Incorporation of label from acetate and laurate into the mannan of Leishmania donovani via the glyoxylate cycle

Apparent isocitrate lyase activity in Leishmania amazonensis

Early reports have demonstrated the occurrence of glyoxylate cycle enzymes in several Leishmania species. However, these results have been underestimated because genes for the two key enzymes of the cycle, isocitrate lyase (ICL) and malate synthase (MS), are not annotated in Leishmania genomes. We have re-examined this issue in promastigotes of Leishmania amazonensis. Enzyme activities were assayed spectrophotometrically in cellular extracts and characterized partially. A 40 kDa band displaying ICL activity was visualized on zymograms of the extracts. By immunoblotting with mouse antibodies against ICL from Bacillus stearothermophilus, a band of approximately 40 kDa was identified, coincident with the relative molecular mass of the activity band revealed on zymograms. Indirect immunofluorescence of intact promastigotes showed that the recognized antigen is distributed as a punctuated pattern, mainly distributed beneath the subpellicular microtubules, over a diffused cytoplasmic stain. These results clearly demonstrate the existence of an apparent ICL activity in L. amazonensis promastigotes, which is associated to a 40 kDa polypeptide and distributed both diffused and as punctuate aggregates in the cytoplasm. The relevance of this activity is discussed.

Functional analysis of an AMP forming acetyl CoA synthetase from Leishmania donovani by gene overexpression and targeted gene disruption approaches

Leishmaniasis, a neglected tropical disease is endemic in 98 countries and >350 million people are at risk of getting the infection. The existing chemotherapy of Leishmaniasis is limited due to adverse effects, resistance to existing drugs and increasing cases of HIV-Leishmaniasis co-infection. Hence, there is a need to identify novel metabolic pathways for design of new chemical entities. Acetyl-CoA synthetase (AceCS) is an enzyme of acetate metabolic pathway whose functions are unknown in Leishmania parasite. AceCS from Leishmania donovani (LdAceCS) is significantly different from human host to be explored as a potential drug candidate to develop parasite specific inhibitors. To dissect the functions of LdAceCS in Leishmania promastigotes, two approaches were followed. LdAceCS overexpressing parasites were generated by episomal expression of LdAceCS in promastigotes and single knockout (SKO) cell lines of LdAceCS were generated by targeted gene disruption. An insight into the phenotypic changes undergone by the overexpressors revealed an increase in LdAceCS activity, total lipid content, infectivity and ergosterol levels by ~2.2, 2.2, 1.65 and 3 fold respectively with respect to wild type. Similarly SKO transgenic parasites exhibited ~2.5, 3, 1.5 and 3 fold decrease in activity, total lipid content, infectivity and ergosterol respectively. Repeated attempts to generate null mutants failed thus indicating that LdAceCS is essential for the parasite and can be selectively targeted to combat Leishmania infection. The present study demonstrates that LdAceCS is important for in vitro macrophage infection and is also essential for biosynthesis of total lipids and ergosterol.

Metabolism of Leishmania: proven and predicted

The complete analysis of the genomes of three major trypanosomatid parasites has facilitated comparison of the metabolic capabilities of each, as predicted from gene sequences. Not surprisingly, there are differences but is it possible to correlate these with the lives of the parasites themselves and make further predictions of the meaning and physiological importance of the apparently parasite-specific metabolism? In this article, we relate gene predictions with the results from experimental studies. We also speculate on the key metabolic adaptations of Leishmania and reasons why it differs from Trypanosoma brucei and Trypanosoma cruzi.

Niche metabolism in parasitic protozoa

Complete or partial genome sequences have recently become available for several medically and evolutionarily important parasitic protozoa. Through the application of bioinformatics complete metabolic repertoires for these parasites can be predicted. For experimentally intractable parasites insight provided by metabolic maps generated in silico has been startling. At its more extreme end, such bioinformatics reckoning facilitated the discovery in some parasites of mitochondria remodelled beyond previous recognition, and the identification of a non-photosynthetic chloroplast relic in malarial parasites. However, for experimentally tractable parasites, mapping of the general metabolic terrain is only a first step in understanding how the parasite modulates its streamlined, yet still often puzzlingly complex, metabolism in order to complete life cycles within host, vector, or environment. This review provides a comparative overview and discussion of metabolic strategies used by several different parasitic protozoa in order to subvert and survive host defences, and illustrates how genomic data contribute to the elucidation of parasite metabolism.

