphatase (MBAP), a Type I membrane protein predominantly associated with endosomal/lysosomal structures of Leishmania mexicana promastigotes, contains motifs in its cytosolic COOH-terminal tail (-MEVWRRYMKFKNKQSEAIIV-COOH) akin to tyrosine- and di-leucine-based sorting signals in multicellular organisms. Here, we first show that the COOH-terminal residues IIV of MBAP, but not the Y-residue, are required for endosomal targeting, suggesting specific binding to an adaptor complex at the cell surface. We then determine whether specific binding can be saturated by analysing the efficiency of endosomal targeting for increasing numbers of MBAP molecules per cell. The ratio of the steady-state abundance of wild-type MBAP on the cell surface to MBAP on endosomes increases until the distribution is no longer different from that observed for a mutant MBAP which lacks the IIV-motif or for a glycosylphosphatidylinositol-anchored form, both of which are distributed according to bulk membrane flow. A quantitative analysis of these in vivo results indicates specific binding to a putative adaptor complex with an affinity of about 10-4M to 50,000 sorting sites on the cell surface.

Clathrin-dependent targeting of receptors to the flagellar pocket of procyclic-form Trypanosoma brucei

In trypanosomatids, endocytosis and exocytosis occur exclusively at the flagellar pocket, which represents about 0.43% of the pellicle membrane and is a deep invagination of the plasma membrane where the flagellum extends from the cell. Receptor molecules are selectively retained at the flagellar pocket. We studied the function of clathrin heavy chain (TbCLH) in the trafficking of the flagellar pocket receptors in Trypanosoma brucei by using the double-stranded RNA interference approach. It appears that TbCLH is essential for the survival of both the procyclic form and the bloodstream form of T. brucei, even though structures resembling large coated endocytic vesicles are absent in procyclic-form trypanosomes. Down-regulation of TbCLH by RNA interference (RNAi) for 24 h rapidly and drastically reduced the uptake of macromolecules via receptor-mediated endocytosis in procyclic-form trypanosomes. This result suggested the importance of TbCLH in receptor-mediated endocytosis of the procyclic-form trypanosome, in which the formation of large coated endocytic vesicles may not be required. Surprisingly, induction of TbCLH RNAi in the procyclic T. brucei for a period of 48 h prohibited the export of the flagellar pocket-associated transmembrane receptor CRAM from the endoplasmic reticulum to the flagellar pocket, while trafficking of the glycosylphosphatidylinositol-anchored procyclin coat was not significantly affected. After 72 h of induction of TbCLH RNAi, procyclics exhibited morphological changes to an apolar round shape without a distinct structure of the flagellar pocket and flagellum. Although trypanosomes, like other eukaryotes, use similar organelles and machinery for protein sorting and transport, our studies reveal a novel role for clathrin in the secretory pathway of trypanosomes. We speculate that the clathrin-dependent trafficking of proteins to the flagellar pocket may be essential for the biogenesis and maintenance of the flagellar pocket in trypanosomes.

Subcellular localization of an intracellular serine protease of 68 kDa in Leishmania (Leishmania) amazonensis promastigotes

Here we report the subcellular localization of an intracellular serine protease of 68 kDa in axenic promastigotes of Leishmania (Leishmania) amazonensis, using subcellular fractionation, enzymatic assays, immunoblotting, and immunocytochemistry. All fractions were evaluated by transmission electron microscopy and the serine protease activity was measured during the cell fractionation procedure using alpha-N-r-tosyl-L-arginine methyl ester (L-TAME) as substrate, phenylmethylsulphone fluoride (PMSF) and L-1-tosylamino-2-phenylethylchloromethylketone (TPCK) as specific inhibitors. The enzymatic activity was detected mainly in a membranous vesicular fraction (6.5-fold enrichment relative to the whole homogenate), but also in a crude plasma membrane fraction (2.0-fold). Analysis by SDS-PAGE gelatin under reducing conditions demonstrated that the major proteolytic activity was found in a 68 kDa protein in all fractions studied. A protein with identical molecular weight was also recognized in immunoblots by a polyclonal antibody against serine protease (anti-SP), with higher immunoreactivity in the vesicular fraction. Electron microscopic immunolocalization using the same polyclonal antibody showed the enzyme present at the cell surface, as well as in cytoplasmic membranous compartments of the parasite. Our findings indicate that the internal location of this serine protease in L. amazonensis is mainly restricted to the membranes of intracellular compartments resembling endocytic/exocytic elements.

