Heme biosynthesis in bacterium-protozoon symbioses: enzymic defects in host hemoflagellates and complemental role of their intracellular symbiotes

Heme metabolism in Strigomonas culicis: Implications of H2O2 resistance induction and symbiont elimination

Monoxenous trypanosomatid Strigomonas culicis harbors an endosymbiotic bacterium, which enables the protozoa to survive without heme supplementation. The impact of H2O2 resistance and symbiont elimination on intracellular heme and Fe2+ availability was analyzed through a comparison of WT strain with both WT H2O2-resistant (WTR) and aposymbiotic (Apo) protozoa. The relative quantification of the heme biosynthetic pathway through label-free parallel reaction monitoring targeted mass spectrometry revealed that H2O2 resistance does not influence the abundance of tryptic peptides. However, the Apo strain showed increased coproporphyrinogen III oxidase and ferrochelatase levels. A putative ferrous iron transporter, homologous to LIT1 and TcIT from Leishmania major and Trypanosoma cruzi, was identified for the first time. Label-free parallel reaction monitoring targeted mass spectrometry also showed that S. culicis Iron Transporter (ScIT) increased 1.6- and 16.4-fold in WTR and Apo strains compared to WT. Accordingly, antibody-mediated blockage of ScIT decreased by 28.0% and 40.0% intracellular Fe2+concentration in both WTR and Apo strains, whereas no effect was detected in WT. In a heme-depleted medium, adding 10 μM hemin decreased ScIT transcript levels in Apo, whereas 10 μM PPIX, the substrate of ferrochelatase, increased intracellular Fe2+ concentration and ferric iron reduction. Overall, the data suggest mechanisms dependent on de novo heme synthesis (and its substrates) in the Apo strain to overcome reduced heme availability. Given the importance of heme and Fe2+ as cofactors in metabolic pathways, including oxidative phosphorylation and antioxidant systems, this study provides novel mechanistic insights associated with H2O2 resistance in S. culicis.

Lipidomic Analysis Reveals Branched-Chain and Cyclic Fatty Acids from Angomonas deanei Grown under Different Nutritional and Physiological Conditions

Angomonas deanei belongs to Trypanosomatidae family, a family of parasites that only infect insects. It hosts a bacterial endosymbiont in a mutualistic relationship, constituting an excellent model for studying organelle origin and cellular evolution. A lipidomic approach, which allows for a comprehensive analysis of all lipids in a biological system (lipidome), is a useful tool for identifying and measuring different expression patterns of lipid classes. The present study applied GC-MS and NMR techniques, coupled with principal component analysis (PCA), in order to perform a comparative lipidomic study of wild and aposymbiotic A. deanei grown in the presence or absence of FBS. Unusual contents of branched-chain iso C17:0 and C19:0-cis-9,10 and-11,12 fatty acids were identified in A. deanei cultures, and it was interesting to note that their content slightly decreased at the log phase culture, indicating that in the latter growth stages the cell must promote the remodeling of lipid synthesis in order to maintain the fluidity of the membrane. The combination of analytical techniques used in this work allowed for the detection and characterization of lipids and relevant contributors in a variety of A. deanei growth conditions.

Notes from the Underground: Heme Homeostasis in C. elegans

Heme is an iron-containing tetrapyrrole that plays a critical role in various biological processes, including oxygen transport, electron transport, signal transduction, and catalysis. However, free heme is hydrophobic and potentially toxic to cells. Organisms have evolved specific pathways to safely transport this essential but toxic macrocycle within and between cells. The bacterivorous soil-dwelling nematode Caenorhabditis elegans is a powerful animal model for studying heme-trafficking pathways, as it lacks the ability to synthesize heme but instead relies on specialized trafficking pathways to acquire, distribute, and utilize heme. Over the past 15 years, studies on this microscopic animal have led to the identification of a number of heme-trafficking proteins, with corresponding functional homologs in vertebrates. In this review, we provide a comprehensive overview of the heme-trafficking proteins identified in C. elegans and their corresponding homologs in related organisms.

Hemoglobin uptake and utilization by human protozoan parasites: a review

The protozoan disease is a major global health concern. Amoebiasis, leishmaniasis, Chagas disease, and African sleeping sickness affect several million people worldwide, leading to millions of deaths annually and immense social and economic problems. Iron is an essential nutrient for nearly all microbes, including invading pathogens. The majority of iron in mammalian hosts is stored intracellularly in proteins, such as ferritin and hemoglobin (Hb). Hb, present in blood erythrocytes, is a very important source of iron and amino acids for pathogenic microorganisms ranging from bacteria to eukaryotic pathogens, such as worms, protozoa, yeast, and fungi. These organisms have developed adequate mechanisms to obtain Hb or its byproducts (heme and globin) from the host. One of the major virulence factors identified in parasites is parasite-derived proteases, essential for host tissue degradation, immune evasion, and nutrient acquisition. The production of Hb-degrading proteases is a Hb uptake mechanism that degrades globin in amino acids and facilitates heme release. This review aims to provide an overview of the Hb and heme-uptake mechanisms utilized by human pathogenic protozoa to survive inside the host.

Hemoglobin Endocytosis and Intracellular Trafficking: A Novel Way of Heme Acquisition by Leishmania

Leishmania species are causative agents of human leishmaniasis, affecting 12 million people annually. Drugs available for leishmaniasis are toxic, and no vaccine is available. Thus, the major thrust is to identify new therapeutic targets. Leishmania is an auxotroph for heme and must acquire heme from the host for its survival. Thus, the major focus has been to understand the heme acquisition process by the parasites in the last few decades. It is conceivable that the parasite is possibly obtaining heme from host hemoprotein, as free heme is not available in the host. Current understanding indicates that Leishmania internalizes hemoglobin (Hb) through a specific receptor by a clathrin-mediated endocytic process and targets it to the parasite lysosomes via the Rab5 and Rab7 regulated endocytic pathway, where it is degraded to generate intracellular heme that is used by the parasite. Subsequently, intra-lysosomal heme is initially transported to the cytosol and is finally delivered to the mitochondria via different heme transporters. Studies using different null mutant parasites showed that these receptors and transporters are essential for the survival of the parasite. Thus, the heme acquisition process in Leishmania may be exploited for the development of novel therapeutics.