Evidence that intracellular beta1-2 mannan is a virulence factor in Leishmania parasites

The protozoan parasite Leishmania mexicana proliferates within macrophage phagolysosomes in the mammalian host. In this study we provide evidence that a novel class of intracellular beta1-2 mannan oligosaccharides is important for parasite survival in host macrophages. Mannan (degree of polymerization 4-40) is expressed at low levels in non-pathogenic promastigote stages but constitutes 80 and 90% of the cellular carbohydrate in the two developmental stages that infect macrophages, non-dividing promastigotes, and lesion-derived amastigotes, respectively. Mannan is catabolized when parasites are starved of glucose, suggesting a reserve function, and developmental stages having low mannan levels or L. mexicana GDPMP mutants lacking all mannose molecules are highly sensitive to glucose starvation. Environmental stresses, such as mild heat shock or the heat shock protein-90 inhibitor, geldanamycin, that trigger the differentiation of promastigotes to amastigotes, result in a 10-25-fold increase in mannan levels. Developmental stages with low mannan levels or L. mexicana mutants lacking mannan do not survive heat shock and are unable to differentiate to amastigotes or infect macrophages in vitro. In contrast, a L. mexicana mutant deficient only in components of the mannose-rich surface glycocalyx differentiates normally and infects macrophages in vitro. Collectively, these data provide strong evidence that mannan accumulation is important for parasite differentiation and survival in macrophages.

A developmentally regulated aconitase related to iron-regulatory protein-1 is localized in the cytoplasm and in the mitochondrion of Trypanosoma brucei

Mitochondrial energy metabolism and Krebs cycle activities are developmentally regulated in the life cycle of the protozoan parasite Trypanosoma brucei. Here we report cloning of a T. brucei aconitase gene that is closely related to mammalian iron-regulatory protein 1 (IRP-1) and plant aconitases. Kinetic analysis of purified recombinant TbACO expressed in Escherichia coli resulted in a K(m) (isocitrate) of 3 +/- 0.4 mM, similar to aconitases of other organisms. This was unexpected since an arginine conserved in the aconitase protein family and crucial for substrate positioning in the catalytic center and for activity of pig mitochondrial aconitase (Zheng, L., Kennedy, M. C., Beinert, H., and Zalkin, H. (1992) J. Biol. Chem. 267, 7895-7903) is substituted by leucine in the TbACO sequence. Expression of the 98-kDa TbACO was shown to be lowest in the slender bloodstream stage of the parasite, 8-fold elevated in the stumpy stage, and increased a further 4-fold in the procyclic stage. The differential expression of TbACO protein contrasted with only minor changes in TbACO mRNA, indicating translational or post-translational mechanisms of regulation. Whereas animal cells express two distinct compartmentalized aconitases, mitochondrial aconitase and cytoplasmic aconitase/IRP-1, TbACO accounts for total aconitase activity in trypanosomes. By cell fractionation and immunofluorescence microscopy, we show that native as well as a transfected epitope-tagged TbACO localizes in both the mitochondrion (30%) and in the cytoplasm (70%). Together with phylogenetic reconstructions of the aconitase family, this suggests that animal IRPs have evolved from a multicompartmentalized ancestral aconitase. The possible functions of a cytoplasmic aconitase in trypanosomes are discussed.