A bioinformatic analysis of the RAB genes of Trypanosoma brucei

RAB proteins are small GTPases with vital roles in eukaryotic intracellular transport; orthologous RABs appear to fulfil similar functions in diverse organisms. Trypanosoma brucei spp., the causative organisms of Old World trypanosomiasis of humans and domestic animals, have extremely effective endocytic and exocytic mechanisms that are likely to be involved in maintenance of infection, making study of these systems of importance. Taking advantage of the essential completion of the T. brucei genome, we have re-examined the T. brucei RABs (TbRABs) so far described and identified a total of 16. BLAST searches and phylogenetic analysis show that nine of the TbRABs can confidently be assigned as orthologues or homologues of known RAB proteins from higher eukaryotes, and four more with reasonable probability. The core endocytic pathway is probably similar in complexity to yeast, whilst the early exocytic pathway appears to be more complex than in yeast. Two of the TbRAB family (RAB23 and 28) with clear mammalian orthologues appear to be unusual, and may be involved in nuclear processes and are described in more detail in an accompanying paper. Three TbRABs appear, however, to have no close homologues and may fulfil specialised functions in this organism. The availability of a complete set of TbRABs--which includes orthologues of the RABs responsible for control of the core of the endomembrane system (i.e. RAB1, 2, 4-7 and 11)--provides a first overview of the trafficking complexity that is present within a kinetoplastid parasite. Based on these homologies we suggest a systematic nomenclature for the TbRABs to reflect their functional homologies. This information is of importance both from the perspective of understanding the evolution and diversity of eukaryotic trafficking, but also in providing a framework by which to understand protein processing, trafficking, endocytosis and other related processes in these parasites.

Structural analysis of the N-glycans of the major cysteine proteinase of Trypanosoma cruzi

Trypanosoma cruzi, the parasitic protozoan that causes Chagas disease, contains a major cysteine proteinase, cruzipain. This lysosomal enzyme bears an unusual C-terminal extension that contains a number of post-translational modifications, and most antibodies in natural and experimental infections are directed against it. In this report we took advantage of UV-MALDI-TOF mass spectrometry in conjunction with peptide N-glycosidase F deglycosylation and high performance anion exchange chromatography analysis to address the structure of the N-linked oligosaccharides present in this domain. The UV-MALDI-TOF MS analysis in the negative-ion mode, using nor-harmane as matrix, allowed us to determine a new striking feature in cruzipain: sulfated high-mannose type oligosaccharides. Sulfated GlcNAc2Man3 to GlcNAc2Man9 species were identified. In accordance, after chemical or enzymatic desulfation, the corresponding signals disappeared. In addition, by UV-MALDI-TOF MS analysis (a) a main population of high-mannose type oligosaccharides was shown in the positive-ion mode, (b) lactosaminic glycans were also identified, among them, structures corresponding to monosialylated species were detected, and (c) as an interesting fact a fucosylated oligosaccharide was also detected. The presence of the deoxy sugar was further confirmed by high performance anion exchange chromatography. In conclusion, the total number of oligosaccharides occurring in cruzipain was shown to be much higher than previous estimates. This constitutes the first report on the presence of sulfated glycoproteins in Trypanosomatids.

The overexpression of an intracellular ABCA-like transporter alters phospholipid trafficking in Leishmania

In parasites, ATP-binding cassette (ABC) transporters represent an important family of proteins related to drug resistance and other biological activities. Here we report the characterization of LtrABCA2, a new ABC transporter of the ABCA subfamily in the protozoan parasite Leishmania tropica, localized at the flagellar pocket region and in internal vesicles. The overexpression of this transporter reduced the accumulation of fluorescent glycerophospholipid analogs, increased the exocytic activity, and decreased infectivity of macrophage, but did not confer resistance to drugs. Together, these results suggest that this new ABC transporter plays a role in phospholipid trafficking, which may be modifying the vesicular trafficking and the infectivity of the parasite.

