Expression of heat shock protein 83 in Leishmania is regulated post-transcriptionally

The protein map of the protozoan parasite Leishmania (Leishmania) amazonensis, Leishmania (Viannia) braziliensis and Leishmania (Leishmania) infantum during growth phase transition and temperature stress

Leishmania parasites cause a spectrum of diseases termed leishmaniasis, which manifests in two main clinical forms, cutaneous and visceral leishmaniasis. Leishmania promastigotes transit from proliferative exponential to quiescent stationary phases inside the insect vector, a relevant step that recapitulates early molecular events of metacyclogenesis. During the insect blood meal of the mammalian hosts, the released parasites interact initially with the skin, an event marked by temperature changes. Deep knowledge on the molecular events activated during Leishmania-host interactions in each step is crucial to develop better therapies and to understand the pathogenesis. In this study, the proteomes of Leishmania (Leishmania) amazonensis (La), Leishmania (Viannia) braziliensis (Lb), and Leishmania (Leishmania) infantum (syn L. L. chagasi) (Lc) were analyzed using quantitative proteomics to uncover the proteome modulation in three different conditions related to growth phases and temperature shifts: 1) exponential phase (Exp); 2) stationary phase (Sta25) and; 3) stationary phase subjected to heat stress (Sta34). Functional validations were performed using orthogonal techniques, focusing on α-tubulin, gp63 and heat shock proteins (HSPs). Species-specific and condition-specific modulation highlights the plasticity of the Leishmania proteome, showing that pathways related to metabolism and cytoskeleton are significantly modulated from exponential to stationary growth phases, while protein folding, unfolded protein binding, signaling and microtubule-based movement were differentially altered during temperature shifts. This study provides an in-depth proteome analysis of three Leishmania spp., and contributes compelling evidence of the molecular alterations of these parasites in conditions mimicking the interaction of the parasites with the insect vector and vertebrate hosts. SIGNIFICANCE: Leishmaniasis disease manifests in two main clinical forms according to the infecting Leishmania species and host immune responses, cutaneous and visceral leishmaniasis. In Brazil, cutaneous leishmaniasis (CL) is associated with L. braziliensis and L. amazonensis, while visceral leishmaniasis, also called kala-azar, is caused by L. infantum. Leishmania parasites remodel their proteomes during growth phase transition and changes in their mileu imposed by the host, including temperature. In this study, we performed a quantitative mass spectrometry-based proteomics to compare the proteome of three New world Leishmania species, L. amazonensis (La), L. braziliensis (Lb) and L. infantum (syn L. chagasi) (Lc) in three conditions: a) exponential phase at 25 °C (Exp); b) stationary phase at 25 °C (Sta25) and; c) stationary phase subjected to temperature stress at 34 °C (Sta34). This study provides an in-depth proteome analysis of three Leishmania spp. with varying pathophysiological outcomes, and contributes compelling evidence of the molecular alterations of these parasites in conditions mimicking the interaction of the parasites with the insect vector and vertebrate hosts.

De Novo Transcriptome Analysis Reveals Potential Thermal Adaptation Mechanisms in the Cicada Hyalessa fuscata

Simple Summary

In metropolitan Seoul and its vicinity, cicadas of the species Hyalessa fuscata living in warmer areas could tolerate the heat better than those living in cooler areas, but genetic mechanisms involved in better heat tolerance remained unclear. In this study, we examined differences in gene expression of cicadas living in a warm urban area, a cool urban area and a suburban area in three experimental treatments: no heating, 10 min heating and heating until the cicadas lost their mobility. Cicadas from the warm urban area changed their gene expressions the most. Activated genes were mostly related to heat shock, energy metabolism, and detoxification. These results suggested that under heat stress, cicadas inhabiting warm areas could differentially express genes to increase their thermal tolerance.