Aedes aegypti Infection With Trypanosomatid Strigomonas culicis Alters Midgut Redox Metabolism and Reduces Mosquito Reproductive Fitness

Aedes aegypti mosquitoes transmit arboviruses of important global health impact, and their intestinal microbiota can influence vector competence by stimulating the innate immune system. Midgut epithelial cells also produce toxic reactive oxygen species (ROS) by dual oxidases (DUOXs) that are essential players in insect immunity. Strigomonas culicis is a monoxenous trypanosomatid that naturally inhabits mosquitoes; it hosts an endosymbiotic bacterium that completes essential biosynthetic pathways of the parasite and influences its oxidative metabolism. Our group previously showed that S. culicis hydrogen peroxide (H₂O₂)-resistant (WTR) strain is more infectious to A. aegypti mosquitoes than the wild-type (WT) strain. Here, we investigated the influence of both strains on the midgut oxidative environment and the effect of infection on mosquito fitness and immunity. WT stimulated the production of superoxide by mitochondrial metabolism of midgut epithelial cells after 4 days post-infection, while WTR exacerbated H₂O₂ production mediated by increased DUOX activity and impairment of antioxidant system. The infection with both strains also disrupted the fecundity and fertility of the females, with a greater impact on reproductive fitness of WTR-infected mosquitoes. The presence of these parasites induced specific transcriptional modulation of immune-related genes, such as attacin and defensin A during WTR infection (11.8- and 6.4-fold, respectively) and defensin C in WT infection (7.1-fold). Thus, we propose that A. aegypti oxidative response starts in early infection time and does not affect the survival of the H₂O₂-resistant strain, which has a more efficient antioxidant system. Our data provide new biological aspects of A. aegypti–S. culicis relationship that can be used later in alternative vector control strategies.

Quantitative Proteomic Map of the Trypanosomatid Strigomonas culicis: The Biological Contribution of its Endosymbiotic Bacterium

Strigomonas culicis is a kinetoplastid parasite of insects that maintains a mutualistic association with an intracellular symbiotic bacterium, which is highly integrated into the protist metabolism: it furnishes essential compounds and divides in synchrony with the eukaryotic nucleus. The protist, conversely, can be cured of the endosymbiont, producing an aposymbiotic cell line, which presents a diminished ability to colonize the insect host. This obligatory association can represent an intermediate step of the evolution towards the formation of an organelle, therefore representing an interesting model to understand the symbiogenesis theory. Here, we used shotgun proteomics to compare the S. culicis endosymbiont-containing and aposymbiotic strains, revealing a total of 11,305 peptides, and up to 2,213 proteins (2,029 and 1,452 for wild type and aposymbiotic, respectively). Gene ontology associated to comparative analysis between both strains revealed that the biological processes most affected by the elimination of the symbiont were the amino acid synthesis, as well as protein synthesis and folding. This large-scale comparison of the protein expression in S. culicis marks a step forward in the comprehension of the role of endosymbiotic bacteria in monoxenous trypanosomatid biology, particularly because trypanosomatids expression is mostly post-transcriptionally regulated.

Comparative genomics of Leishmania (Mundinia)

Background

Trypanosomatids of the genus Leishmania are parasites of mammals or reptiles transmitted by bloodsucking dipterans. Many species of these flagellates cause important human diseases with clinical symptoms ranging from skin sores to life-threatening damage of visceral organs. The genus Leishmania contains four subgenera: Leishmania, Sauroleishmania, Viannia, and Mundinia. The last subgenus has been established recently and remains understudied, although Mundinia contains human-infecting species. In addition, it is interesting from the evolutionary viewpoint, representing the earliest branch within the genus and possibly with a different type of vector. Here we analyzed the genomes of L. (M.) martiniquensis, L. (M.) enriettii and L. (M.) macropodum to better understand the biology and evolution of these parasites.

Results

All three genomes analyzed were approximately of the same size (~ 30 Mb) and similar to that of L. (Sauroleishmania) tarentolae, but smaller than those of the members of subgenera Leishmania and Viannia, or the genus Endotrypanum (~ 32 Mb). This difference was explained by domination of gene losses over gains and contractions over expansions at the Mundinia node, although only a few of these genes could be identified. The analysis predicts significant changes in the Mundinia cell surface architecture, with the most important ones relating to losses of LPG-modifying side chain galactosyltransferases and arabinosyltransferases, as well as β-amastins. Among other important changes were gene family contractions for the oxygen-sensing adenylate cyclases and FYVE zinc finger-containing proteins.

Conclusions

We suggest that adaptation of Mundinia to different vectors and hosts has led to alternative host-parasite relationships and, thereby, made some proteins redundant. Thus, the evolution of genomes in the genus Leishmania and, in particular, in the subgenus Mundinia was mainly shaped by host (or vector) switches.

Heme synthesis through the life cycle of the heme auxotrophic parasite Leishmania major

Heme is an essential molecule synthetized through a broadly conserved 8-step route that has been lost in trypanosomatid parasites. Interestingly, Leishmania reacquired by horizontal gene transfer from γ-proteobacteria the genes coding for the last 3 enzymes of the pathway. Here we show that intracellular amastigotes of Leishmania major can scavenge heme precursors from the host cell to fulfill their heme requirements, demonstrating the functionality of this partial pathway. To dissect its role throughout the L. major life cycle, the significance of L. major ferrochelatase (LmFeCH), the terminal enzyme of the route, was evaluated. LmFeCH expression in a heterologous system demonstrated its activity. Knockout promastigotes lacking lmfech were not able to use the ferrochelatase substrate protoporphyrin IX as a source of heme. In vivo infection of Phlebotomus perniciosus with knockout promastigotes shows that LmFeCH is not required for their development in the sandfly. In contrast, the replication of intracellular amastigotes was hampered in vitro by the deletion of lmfech. However, LmFeCH^-/- parasites produced disease in a cutaneous leishmaniasis murine model in a similar way as control parasites. Therefore, although L. major can synthesize de novo heme from macrophage precursors, this activity is dispensable being an unsuited target for leishmaniasis treatment.-Orrego, L. M., Cabello-Donayre, M., Vargas, P., Martínez-García, M., Sánchez, C., Pineda-Molina, E., Jiménez, M., Molina, R., Pérez-Victoria, J. M. Heme synthesis through the life cycle of the heme auxotrophic parasite Leishmania major.

Recent advances in trypanosomatid research: genome organization, expression, metabolism, taxonomy and evolution

Unicellular flagellates of the family Trypanosomatidae are obligatory parasites of invertebrates, vertebrates and plants. Dixenous species are aetiological agents of a number of diseases in humans, domestic animals and plants. Their monoxenous relatives are restricted to insects. Because of the high biological diversity, adaptability to dramatically different environmental conditions, and omnipresence, these protists have major impact on all biotic communities that still needs to be fully elucidated. In addition, as these organisms represent a highly divergent evolutionary lineage, they are strikingly different from the common 'model system' eukaryotes, such as some mammals, plants or fungi. A number of excellent reviews, published over the past decade, were dedicated to specialized topics from the areas of trypanosomatid molecular and cell biology, biochemistry, host-parasite relationships or other aspects of these fascinating organisms. However, there is a need for a more comprehensive review that summarizing recent advances in the studies of trypanosomatids in the last 30 years, a task, which we tried to accomplish with the current paper.