A potential role for ICP, a Leishmanial inhibitor of cysteine peptidases, in the interaction between host and parasite

The biological role of a natural inhibitor of cysteine peptidases (designated ICP) of Leishmania has been investigated by genetic manipulation of the parasite. Null mutants grew normally in vitro, were as infective to macrophages in vitro as wild-type parasites, but had reduced infectivity to mice. Mutants re-expressing ICP from a single gene gave partial restoration of virulence in vivo, whereas mutants overexpressing ICP secreted the inhibitor and showed markedly reduced virulence in mice. Promastigotes of the null mutants had similar cysteine peptidase activities as the wild-type parasites, suggesting that ICP is not required for the expression or processing of the enzymes. The only proteins found to bind to ICP in promastigote cell lysates were fully processed forms of CPA and CPB, showing that ICP does not bind in abundance either to zymogens of the cysteine peptidases or other leishmanial proteins. However, only a small proportion of ICP colocalized with CPA and CPB in the promastigote (in the endoplasmic reticulum and Golgi) and the majority of ICP resided in vesicles that are apparently distinct from endosomes and the multivesicular tubule (MVT)-lysosome. These data suggest that ICP has a role other than modulation of the activity of the parasite's own cysteine peptidases and their normal trafficking to the MVT-lysosome via the flagellar pocket. The finding that ICP partially colocalized with an endocytosed cysteine peptidase leads us to postulate that ICP has a role in protection of the parasite against the hydrolytic environment of the sandfly gut and/or the parasitophorous vacuole of host macrophages.

Homologous and heterologous reconstitution of Golgi to chloroplast transport and protein import into the complex chloroplasts of Euglena

Euglena complex chloroplasts evolved through secondary endosymbiosis between a phagotrophic trypanosome host and eukaryotic algal endosymbiont. Cytoplasmically synthesized chloroplast proteins are transported in vesicles as integral membrane proteins from the ER to the Golgi apparatus to the Euglena chloroplast. Euglena chloroplast preprotein pre-sequences contain a functional N-terminal ER-targeting signal peptide and a domain having characteristics of a higher plant chloroplast targeting transit peptide, which contains a hydrophobic stop-transfer membrane anchor sequence that anchors the precursor in the vesicle membrane. Pulse-chase subcellular fractionation studies showed that (35)S-labeled precursor to the light harvesting chlorophyll a/b binding protein accumulated in the Golgi apparatus of Euglena incubated at 15 degrees C and transport to the chloroplast resumed after transfer to 26 degrees C. Transport of the (35)S-labeled precursor to the chlorophyll a/b binding protein from Euglena Golgi membranes to Euglena chloroplasts and import into chloroplasts was reconstituted using Golgi membranes isolated from 15 degrees C cells returned to 26 degrees C. Transport was dependent upon extra- and intrachloroplast ATP and GTP hydrolysis. Golgi to chloroplast transport was not inhibited by N-ethylmaleimide indicating that fusion of Golgi vesicles to the chloroplast envelope does not require N-ethylmaleimide-sensitive factor (NSF). This suggests that N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are not utilized in the targeting fusion reaction. The Euglena precursor to the chloroplast-localized small subunit of ribulose-1,5-bisphosphate carboxylase was not imported into isolated pea chloroplasts. A precursor with the N-terminal signal peptide deleted was imported, indicating that the Euglena pre-sequence has a transit peptide that functions in pea chloroplasts. A precursor to the small subunit of ribulose-1,5-bisphosphate carboxylase with the hydrophobic membrane anchor and the pre-sequence region C-terminal to the hydrophobic membrane anchor deleted was imported localizing the functional transit peptide to the Euglena pre-sequence region between the signal peptidase cleavage site and the hydrophobic membrane anchor. The Euglena precursor to the small subunit of ribulose-1,5-bisphosphate carboxylase and the deletion constructs were not post-translationally imported into isolated Euglena chloroplasts indicating that vesicular transport is the obligate import mechanism. Taken together, these studies suggest that protein import into complex Euglena chloroplasts evolved by developing a novel vesicle fusion targeting system to link the host secretory system to the transit peptide-dependent chloroplast protein import system of the endosymbiont.