Abstract

In metropolitan Seoul, populations of the cicada Hyalessa fuscata in hotter urban heat islands (“high UHIs”) exhibit higher thermal tolerance than those in cooler UHIs (“low UHIs”). We hypothesized that heat stress may activate the expression of genes that facilitate greater thermal tolerance in high-UHI cicadas than in those from cooler areas. Differences in the transcriptomes of adult female cicadas from high-UHI, low-UHI, and suburban areas were analyzed at the unheated level, after acute heat stress, and after heat torpor. No noticeable differences in unheated gene expression patterns were observed. After 10 min of acute heat stress, however, low-UHI and suburban cicadas expressed more heat shock protein genes than high-UHI counterparts. More specifically, remarkable changes in the gene expression of cicadas across areas were observed after heat torpor stimulus, as represented by a large number of up- and downregulated genes in the heat torpor groups compared with the 10 min acute heat stress and control groups. High-UHI cicadas expressed the most differentially expressed genes, followed by the low-UHI and suburban cicadas. There was a notable increase in the expression of heat shock, metabolism, and detoxification genes; meanwhile, immune-related, signal transduction, and protein turnover genes were downregulated in high-UHI cicadas versus the other cicada groups. These results suggested that under heat stress, cicadas inhabiting high-UHIs could rapidly express genes related to heat shock, energy metabolism, and detoxification to protect cells from stress-induced damage and to increase their thermal tolerance toward heat stress. The downregulation of apoptosis mechanisms in high-UHI cicadas suggested that there was less cellular damage, which likely contributed to their high tolerance of heat stress.

Heat Shock Proteins as the Druggable Targets in Leishmaniasis: Promises and Perils

Leishmania, the causative agent of leishmaniasis, is an intracellular pathogen that thrives in the insect gut and mammalian macrophages to complete its life cycle. Apart from temperature difference (26 to 37°C), it encounters several harsh conditions, including oxidative stress, inflammatory reactions, and low pH. Heat shock proteins (HSPs) play essential roles in cell survival by strategically reprogramming cellular processes and signaling pathways. HSPs assist cells in multiple functions, including differentiation, adaptation, virulence, and persistence in the host cell. Due to cyclical epidemiological patterns, limited chemotherapeutic options, drug resistance, and the absence of a vaccine, control of leishmaniasis remains a far-fetched dream. The essential roles of HSPs in parasitic differentiation and virulence and increased expression in drug-resistant strains highlight their importance in combating the disease. In this review, we highlighted the diverse physiological importance of HSPs present in Leishmania, emphasizing their significance in disease pathogenesis. Subsequently, we assessed the potential of HSPs as a chemotherapeutic target and underlined the challenges associated with it. Furthermore, we have summarized a few ongoing drug discovery initiatives that need to be explored further to develop clinically successful chemotherapeutic agents in the future.

The Leishmania donovani SENP Protease Is Required for SUMO Processing but Not for Viability

The protozoan parasite Leishmania donovani is part of an early eukaryotic branch and depends on post-transcriptional mechanisms for gene expression regulation. This includes post-transcriptional protein modifications, such as protein phosphorylation. The presence of genes for protein SUMOylation, i.e., the covalent attachment of small ubiquitin-like modifier (SUMO) polypeptides, in the Leishmania genomes prompted us to investigate the importance of the sentrin-specific protease (SENP) and its putative client, SUMO, for the vitality and infectivity of Leishmania donovani. While SENP null mutants are viable with reduced vitality, viable SUMO null mutant lines could not be obtained. SUMO C-terminal processing is disrupted in SENP null mutants, preventing SUMO from covalent attachment to proteins and nuclear translocation. Infectivity in vitro is not affected by the loss of SENP-dependent SUMO processing. We conclude that SENP is required for SUMO processing, but that functions of unprocessed SUMO are critical for Leishmania viability.