The reduced genome of Candidatus Kinetoplastibacterium sorsogonicusi, the endosymbiont of Kentomonas sorsogonicus (Trypanosomatidae): loss of the haem-synthesis pathway

Trypanosomatids of the genera Angomonas and Strigomonas (subfamily Strigomonadinae) have long been known to contain intracellular beta-proteobacteria, which provide them with many important nutrients such as haem, essential amino acids and vitamins. Recently, Kentomonas sorsogonicus, a divergent member of Strigomonadinae, has been described. Herein, we characterize the genome of its endosymbiont, Candidatus Kinetoplastibacterium sorsogonicusi. This genome is completely syntenic with those of other known Ca. Kinetoplastibacterium spp., but more reduced in size (~742 kb, compared with 810-833 kb, respectively). Gene losses are not concentrated in any hot-spots but are instead distributed throughout the genome. The most conspicuous loss is that of the haem-synthesis pathway. For long, removing haemin from the culture medium has been a standard procedure in cultivating trypanosomatids isolated from insects; continued growth was considered as an evidence of endosymbiont presence. However, we demonstrate that, despite bearing the endosymbiont, K. sorsogonicus cannot grow in culture without haem. Thus, the traditional test cannot be taken as a reliable criterion for the absence or presence of endosymbionts in trypanosomatid flagellates. It remains unclear why the ability to synthesize such an essential compound was lost in Ca. K. sorsogonicusi, whereas all other known bacterial endosymbionts of trypanosomatids retain them.

Hydrogen peroxide resistance in Strigomonas culicis: Effects on mitochondrial functionality and Aedes aegypti interaction

Reactive oxygen species (ROS) are toxic molecules involved in several biological processes such as cellular signaling, proliferation, differentiation and cell death. Adaptations to oxidative environments are crucial for the success of the colonization of insects by protozoa. Strigomonas culicis is a monoxenic trypanosomatid found in the midgut of mosquitoes and presenting a life cycle restricted to the epimastigote form. Among S. culicis peculiarities, there is an endosymbiotic bacterium in the cytoplasm, which completes essential biosynthetic routes of the host cell and may represent an intermediary evolutive step in organelle origin, thus constituting an interesting model for evolutive researches. In this work, we induced ROS resistance in wild type S. culicis epimastigotes by the incubation with increasing concentrations of hydrogen peroxide (H₂O₂), and compared the oxidative and energetic metabolisms among wild type, wild type-H₂O₂ resistant and aposymbiotic strains. Resistant protozoa were less sensitive to the oxidative challenge and more dependent on oxidative phosphorylation, which was demonstrated by higher oxygen consumption and mitochondrial membrane potential, increased activity of complexes II-III and IV, increased complex II gene expression and higher ATP production. Furthermore, the wild type-H₂O₂ resistant strain produced reduced ROS levels and showed lower lipid peroxidation, as well as an increase in gene expression of antioxidant enzymes and thiol-dependent peroxidase activity. On the other hand, the aposymbiotic strain showed impaired mitochondrial function, higher H₂O₂ production and deficient antioxidant response. The induction of H₂O₂ resistance also led to a remarkable increase in Aedes aegypti midgut binding in vitro and colonization in vivo, indicating that both the pro-oxidant environment in the mosquito gut and the oxidative stress susceptibility regulate S. culicis population in invertebrates.

The Trypanosoma brucei TbHrg protein is a heme transporter involved in the regulation of stage-specific morphological transitions

The human parasite Trypanosoma brucei does not synthesize heme de novo and instead relies entirely on heme supplied by its vertebrate host or its insect vector, the tsetse fly. In the host bloodstream T. brucei scavenges heme via haptoglobin-hemoglobin (HpHb) receptor-mediated endocytosis occurring in the flagellar pocket. However, in the procyclic developmental stage, in which T. brucei is confined to the tsetse fly midgut, this receptor is apparently not expressed, suggesting that T. brucei takes up heme by a different, unknown route. To define this alternative route, we functionally characterized heme transporter TbHrg in the procyclic stage. RNAi-induced down-regulation of TbHrg in heme-limited culture conditions resulted in slower proliferation, decreased cellular heme, and marked changes in cellular morphology so that the cells resemble mesocyclic trypomastigotes. Nevertheless, the TbHrg KO developed normally in the tsetse flies at rates comparable with wild-type cells. T. brucei cells overexpressing TbHrg displayed up-regulation of the early procyclin GPEET and down-regulation of the late procyclin EP1, two proteins coating the T. brucei surface in the procyclic stage. Light microscopy of immunostained TbHrg indicated localization to the flagellar membrane, and scanning electron microscopy revealed more intense TbHrg accumulation toward the flagellar pocket. Based on these findings, we postulate that T. brucei senses heme levels via the flagellar TbHrg protein. Heme deprivation in the tsetse fly anterior midgut might represent an environmental stimulus involved in the transformation of this important human parasite, possibly through metabolic remodeling.

Novel Trypanosomatid-Bacterium Association: Evolution of Endosymbiosis in Action

We describe a novel symbiotic association between a kinetoplastid protist, Novymonas esmeraldas gen. nov., sp. nov., and an intracytoplasmic bacterium, “Candidatus Pandoraea novymonadis” sp. nov., discovered as a result of a broad-scale survey of insect trypanosomatid biodiversity in Ecuador. We characterize this association by describing the morphology of both organisms, as well as their interactions, and by establishing their phylogenetic affinities. Importantly, neither partner is closely related to other known organisms previously implicated in eukaryote-bacterial symbiosis. This symbiotic association seems to be relatively recent, as the host does not exert a stringent control over the number of bacteria harbored in its cytoplasm. We argue that this unique relationship may represent a suitable model for studying the initial stages of establishment of endosymbiosis between a single-cellular eukaryote and a prokaryote. Based on phylogenetic analyses, Novymonas could be considered a proxy for the insect-only ancestor of the dixenous genus Leishmania and shed light on the origin of the two-host life cycle within the subfamily Leishmaniinae.

The parasitic trypanosomatid protist Novymonas esmeraldas gen. nov., sp. nov. entered into endosymbiosis with the bacterium “Ca. Pandoraea novymonadis” sp. nov. This novel and rather unstable interaction shows several signs of relatively recent establishment, qualifying it as a potentially unique transient stage in the increasingly complex range of eukaryotic-prokaryotic relationships.