Acidocalcisomes - conserved from bacteria to man

Recent work has shown that acidocalcisomes, which are electron-dense acidic organelles rich in calcium and polyphosphate, are the only organelles that have been conserved during evolution from prokaryotes to eukaryotes. Acidocalcisomes were first described in trypanosomatids and have been characterized in most detail in these species. Acidocalcisomes have been linked with several functions, including storage of cations and phosphorus, polyphosphate metabolism, calcium homeostasis, maintenance of intracellular pH homeostasis and osmoregulation. Here, we review acidocalcisome ultrastructure, composition and function in different trypanosomatids and other organisms.

LdARF1 in trafficking and structural maintenance of the trans-Golgi cisternal network in the protozoan pathogen Leishmania donovani

Adenosine diphosphate ribosylation factors (ARFs) are small guanosine-5'-triphosphatases that are essential in vesicular trafficking and in the maintenance of the Golgi network. In this report, we identified a homolog of the mammalian ARF1 in the human pathogenic protozoan parasite, Leishmania donovani (Ld). Ld ARF1 is a 549 bp gene encoding a 183-amino acid deduced protein of approximately 20 kDa. We demonstrated by Southern blot analysis that there are at least two copies of ARF1 in the Ld genome. Moreover, Northern blot analysis revealed that Ld ARF1 is expressed on a 1.35 kb transcript in both the insect vector (promastigotes) and mammalian host (amastigotes) forms of this parasite. Fluorescent microscopy studies using Ld promastigotes episomally transfected with an ARF1::GFP (green fluorescent protein) chimeric construct showed that such chimeras appeared to localize to the Golgi region of these organisms. This observation was verified by immunoelectron microscopy using an anti-GFP antibody. Such studies also revealed that Ld ARF1::GFP chimeras localized to trans-Golgi vesicles, the flagellar pocket/reservoir and other vesicles located between the trans-Golgi network and flagellar pocket in these apically polarized cells. Fluorescence recovery after photobleaching and fluorescence loss in photobleaching experiments revealed both the dynamic binding and releasing activity of Ld ARF1 from the Golgi network in these parasites. Further, episomal expression of a constitutively active ("on") ARF1 (Q71L mutation) resulted in the aberrant swelling and distended-structure of the trans-Golgi cisternae in these cells. These results show that Ld ARF1 is transiently associated with the Golgi network and plays a role in the structural maintenance of this organelle in these important human pathogens.

Immunoblotting analyses using two-dimensional gel electrophoresis of Trypanosoma cruzi excreted-secreted antigens

Trypanosoma cruzi trypomastigotes excrete-secrete a complex mixture of antigenic molecules. This antigenic mixture denominated trypomastigote excreted-secreted antigens contains a 150-160 kDa band that shows excellent performance in Chagas' disease diagnosis by immunoblotting. The present study partially characterized by two-dimensional gel electrophoresis the immunoreactivity against the 150-160 kDa protein using sera samples from chagasic patients in different phases of the disease. Trypomastigote excreted-secreted antigen preparations were subjected to high-resolution two-dimensional (2D) gel electrophoresis followed by immunoblotting with sera from chagasic and non-chagasic patients. The 150-160 kDa protein presented four isoforms with isoelectric focusing ranging from 6.2 to 6.7. The four isoforms were recognized by IgM from acute phase and IgG from chronic phase sera of chagasic patients. The 150-160 kDa isoform with IF of approximately 6.4 became the immunodominant spot with the progression of the disease. No cross-reactivity was observed with non-chagasic or patients infected with Leishmania sp. In this study we provide basic knowledge that supports the validation of trypomastigote excreted-secreted antigens for serological diagnosis of Chagas' disease.