Purification and properties of phosphoglucose isomerases of Trypanosoma cruzi

Glucosephosphate isomerase (PGI; EC 5.3.1.9) of Trypanosoma cruzi epimastigotes was found in about the same proportion in the glycosome and the cytosol. This subcellular distribution is similar to that of Leishmania mexicana, but contrasts with that of T. brucei bloodstream form, where the enzyme is essentially restricted to the glycosome. Glucosephosphate isomerase was highly purified from a glycosome-enriched fraction and to about 70% purity from the soluble extract. Both enzymes displayed Michaelis-Menten-Henri kinetics. Km values for fructose 6-phosphate were 0.125 +/- 0.07 and 0.80 +/- 0.10 mM for the glycosomal and the cytosolic PGIs, respectively. Erythrose-4-phosphate, 6-phosphogluconate and mannose-6-phosphate were inhibitors for both PGIs. Phosphogluconate and erythrose phosphate showed higher affinity for cytosolic PGI than for glycosomal PGI, by 2.5- and 4-fold respectively. The PGIs differed slightly in their isoelectric point (7.1 +/- 0.15 and 7.5 +/- 0.12) and optimum pH range. Both PGIs also differed in their chromatographic properties (ion-exchange and phenyl Sepharose), indicating a difference in charge and hydrophobicity, with the glycosomal enzyme being more hydrophobic. The molecular mass of both PGIs was 186,000 +/- 9000 Da, which is higher than that of other known PGIs, including those from T. brucei and other trypanosomatids. The molecular mass of the subunit, 63 kDa, is similar to that of PGIs from other sources. It appears that PGIs from T. cruzi are trimeric, in contrast with all other known PGIs which are dimeric.

Characterization of the respiratory chain of Leishmania donovani promastigotes

Inhibition analysis of respiration of Leishmania donovani promastigotes in resting, starved and permeabilized cells in the presence of classical electron transfer complex inhibitors such as rotenone, thenoyltrifluoroacetone and antimycin demonstrated the absence of complex I component of the respiratory chain in this organism. Cyanide failed to completely block the oxygen uptake (residual 25-30%) even at high concentrations. The alternative oxidase inhibitor for Trypanosoma brucei, salicylhydroxamic acid (SHAM) had no effect on respiration while the cytochrome o inhibitor orthohydroxydiphenyl (OHD) could block cyanide-insensitive respiration at low concentrations. Succinate-dependent O2 uptake in permeabilized cells follows the classical pathway. Oxidation of NADH by a membrane-rich fraction produced H2O2 as the end product and was insensitive to respiratory chain inhibitors. The presence of NADH-fumarate reductase was demonstrated in membrane-rich fraction and fumarate could reduce H2O2 production from NADH indicating fumarate to be an endogenous substrate for accepting electrons from NADH. A differential route for NADH oxidation was further confirmed by NADH cytochrome c reductase insensitivity to antimycin. A tentative scheme for electron transfer pathway in this organism is proposed in which a reversal of Krebs cycle enzymes occur producing succinate that can be excreted or oxidized depending upon the energy demands of the cell. Inhibition studies also suggest bifurcations of the respiratory chain that can be of minor importance for the organism.

Molecular analysis of glyceraldehyde-3-phosphate dehydrogenase in Trypanoplasma borelli: an evolutionary scenario of subcellular compartmentation in kinetoplastida