Modulation of Na+/K+ ATPase Activity by Hydrogen Peroxide Generated through Heme in L. amazonensis

Leishmania amazonensis is a protozoan parasite that occurs in many areas of Brazil and causes skin lesions. Using this parasite, our group showed the activation of Na⁺/K⁺ ATPase through a signaling cascade that involves the presence of heme and protein kinase C (PKC) activity. Heme is an important biomolecule that has pro-oxidant activity and signaling capacity. Reactive oxygen species (ROS) can act as second messengers, which are required in various signaling cascades. Our goal in this work is to investigate the role of hydrogen peroxide (H₂O₂) generated in the presence of heme in the Na⁺/K⁺ ATPase activity of L. amazonensis. Our results show that increasing concentrations of heme stimulates the production of H₂O₂ in a dose-dependent manner until a concentration of 2.5 μM heme. To confirm that the effect of heme on the Na⁺/K⁺ ATPase is through the generation of H₂O₂, we measured enzyme activity using increasing concentrations of H₂O₂ and, as expected, the activity increased in a dose-dependent manner until a concentration of 0.1 μM H₂O₂. To investigate the role of PKC in this signaling pathway, we observed the production of H₂O₂ in the presence of its activator phorbol 12-myristate 13-acetate (PMA) and its inhibitor calphostin C. Both showed no effect on the generation of H₂O₂. Furthermore, we found that PKC activity is increased in the presence of H₂O₂, and that in the presence of calphostin C, H₂O₂ is unable to activate the Na⁺/K⁺ ATPase. 100 μM of Mito-TEMPO was capable of abolishing the stimulatory effect of heme on Na⁺/K⁺ ATPase activity, indicating that mitochondria might be the source of the hydrogen peroxide production induced by heme. The modulation of L. amazonensis Na⁺/K⁺ ATPase by H₂O₂ opens new possibilities for understanding the signaling pathways of this parasite.

Biochemical and phylogenetic analyses of phosphatidylinositol production in Angomonas deanei, an endosymbiont-harboring trypanosomatid

BACKGROUND

The endosymbiosis in trypanosomatids is characterized by co-evolution between one bacterium and its host protozoan in a mutualistic relationship, thus constituting an excellent model to study organelle origin in the eukaryotic cell. In this association, an intense metabolic exchange is observed between both partners: the host provides energetic molecules and a stable environment to a reduced wall symbiont, while the bacterium is able to interfere in host metabolism by enhancing phospholipid production and completing essential biosynthesis pathways, such as amino acids and hemin production. The bacterium envelope presents a reduced cell wall which is mainly composed of cardiolipin and phosphatidylcholine, being the latter only common in intracellular prokaryotes. Phosphatidylinositol (PI) is also present in the symbiont and host cell membranes. This phospholipid is usually related to cellular signaling and to anchor surface molecules, which represents important events for cellular interactions.

METHODS

In order to investigate the production of PI and its derivatives in symbiont bearing trypanosomatids, aposymbiotic and wild type strains of Angomonas deanei, as well as isolated symbionts, were incubated with [(3)H]myo-inositol and the incorporation of this tracer was analyzed into inositol-containing molecules, mainly phosphoinositides and lipoproteins. Gene searches and their phylogenies were also performed in order to investigate the PI synthesis in symbiontbearing trypanosomatids.

RESULTS

Our results showed that the bacterium did not incorporate the tracer and that both strains produced similar quantities of PI and its derivatives, indicating that the symbiont does not influence the production of these metabolites. Gene searches related to PI synthesis revealed that the trypanosomatid genome contains an inositol transporter, PI synthase and the myo-inositol synthase. Thus, the host is able to produce PI either from exogenous myo-inositol (inositol transporter) or from myo-inositol synthesized de novo. Phylogenetic analysis using other organisms as references indicated that, in trypanosomatids, the genes involved in PI synthesis have a monophyletic origin. In accordance with experimental data, sequences for myo-inositol transport or for myo-inositol and PI biosynthesis were not found in the symbiont.

CONCLUSIONS

Altogether, our results indicate that the bacterium depends on the host to obtain PI.

Lessons from bloodless worms: heme homeostasis in C. elegans

Heme is an essential cofactor for proteins involved in diverse biological processes such as oxygen transport, electron transport, and microRNA processing. Free heme is hydrophobic and cytotoxic, implying that specific trafficking pathways must exist for the delivery of heme to target hemoproteins which reside in various subcellular locales. Although heme biosynthesis and catabolism have been well characterized, the pathways for trafficking heme within and between cells remain poorly understood. Caenorhabditis elegans serves as a unique animal model for uncovering these pathways because, unlike vertebrates, the worm lacks enzymes to synthesize heme and therefore is crucially dependent on dietary heme for sustenance. Using C. elegans as a genetic animal model, several novel heme trafficking molecules have been identified. Importantly, these proteins have corresponding homologs in vertebrates underscoring the power of using C. elegans, a bloodless worm, in elucidating pathways in heme homeostasis and hematology in humans. Since iron deficiency and anemia are often exacerbated by parasites such as helminths and protozoa which also rely on host heme for survival, C. elegans will be an ideal model to identify anti-parasitic drugs that target heme transport pathways unique to the parasite.

Kentomonas gen. n., a new genus of endosymbiont-containing trypanosomatids of Strigomonadinae subfam. n

Compared to their relatives, the diversity of endosymbiont-containing Trypanosomatidae remains under-investigated, with only two new species described in the past 25 years, bringing the total to six. The possible reasons for such a poor representation of this group are either their overall scarcity or susceptibility of their symbionts to antibiotics that are traditionally used for cultivation of flagellates. In this work we describe the isolation, cultivation, as well as morphological and molecular characterization of a novel endosymbiont-harboring trypanosomatid species, Kentomonas sorsogonicus sp. n. The newly erected genus Kentomonas gen. n. shares many common features with the genera Angomonas and Strigomonas, such as the presence of an extensive system of peripheral mitochondrial branches distorting the corset of subpellicular microtubules, large and loosely packed kinetoplast, and a rudimentary paraflagellar rod. Here we also propose to unite all endosymbiont-bearing trypanosomatids into the new subfamily Strigomonadinae subfam. n.

Mitochondrial respiration and genomic analysis provide insight into the influence of the symbiotic bacterium on host trypanosomatid oxygen consumption

Certain trypanosomatids co-evolve with an endosymbiotic bacterium in a mutualistic relationship that is characterized by intense metabolic exchanges. Symbionts were able to respire for up to 4 h after isolation from Angomonas deanei. FCCP (carbonyl cyanide-4-(trifluoromethoxy)phenylhydrazone) similarly increased respiration in wild-type and aposymbiotic protozoa, though a higher maximal O2 consumption capacity was observed in the symbiont-containing cells. Rotenone, a complex I inhibitor, did not affect A. deanei respiration, whereas TTFA (thenoyltrifluoroacetone), a complex II activity inhibitor, completely blocked respiration in both strains. Antimycin A and cyanide, inhibitors of complexes III and IV, respectively, abolished O2 consumption, but the aposymbiotic protozoa were more sensitive to both compounds. Oligomycin did not affect cell respiration, whereas carboxyatractyloside (CAT), an inhibitor of the ADP-ATP translocator, slightly reduced O2 consumption. In the A. deanei genome, sequences encoding most proteins of the respiratory chain are present. The symbiont genome lost part of the electron transport system (ETS), but complex I, a cytochrome d oxidase, and FoF1-ATP synthase remain. In conclusion, this work suggests that the symbiont influences the mitochondrial respiration of the host protozoan.