Antileishmanial activity of parthenolide, a sesquiterpene lactone isolated from Tanacetum parthenium

The in vitro activity of parthenolide against Leishmania amazonensis was investigated. Parthenolide is a sesquiterpene lactone purified from the hydroalcoholic extract of aerial parts of Tanacetum parthenium. This isolated compound was identified through spectral analyses by UV, infrared, (1)H and (13)C nuclear magnetic resonance imaging, DEPT (distortionless enhancement by polarization transfer), COSY (correlated spectroscopy), HMQC (heteronuclear multiple-quantum coherence), and electron spray ionization-mass spectrometry. Parthenolide showed significant activity against the promastigote form of L. amazonensis, with 50% inhibition of cell growth at a concentration of 0.37 microg/ml. For the intracellular amastigote form, parthenolide reduced by 50% the survival index of parasites in macrophages when it was used at 0.81 microg/ml. The purified compound showed no cytotoxic effects against J774G8 macrophages in culture and did not cause lysis in sheep blood when it was used at higher concentrations that inhibited promastigote forms. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis with gelatin as the substrate showed that the enzymatic activity of the enzyme cysteine protease increased following treatment of the promastigotes with the isolated compound. This finding was correlated with marked morphological changes induced by parthenolide, such as the appearance of structures similar to large lysosomes and intense exocytic activity in the region of the flagellar pocket, as seen by electron microscopy. These results provide new perspectives on the development of novel drugs with leishmanicidal activities obtained from natural products.

Temporins, small antimicrobial peptides with leishmanicidal activity

Leishmaniasis encompasses a wide range of infections caused by the human parasitic protozoan species belonging to the Leishmania genus. It appears frequently as an opportunistic disease, especially in virus-infected immunodepressed people. Similarly to other pathogens, parasites became resistant to most of the first-line drugs. Therefore, there is an urgent need to develop antiparasitic agents with new modes of action. Gene-encoded antimicrobial peptides are promising candidates, but so far only a few of them have shown anti-protozoa activities. Here we found that temporins A and B, 13-amino acid antimicrobial peptides secreted from the skin of the European red frog Rana temporaria, display anti-Leishmania activity at micromolar concentrations, with no cytolytic activity against human erythrocytes. To the best of our knowledge, temporins represent the shortest natural peptides having the highest leishmanicidal activity and the lowest number of positively charged amino acids (a single lysine/arginine) and maintain biological function in serum. Their lethal mechanism involves plasma membrane permeation based on the following data. (i) They induce a rapid collapse of the plasma membrane potential. (ii) They induce the influx of the vital dye SYTOX Green. (iii) They reduce intracellular ATP levels. (iv) They severely damage the membrane of the parasite, as shown by transmission electron microscopy. Besides giving us basic important information, the unique properties of temporins, as well as their membranolytic effect, which should make it difficult for the pathogen to develop resistance, suggest them as potential candidates for the future design of antiparasitic drugs with a new mode of action.

Cysteine peptidases as virulence factors of Leishmania

Leishmania mexicana amastigotes are particularly rich in cysteine peptidases (CPs), which play important roles in facilitating the survival and growth of the parasites in mammals. The importance of the CPs as virulence factors and their potential as drug targets and vaccine candidates has been investigated extensively. Recent years, however, have heralded advances in our knowledge and understanding of leishmanial CPs on two fronts. Firstly, genome analysis has revealed the great diversity of CPs, and, secondly, the ways in which the most widely studied CPs, designated CPB, influence the interaction between parasite and mammalian host have been elucidated. These topics are the focus of this review.