In Trypanoplasma borelli, a representative of the Bodonina within the Kinetoplastida, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) activity was detected in both the cytosol and glycosomes. This situation is similar to that previously found in Trypanosomatidae, belonging to a different Kinetoplastida suborder. In Trypanosomatidae different isoenzymes, only distantly related, are responsible for the activity in the two cell compartments. In contrast, immunoblot analysis indicated that the GAPDH activity in cytosol and glycosomes of T. borelli should be attributed to identical or at least very similar proteins related to the glycosomal GAPDH of Trypanosomatidae. Moreover, only genes related to the glycosomal GAPDH genes of Trypanosomatidae could be detected. All attempts to identify a gene related to the one coding for the trypanosomatid cytosolic GAPDH remained unsuccessful. Two tandemly arranged genes were found which are 95% identical. The two encoded polypeptides differ in 17 residues. Their sequences are 72-77% identical to the glycosomal GAPDH of the other Kinetoplastida and share with them some characteristic features: an excess of positively charged residues, specific insertions, and a small carboxy-terminal extension containing the sequence -AKL. This tripeptide conforms to the consensus signal for targeting of proteins to glycosomes. One of the two gene copies has undergone some mutations at positions coding for highly conserved residues of the active site and the NAD(+)-binding domain of GAPDH. Modeling of the protein's three-dimensional structure suggested that several of the substitutions compensate each other, retaining the functional coenzyme-binding capacity, although this binding may be less tight. The presented analysis of GAPDH in T. borelli gives further support to the assertion that one isoenzyme, the cytosolic one, was acquired by horizontal gene transfer during the evolution of the Kinetoplastida, in the lineage leading to the suborder Trypanosomatina (Trypanosoma, Leishmania), after the divergence from the Bodonina (Trypanoplasma). Furthermore, the data clearly suggest that the original GAPDH of the Kinetoplastida has been compartmentalized during evolution.

Subcellular distribution and characterization of glucosephosphate isomerase in Leishmania mexicana mexicana

The glycolytic enzyme glucosephosphate isomerase (PGI) is present in two different cell compartments of Leishmania mexicana promastigotes; more than 90% of the activity was detected in the cytosol, the remainder in glycosomes. This subcellular distribution contrasts with that in Trypanosoma brucei, in which the enzyme activity has been mainly located in the glycosomes. PGI was partially purified from L. mexicana cell extracts. Throughout the purification procedure only one single PGI activity could be detected. The partially purified protein had the same subunit molecular mass (65 kDa) as the previously characterized glycosomal protein of T. brucei. Both proteins were also very similar with respect to their kinetic and antigenic properties. Using the T. brucei glycosomal PGI gene as a hybridization probe, we cloned the corresponding gene of L. mexicana. Only a single PGI locus could be detected in the L. mexicana genome. Characterization of the cloned gene showed that it codes for a polypeptide of 604 amino acids, with a molecular mass of 67,113. The sequences of the Leishmania and Trypanosoma polypeptides are 69% identical. They differ in calculated net charge (-8 versus -2, respectively) and isoelectric point (6.65 versus 7.35). Our data strongly suggest that the PGI activity in the two cell compartments of L. mexicana and T. brucei is not attributable to different isoenzymes. We discuss the possible metabolic function of the highly different enzyme distribution in the two organisms, and the molecular mechanism that could be responsible for it.

Energy metabolism in Leishmania

Alanine plays a key role in the response of promastigotes to osmotic stress and to hypoxia. It is rapidly released in response to hypo-osmolality, is consumed from its large intracellular pool under iso-osmotic conditions even in the presence of glucose, and is synthesized under hyperosmotic conditions even in the absence of glucose. Its rate of oxidation, in the presence or absence of any of ten other amino acids tested, is strongly inhibited by hyperosmolality. Glucose oxidation is also inhibited by hyperosmolality, but to a lesser extent than that of alanine, and is inhibited by alanine, glutamate, and aspartate. Hyperosmolality also inhibits the incorporation of label from [2-14C]acetate into the putative storage carbohydrate, mannan, which occurs via the glyoxylate bypass and the as yet unexplored "mannoneogenic" pathway. The rates of glycolysis and of oxidation of several amino acids decrease with increasing culture age, but the capacity to oxidize fatty acids increases, and in cells from 3-day stationary phase cultures hyperosmolality enhances rather than inhibits alanine oxidation.

The evolution of kinetoplastid glycosomes

The available data on carbohydrate metabolism in Kinetoplastida have been reviewed. Based on the metabolic pattern of different kinetoplastid organisms, on the subcellular distribution of their glycolytic enzymes, and on the structural and regulatory properties of these proteins, we propose that the glycosome developed from an endosymbiont, as a specific manner to control carbohydrate and energy metabolism. It is discussed how the enzymes were subcellularly recompartmentalized during evolution as adaptation to the environment encountered by the organisms.