Leishmania major possesses a unique HemG-type protoporphyrinogen IX oxidase

Leishmania major was proposed to either utilize haem from its host or partially synthesize the tetrapyrrole from host provided precursors. However, only indirect evidence was available for this partial late haem biosynthetic pathway. Here, we demonstrate that the LMJF_06_1280 gene of L. major encodes a HemG-type PPO (protoporphyrinogen IX oxidase) catalysing the oxidation of protoporphyrinogen IX to protoporphyrin IX. Interestingly, trypanosomatids are currently the only known eukaryotes possessing HemG-type enzymes. The LMJF_06_1280 gene forms a potential transcriptional unit with LMJF_06_1270 encoding CPO (coproporphyrinogen III oxidase) and with LMJF_06_1290 for a cytochrome b5. In vivo function of the L. major hemG gene was shown by the functional complementation of the Escherichia coli ΔhemG strain LG285. Restored haem formation in E. coli was observed using HPLC analyses. Purified recombinant L. major HemG revealed PPO activity in vitro using different ubiquinones and triphenyltetrazolium as electron acceptors. FMN was identified as the L. major HemG cofactor. Active site residues were found to be essential for HemG catalysis. These data in combination with the solved crystal structures of L. major CPO and the physiological proof of a ferrochelatase activity provide clear-cut evidence for a partial haem biosynthetic pathway in L. major.

Genome evolution in trypanosomatid parasites

A decade of genome sequencing has transformed our understanding of how trypanosomatid parasites have evolved and provided fresh impetus to explaining the origins of parasitism in the Kinetoplastida. In this review, I will consider the many ways in which genome sequences have influenced our view of genomic reduction in trypanosomatids; how species-specific genes, and the genomic domains they occupy, have illuminated the innovations in trypanosomatid genomes; and how comparative genomics has exposed the molecular mechanisms responsible for innovation and adaptation to a parasitic lifestyle.

Trypanosomatid essential metabolic pathway: new approaches about heme fate in Trypanosoma cruzi

Trypanosoma cruzi, the causal agent of Chagas disease, has a complex life cycle and depends on hosts for its nutritional needs. Our group has investigated heme (Fe-protoporphyrin IX) internalization and the effects on parasite growth, following the fate of this porphyrin in the parasite. Here, we show that epimastigotes cultivated with heme yielded the compounds α-meso-hydroxyheme, verdoheme and biliverdin (as determined by HPLC), suggesting an active heme degradation pathway in this parasite. Furthermore, through immunoprecipitation and immunoblotting assays of epimastigote extracts, we observed recognition by an antibody against mammalian HO-1. We also detected the localization of the HO-1-like protein in the parasite using immunocytochemistry, with antibody staining primarily in the cytoplasm. Although HO has not been described in the parasite's genome, our results offer new insights into heme metabolism in T. cruzi, revealing potential future therapeutic targets.

Endosymbiosis in trypanosomatids: the genomic cooperation between bacterium and host in the synthesis of essential amino acids is heavily influenced by multiple horizontal gene transfers

BACKGROUND

Trypanosomatids of the genera Angomonas and Strigomonas live in a mutualistic association characterized by extensive metabolic cooperation with obligate endosymbiotic Betaproteobacteria. However, the role played by the symbiont has been more guessed by indirect means than evidenced. Symbiont-harboring trypanosomatids, in contrast to their counterparts lacking symbionts, exhibit lower nutritional requirements and are autotrophic for essential amino acids. To evidence the symbiont's contributions to this autotrophy, entire genomes of symbionts and trypanosomatids with and without symbionts were sequenced here.

RESULTS

Analyses of the essential amino acid pathways revealed that most biosynthetic routes are in the symbiont genome. By contrast, the host trypanosomatid genome contains fewer genes, about half of which originated from different bacterial groups, perhaps only one of which (ornithine cyclodeaminase, EC:4.3.1.12) derived from the symbiont. Nutritional, enzymatic, and genomic data were jointly analyzed to construct an integrated view of essential amino acid metabolism in symbiont-harboring trypanosomatids. This comprehensive analysis showed perfect concordance among all these data, and revealed that the symbiont contains genes for enzymes that complete essential biosynthetic routes for the host amino acid production, thus explaining the low requirement for these elements in symbiont-harboring trypanosomatids. Phylogenetic analyses show that the cooperation between symbionts and their hosts is complemented by multiple horizontal gene transfers, from bacterial lineages to trypanosomatids, that occurred several times in the course of their evolution. Transfers occur preferentially in parts of the pathways that are missing from other eukaryotes.

CONCLUSION

We have herein uncovered the genetic and evolutionary bases of essential amino acid biosynthesis in several trypanosomatids with and without endosymbionts, explaining and complementing decades of experimental results. We uncovered the remarkable plasticity in essential amino acid biosynthesis pathway evolution in these protozoans, demonstrating heavy influence of horizontal gene transfer events, from Bacteria to trypanosomatid nuclei, in the evolution of these pathways.

Genome evolution and phylogenomic analysis of Candidatus Kinetoplastibacterium, the betaproteobacterial endosymbionts of Strigomonas and Angomonas

It has been long known that insect-infecting trypanosomatid flagellates from the genera Angomonas and Strigomonas harbor bacterial endosymbionts (Candidatus Kinetoplastibacterium or TPE [trypanosomatid proteobacterial endosymbiont]) that supplement the host metabolism. Based on previous analyses of other bacterial endosymbiont genomes from other lineages, a stereotypical path of genome evolution in such bacteria over the duration of their association with the eukaryotic host has been characterized. In this work, we sequence and analyze the genomes of five TPEs, perform their metabolic reconstruction, do an extensive phylogenomic analyses with all available Betaproteobacteria, and compare the TPEs with their nearest betaproteobacterial relatives. We also identify a number of housekeeping and central metabolism genes that seem to have undergone positive selection. Our genome structure analyses show total synteny among the five TPEs despite millions of years of divergence, and that this lineage follows the common path of genome evolution observed in other endosymbionts of diverse ancestries. As previously suggested by cell biology and biochemistry experiments, Ca. Kinetoplastibacterium spp. preferentially maintain those genes necessary for the biosynthesis of compounds needed by their hosts. We have also shown that metabolic and informational genes related to the cooperation with the host are overrepresented amongst genes shown to be under positive selection. Finally, our phylogenomic analysis shows that, while being in the Alcaligenaceae family of Betaproteobacteria, the closest relatives of these endosymbionts are not in the genus Bordetella as previously reported, but more likely in the Taylorella genus.