Trypanosoma brucei glycoproteins contain novel giant poly-N-acetyllactosamine carbohydrate chains

The flagellar pocket of the bloodstream form of the African sleeping sickness parasite Trypanosoma brucei contains material that binds the beta-d-galactose-specific lectin ricin (Brickman, M. J., and Balber, A. E. (1990) J. Protozool. 37, 219-224). Glycoproteins were solubilized from bloodstream form T. brucei cells in 8 M urea and 3% SDS and purified by ricin affinity chromatography. Essentially all binding of ricin to these glycoproteins was abrogated by treatment with peptide N-glycosidase, showing that the ricin ligands are attached to glycoproteins via N-glycosidic linkages to asparagine residues. Glycans released by peptide N-glycosidase were resolved by Bio-Gel P-4 gel filtration into two fractions: a low molecular mass mannose-rich fraction and a high molecular mass galactose and N-acetylglucosamine-rich fraction. The latter fraction was further separated by high pH anion exchange chromatography and analyzed by gas chromatography mass spectrometry, one- and two-dimensional NMR, electrospray mass spectrometry, and methylation linkage analysis. The high molecular mass ricin-binding N-glycans are based on a conventional Manalpha1-3(Manalpha1-6)Manbeta1-4-GlcNAcbeta1-4GlcNAc core structure and contain poly-N-acetyllactosamine chains. A significant proportion of these structures are extremely large and of unusual structure. They contain an average of 54 N-acetyllactosamine (Galbeta1-4GlcNAc) repeats per glycan, linked mostly by -4GlcNAcbeta1-6Galbeta1-interrepeat linkages, with an average of one -4GlcNAcbeta1-3(-4GlcNAcbeta1-6)Galbeta1- branch point in every six repeats. These structures, which also bind tomato lectin, are twice the size reported for the largest mammalian poly-N-acetyllactosamine N-linked glycans and also differ in their preponderance of -4GlcNAcbeta1-6Galbeta1- over -4GlcNacbeta1-3Galbeta1- interrepeat linkages. Molecular modeling suggests that -4GlcNAcbeta1-6Galbeta1- interrepeat linkages produce relatively compact structures that may give these giant N-linked glycans unique physicochemical properties. Fluorescence microscopy using fluorescein isothiocyanatericin indicates that ricin ligands are located mainly in the flagellar pocket and in the endosomal/lysosomal system of the trypanosome.

Roles of N-linked glycans in the endoplasmic reticulum

From a process involved in cell wall synthesis in archaea and some bacteria, N-linked glycosylation has evolved into the most common covalent protein modification in eukaryotic cells. The sugars are added to nascent proteins as a core oligosaccharide unit, which is then extensively modified by removal and addition of sugar residues in the endoplasmic reticulum (ER) and the Golgi complex. It has become evident that the modifications that take place in the ER reflect a spectrum of functions related to glycoprotein folding, quality control, sorting, degradation, and secretion. The glycans not only promote folding directly by stabilizing polypeptide structures but also indirectly by serving as recognition "tags" that allow glycoproteins to interact with a variety of lectins, glycosidases, and glycosyltranferases. Some of these (such as glucosidases I and II, calnexin, and calreticulin) have a central role in folding and retention, while others (such as alpha-mannosidases and EDEM) target unsalvageable glycoproteins for ER-associated degradation. Each residue in the core oligosaccharide and each step in the modification program have significance for the fate of newly synthesized glycoproteins.

Characterization of an ABCA-like transporter involved in vesicular trafficking in the protozoan parasite Trypanosoma cruzi

Protozoan parasites are responsible of important healthy problems, among others malaria, leishmaniasis and trypanosomiasis. The present work reports the characterization of the first mammalian ATP-binding cassette transporter, subfamily A (ABCA)-like in Trypanosoma cruzi. TcABC1 is a single copy gene differentially expressed along the life cycle of the parasite, being absent in its infective form. TcABC1 localizes to the plasma membrane, flagellar pocket and intracellular vesicles. Functional studies of TcABC1 in transfected parasites suggest that the protein is implicated in intracellular trafficking, as determined by the analysis of endocytosis and exocytosis events. The accumulation of the endocytic markers FM4-64 and NBD-SM is increased in transfected parasites. Similarly, ectophosphatase and ectoATPase activities are increased in TcABC1 overproducers. Indeed, transmission electronic microscopy analysis showed a higher number of intracellular vesicles in TcABC1 transfectants. Taken together, these results suggest that the protein is involved in the endocytic and exocytic pathways of T. cruzi.