Predicting the proteins of Angomonas deanei, Strigomonas culicis and their respective endosymbionts reveals new aspects of the trypanosomatidae family

Endosymbiont-bearing trypanosomatids have been considered excellent models for the study of cell evolution because the host protozoan co-evolves with an intracellular bacterium in a mutualistic relationship. Such protozoa inhabit a single invertebrate host during their entire life cycle and exhibit special characteristics that group them in a particular phylogenetic cluster of the Trypanosomatidae family, thus classified as monoxenics. In an effort to better understand such symbiotic association, we used DNA pyrosequencing and a reference-guided assembly to generate reads that predicted 16,960 and 12,162 open reading frames (ORFs) in two symbiont-bearing trypanosomatids, Angomonas deanei (previously named as Crithidia deanei) and Strigomonas culicis (first known as Blastocrithidia culicis), respectively. Identification of each ORF was based primarily on TriTrypDB using tblastn, and each ORF was confirmed by employing getorf from EMBOSS and Newbler 2.6 when necessary. The monoxenic organisms revealed conserved housekeeping functions when compared to other trypanosomatids, especially compared with Leishmania major. However, major differences were found in ORFs corresponding to the cytoskeleton, the kinetoplast, and the paraflagellar structure. The monoxenic organisms also contain a large number of genes for cytosolic calpain-like and surface gp63 metalloproteases and a reduced number of compartmentalized cysteine proteases in comparison to other TriTryp organisms, reflecting adaptations to the presence of the symbiont. The assembled bacterial endosymbiont sequences exhibit a high A+T content with a total of 787 and 769 ORFs for the Angomonas deanei and Strigomonas culicis endosymbionts, respectively, and indicate that these organisms hold a common ancestor related to the Alcaligenaceae family. Importantly, both symbionts contain enzymes that complement essential host cell biosynthetic pathways, such as those for amino acid, lipid and purine/pyrimidine metabolism. These findings increase our understanding of the intricate symbiotic relationship between the bacterium and the trypanosomatid host and provide clues to better understand eukaryotic cell evolution.

Identification of a bacteria-like ferrochelatase in Strongyloides venezuelensis, an animal parasitic nematode

Heme is an essential molecule for vast majority of organisms serving as a prosthetic group for various hemoproteins. Although most organisms synthesize heme from 5-aminolevulinic acid through a conserved heme biosynthetic pathway composed of seven consecutive enzymatic reactions, nematodes are known to be natural heme auxotrophs. The completely sequenced Caenorhabditis elegans genome, for example, lacks all seven genes for heme biosynthesis. However, genome/transcriptome sequencing of Strongyloides venezuelensis, an important model nematode species for studying human strongyloidiasis, indicated the presence of a gene for ferrochelatase (FeCH), which catalyzes the terminal step of heme biosynthesis, whereas the other six heme biosynthesis genes are apparently missing. Phylogenetic analyses indicated that nematode FeCH genes, including that of S. venezuelensis (SvFeCH) have a fundamentally different evolutionally origin from the FeCH genes of non-nematode metazoa. Although all non-nematode metazoan FeCH genes appear to be inherited vertically from an ancestral opisthokont, nematode FeCH may have been acquired from an alpha-proteobacterium, horizontally. The identified SvFeCH sequence was found to function as FeCH as expected based on both in vitro chelatase assays using recombinant SvFeCH and in vivo complementation experiments using an FeCH-deficient strain of Escherichia coli. Messenger RNA expression levels during the S. venezuelensis lifecycle were examined by real-time RT-PCR. SvFeCH mRNA was expressed at all the stages examined with a marked reduction at the infective third-stage larvae. Our study demonstrates the presence of a bacteria-like FeCH gene in the S. venezuelensis genome. It appeared that S. venezuelensis and some other animal parasitic nematodes reacquired the once-lost FeCH gene. Although the underlying evolutionary pressures that necessitated this reacquisition remain to be investigated, it is interesting that the presence of FeCH genes in the absence of other heme biosynthesis genes has been reported only for animal pathogens, and this finding may be related to nutritional availability in animal hosts.

Heme uptake by Leishmania amazonensis is mediated by the transmembrane protein LHR1

Trypanosomatid protozoan parasites lack a functional heme biosynthetic pathway, so must acquire heme from the environment to survive. However, the molecular pathway responsible for heme acquisition by these organisms is unknown. Here we show that L. amazonensis LHR1, a homolog of the C. elegans plasma membrane heme transporter HRG-4, functions in heme transport. Tagged LHR1 localized to the plasma membrane and to endocytic compartments, in both L. amazonensis and mammalian cells. Heme deprivation in L. amazonensis increased LHR1 transcript levels, promoted uptake of the fluorescent heme analog ZnMP, and increased the total intracellular heme content of promastigotes. Conversely, deletion of one LHR1 allele reduced ZnMP uptake and the intracellular heme pool by approximately 50%, indicating that LHR1 is a major heme importer in L. amazonensis. Viable parasites with correct replacement of both LHR1 alleles could not be obtained despite extensive attempts, suggesting that this gene is essential for the survival of promastigotes. Notably, LHR1 expression allowed Saccharomyces cerevisiae to import heme from the environment, and rescued growth of a strain deficient in heme biosynthesis. Syntenic genes with high sequence identity to LHR1 are present in the genomes of several species of Leishmania and also Trypanosoma cruzi and Trypanosoma brucei, indicating that therapeutic agents targeting this transporter could be effective against a broad group of trypanosomatid parasites that cause serious human disease.

The biological activity of many proteins and enzymes requires heme, a large organic ring containing one iron atom at the center. It has been known for several decades that trypanosomatid protozoa lack several enzymes in the heme biosynthetic pathway. Therefore, unlike mammalian cells that can synthesize heme, these unicellular organisms must acquire heme from the environment. However, the mechanism by which this critical co-factor is transported into trypanosomatid parasites was unknown. In this study we identified LHR1, a trans-membrane protein from Leishmania amazonensis that mediates transport of extracellular heme into the parasites. Parasites partially deficient in LHR1 are impaired in heme import, and strains completely deficient do not survive. Genes highly similar to LHR1 are present in several species of trypanosomatid parasites that cause human disease, identifying this transporter as an important target for the development of anti-parasitic drugs.

Aerobic kinetoplastid flagellate Phytomonas does not require heme for viability

Heme is an iron-coordinated porphyrin that is universally essential as a protein cofactor for fundamental cellular processes, such as electron transport in the respiratory chain, oxidative stress response, or redox reactions in various metabolic pathways. Parasitic kinetoplastid flagellates represent a rare example of organisms that depend on oxidative metabolism but are heme auxotrophs. Here, we show that heme is fully dispensable for the survival of Phytomonas serpens, a plant parasite. Seeking to understand the metabolism of this heme-free eukaryote, we searched for heme-containing proteins in its de novo sequenced genome and examined several cellular processes for which heme has so far been considered indispensable. We found that P. serpens lacks most of the known hemoproteins and does not require heme for electron transport in the respiratory chain, protection against oxidative stress, or desaturation of fatty acids. Although heme is still required for the synthesis of ergosterol, its precursor, lanosterol, is instead incorporated into the membranes of P. serpens grown in the absence of heme. In conclusion, P. serpens is a flagellate with unique metabolic adaptations that allow it to bypass all requirements for heme.

Identification and phylogenetic analysis of heme synthesis genes in trypanosomatids and their bacterial endosymbionts

It has been known for decades that some insect-infecting trypanosomatids can survive in culture without heme supplementation while others cannot, and that this capability is associated with the presence of a betaproteobacterial endosymbiont in the flagellate's cytoplasm. However, the specific mechanisms involved in this process remained obscure. In this work, we sequence and phylogenetically analyze the heme pathway genes from the symbionts and from their hosts, as well as from a number of heme synthesis-deficient Kinetoplastida. Our results show that the enzymes responsible for synthesis of heme are encoded on the symbiont genomes and produced in close cooperation with the flagellate host. Our evidence suggests that this synergistic relationship is the end result of a history of extensive gene loss and multiple lateral gene transfer events in different branches of the phylogeny of the Trypanosomatidae.

Role of heme and heme-proteins in trypanosomatid essential metabolic pathways

Around the world, trypanosomatids are known for being etiological agents of several highly disabling and often fatal diseases like Chagas disease (Trypanosoma cruzi), leishmaniasis (Leishmania spp.), and African trypanosomiasis (Trypanosoma brucei). Throughout their life cycle, they must cope with diverse environmental conditions, and the mechanisms involved in these processes are crucial for their survival. In this review, we describe the role of heme in several essential metabolic pathways of these protozoans. Notwithstanding trypanosomatids lack of the complete heme biosynthetic pathway, we focus our discussion in the metabolic role played for important heme-proteins, like cytochromes. Although several genes for different types of cytochromes, involved in mitochondrial respiration, polyunsaturated fatty acid metabolism, and sterol biosynthesis, are annotated at the Tritryp Genome Project, the encoded proteins have not yet been deeply studied. We pointed our attention into relevant aspects of these protein functions that are amenable to be considered for rational design of trypanocidal agents.

Phylogenetic validation of the genera Angomonas and Strigomonas of trypanosomatids harboring bacterial endosymbionts with the description of new species of trypanosomatids and of proteobacterial symbionts

We comparatively examined the nutritional, molecular and optical and electron microscopical characteristics of reference species and new isolates of trypanosomatids harboring bacterial endosymbionts. Sequencing of the V7V8 region of the small subunit of the ribosomal RNA (SSU rRNA) gene distinguished six major genotypes among the 13 isolates examined. The entire sequences of the SSU rRNA and glycosomal glyceraldehyde phosphate dehydrogenase (gGAPDH) genes were obtained for phylogenetic analyses. In the resulting phylogenetic trees, the symbiont-harboring species clustered as a major clade comprising two subclades that corresponded to the proposed genera Angomonas and Strigomonas. The genus Angomonas comprised 10 flagellates including former Crithidia deanei and C. desouzai plus a new species. The genus Strigomonas included former Crithidia oncopelti and Blastocrithidia culicis plus a new species. Sequences from the internal transcribed spacer of ribosomal DNA (ITS rDNA) and size polymorphism of kinetoplast DNA (kDNA) minicircles revealed considerable genetic heterogeneity within the genera Angomonas and Strigomonas. Phylogenetic analyses based on 16S rDNA and ITS rDNA sequences demonstrated that all of the endosymbionts belonged to the Betaproteobacteria and revealed three new species. The congruence of the phylogenetic trees of trypanosomatids and their symbionts support a co-divergent host-symbiont evolutionary history.

Using heme as an energy boost for lactic acid bacteria

Lactic acid bacteria (LAB) are a phylogenetically diverse group named for their main attribute in food fermentations, that is, production of lactic acid. However, several LAB are genetically equipped for aerobic respiration metabolism when provided with exogenous sources of heme (and menaquinones for some species). Respiration metabolism is energetically favorable and leads to less oxidative and acid stress during growth. As a consequence, the growth and survival of several LAB can be dramatically improved under respiration-permissive conditions. Respiration metabolism already has industrial applications for the production of dairy starter cultures. In view of the growth and survival advantages conferred by respiration, and the availability of heme and menaquinones in natural environments, we recommend that respiration be accepted as a part of the natural lifestyle of numerous LAB.

Heme and blood-feeding parasites: friends or foes?

Hemoparasites, like malaria and schistosomes, are constantly faced with the challenges of storing and detoxifying large quantities of heme, released from their catabolism of host erythrocytes. Heme is an essential prosthetic group that forms the reactive core of numerous hemoproteins with diverse biological functions. However, due to its reactive nature, it is also a potentially toxic molecule. Thus, the acquisition and detoxification of heme is likely to be paramount for the survival and establishment of parasitism. Understanding the underlying mechanism involved in this interaction could possibly provide potential novel targets for drug and vaccine development, and disease treatment. However, there remains a wide gap in our understanding of these mechanisms. This review summarizes the biological importance of heme for hemoparasite, and the adaptations utilized in its sequestration and detoxification.

Evolution of the haem synthetic pathway in kinetoplastid flagellates: an essential pathway that is not essential after all?

For a vast majority of living organisms, haem is an essential compound that is synthesised through a conserved biosynthetic pathway. However, certain organisms are haem auxotrophs and need to obtain this molecule from exogenous sources. Kinetoplastid flagellates represent an interesting group of species, as some of them lost the complete pathway while others possess only the last three biosynthetic steps. We decided to supplement a current view on the phylogeny of these important pathogens with the expected state of haem synthesis in representative species. We propose a scenario in which the ancestor of all trypanosomatids was completely deficient of the synthesis of haem. In trypanosomatids other than members of the genus Trypanosoma, the pathway was partially rescued by genes encoding enzymes for the last three steps, supposedly obtained by horizontal transfer from a gamma-proteobacterium. This event preceded the diversification of the non-Trypanosoma trypanosomatids. Later, some flagellates acquired a beta-proteobacterial endosymbiont which supplied them with haem precursors. On the other hand, the medically important trypanosomes have remained fully deficient of haem synthesis and obtain this compound from the host.

Leishmania spp.: delta-aminolevulinate-inducible neogenesis of porphyria by genetic complementation of incomplete heme biosynthesis pathway

To further develop the Leishmania model for porphyria based on their deficiencies in heme biosynthesis, three Old World species were doubly transfected as before for Leishmania amazonensis with cDNAs, encoding the 2nd and 3rd enzymes in the pathway. Expression of the transgenes was verified immunologically at the protein level and functionally by uroporphyrin neogenesis that occurs only after exposure of the double-transfectants to delta-aminolevulinate. All species examined were equally deficient in heme biosynthesis, as indicated by the accumulation of uroporphyrin as the sole porphyrin and the production of coproporphyrin upon further transfection of one representative species with the downstream gene. The results obtained thus demonstrate that at least the first five enzymes for heme biosynthesis are absent in all species examined, rendering their transfectants inducible with aminolevulinate to accumulate porphyrins and thus useful as cellular models for human porphyrias.

The ubiquitous gp63-like metalloprotease from lower trypanosomatids: in the search for a function

Plant and insect trypanosomatids constitute the "lower trypanosomatids", which have been used routinely as laboratory models for biochemical and molecular studies because they are easily cultured under axenic conditions, and they contain homologues of virulence factors from the classic human trypanosomatid pathogens. Among the molecular factors that contribute to Leishmania spp. virulence and pathogenesis, the major surface protease, alternatively called MSP, PSP, leishmanolysin, EC 3.4.24.36 and gp63, is the most abundant surface protein of Leishmania promastigotes. A myriad of functions have been described for the gp63 from Leishmania spp. when the metacyclic promastigote is inside the mammalian host. However, less is known about the functions performed by this molecule in the invertebrate vector. Intriguingly, gp63 is predominantly expressed in the insect stage of Leishmania, and in all insect and plant trypanosomatids examined so far. The gp63 homologues found in lower trypanosomatids seem to play essential roles in the nutrition as well as in the interaction with the insect epithelial cells. Since excellent reviews were produced in the last decade regarding the roles played by proteases in the vertebrate hosts, we focused in the recent developments in our understanding of the biochemistry and cell biology of gp63-like proteins in lower trypanosomatids.

Antileishmanial drug development: exploitation of parasite heme dependency

A rational approach in the search for new antiparasitic drugs is the exploitation of biochemical differences between the parasite and its mammalian host. One specific example in the case of Leishmania relates to the biosynthesis of heme, a critical prosthetic group for proteins involved in metabolism and electron transport. Like all Trypanosomatids, Leishmania parasites require heme or pre-formed porphyrins for survival because they lack several key enzymes in the heme biosynthetic pathway. Considering their specific nutritional requirements, we speculated that they would be particularly sensitive to the effects of heme-complexing xanthones. In this report, we document the antileishmanial activity of selected nitrogenated xanthones and correlate drug potency with heme affinity. In vitro tests demonstrated that 3,6-bis-omega-diethylaminoamyloxyxanthone, C5, was at least 100 times more active than pentamidine against intracellular amastigotes of Leishmania mexicana. Our findings provide practical guidance for optimizing the antileishmanial activity of the xanthone pharmacophore to better exploit parasite heme salvage processes.

Genetic rescue of Leishmania deficiency in porphyrin biosynthesis creates mutants suitable for analysis of cellular events in uroporphyria and for photodynamic therapy

Leishmania was found deficient in at least five and most likely seven of the eight enzymes in the heme biosynthesis pathway, accounting for their growth requirement for heme compounds. The xenotransfection of this trypanosomatid protozoan led to their expression of the mammalian genes encoding delta-aminolevulinate (ALA) dehydratase and porphobilinogen deaminase, the second and the third enzymes of the pathway, respectively. These transfectants still require hemin or protoporphyrin IX for growth but produce porphyrin when ALA was supplied exogenously. Leishmania is thus deficient in all first three enzymes of the pathway. Uroporphyrin I was produced as the sole intermediate by these transfectants, further indicating that they are also deficient in at least two porphyrinogen-metabolizing enzymes downstream of porphobilinogen deaminase, i.e. uroporphyrinogen III co-synthase and uroporphyrinogen decarboxylase. Pulsing the transfectants with ALA induced their transition from aporphyria to uroporphyria. Uroporphyrin I emerged in these cells initially as diffused throughout the cytosol, rendering them sensitive to UV irradiation. The porphyrin was subsequently sequestered in cytoplasmic vacuoles followed by its release and accumulation in the extracellular milieu, concomitant with a reduced photosensitivity of the cells. These events may represent cellular mechanisms for disposing soluble toxic waste from the cytosol. Monocytic tumor cells were rendered photosensitive by infection with uroporphyric Leishmania, suggestive of their potential application for photodynamic therapy.

Dose-dependent differential effect of hemin on protein synthesis and cell proliferation in Leishmania donovani promastigotes cultured in vitro

Leishmania donovani requires an exogenous source of heme for growth and transformation. In in vitro culture of the free-living promastigotes, exogenously added hemin enhances cell proliferation. In this investigation, the question of the function of heme with particular reference to protein synthesis and cell proliferation has been addressed. The results of in vitro cell culture experiments demonstrated that hemin (10 microM) alone is suitable for supporting optimum level of protein synthesis, and thereby cell proliferation of promastigotes to an extent that it can replace fetal bovine serum. However, in situ labelling experiments along with Western blots revealed that high concentration of hemin (50 microM) reduced the level of protein synthesis in general and of beta-tubulin in particular with a concomitant induction of hsp90, and induced consequent morphological changes that are observed during in situ transformation of promastigotes in mammalian macrophages. These results therefore suggest that sudden exposure to high concentration of heme in mammalian macrophages may be one of the key factors that trigger promastigote to amastigote transformation in L. donovani. Furthermore, hemin with its dual characteristic could be used as a tool to understand molecular mechanism of cell proliferation and transformation in these parasites.

Endosymbiosis in protozoa of the Trypanosomatidae family

A small number of trypanosomatids present bacterium endosymbionts in the cytoplasm, which divide synchronously with the host cell. Crithidia oncopleti, Crithidia deanei. Crithidia desouzai, Blastocrithidia culicis and Herpetomonas roitmani are the best characterized species. The endosymbiont is surrounded by two membranes separated from each other by an electron-lucent space. The presence of the endosymbiont led to the appearance of morphological changes which include the lack of the paraflagellar rod associated to the axoneme, the morphology of the kinetoplast and the association of the sub-pellicular microtubules with portions of the protozoan plasma membrane. Aposymbiotic strains could be obtained by antibiotic treatment, opening the possibility to make comparative analysis of endosymbiont-containing an endosymbiont-free populations of the same species. It is clear that metabolic cycles are established between the prokaryiont and the host cell. The results obtained show that endosymbiont-containing species of trypanosomatids constitute an excellent model to study basic processes on the endosymbiont-host cell relationship and the origin of new organelles.