Casein kinase 1.2 over expression restores stress resistance to Leishmania donovani HSP23 null mutants

Leishmania donovani is a trypanosomatidic parasite and causes the lethal kala-azar fever, a neglected tropical disease. The Trypanosomatida are devoid of transcriptional gene regulation and rely on gene copy number variations and translational control for their adaption to changing conditions. To survive at mammalian tissue temperatures, L. donovani relies on the small heat shock protein HSP23, the loss of which renders the parasites stress sensitive and impairs their proliferation. Here, we analysed a spontaneous escape mutant with wild type-like in vitro growth. Further selection of this escape strains resulted in a complete reversion of the phenotype. Whole genome sequencing revealed a correlation between stress tolerance and the massive amplification of a six-gene cluster on chromosome 35, with further analysis showing over expression of the casein kinase 1.2 gene as responsible. In vitro phosphorylation experiments established both HSP23 and the related P23 co-chaperone as substrates and modulators of casein kinase 1.2, providing evidence for another crucial link between chaperones and signal transduction protein kinases in this early branching eukaryote.

The genetics of Leishmania virulence

The ability of Leishmania parasites to infect and persist in the antigen-presenting cell population of their mammalian hosts is dependent on their ability to gain entry to their host and host cells, to survive the mammalian cell environment, and to suppress or evade the protective immune response mechanisms of their hosts. A multitude of genes and their products have been implicated in each of these virulence-enhancing strategies to date, and we present an overview of the nature and known function of such virulence genes.

LiZIP3 is a cellular zinc transporter that mediates the tightly regulated import of zinc in Leishmania infantum parasites

Cellular zinc homeostasis ensures that the intracellular concentration of this element is kept within limits that enable its participation in critical physiological processes without exerting toxic effects. We report here the identification and characterization of the first mediator of zinc homeostasis in Leishmania infantum, LiZIP3, a member of the ZIP family of divalent metal-ion transporters. The zinc transporter activity of LiZIP3 was first disclosed by its capacity to rescue the growth of Saccharomyces cerevisiae strains deficient in zinc acquisition. Subsequent expression of LiZIP3 in Xenopus laevis oocytes was shown to stimulate the uptake of a broad range of metal ions, among which Zn(2+) was the preferred LiZIP3 substrate (K0.5  ≈ 0.1 μM). Evidence that LiZIP3 functions as a zinc importer in L. infantum came from the observations that the protein locates to the cell membrane and that its overexpression leads to augmented zinc internalization. Importantly, expression and cell-surface location of LiZIP3 are lost when parasites face high zinc bioavailability. LiZIP3 decline in response to zinc is regulated at the mRNA level in a process involving (a) short-lived protein(s). Collectively, our data reveal that LiZIP3 enables L. infantum to acquire zinc in a highly regulated manner, hence contributing to zinc homeostasis.

A review of recent patents on the protozoan parasite HSP90 as a drug target

Diseases caused by protozoan parasites are still an important health problem. These parasites can cause a wide spectrum of diseases, some of which are severe and have high morbidity or mortality if untreated. Since they are still uncontrolled, it is important to find novel drug targets and develop new therapies to decrease their remarkable social and economic impact on human societies. In the past years, human HSP90 has become an interesting drug target that has led to a large number of investigations both at state organizations and pharmaceutical companies, followed by clinical trials. The finding that HSP90 has important biological roles in some protozoan parasites like Plasmodium spp, Toxoplasma gondii and trypanosomatids has allowed the expansion of the results obtained in human cancer to these infections. This review summarizes the latest important findings showing protozoan HSP90 as a drug target and presents three patents targeting T. gondii, P. falciparum and trypanosomatids HSP90.

No stress--Hsp90 and signal transduction in Leishmania

SUMMARY Hsp90 (a.k.a. Hsp83) plays a significant role in the life cycle control of the protozoan parasite Leishmania donovani. Rather than protecting Leishmania spp. against adverse and stressful environs, Hsp90 is required for the maintenance of the motile, highly proliferative insect stage, the promastigote. However, Hsp90 is also essential for survival and proliferation of the intracellular mammalian stage, the amastigote. Moreover, recent evidence shows Hsp90 and other components of large multi-chaperone complexes as substrates of stage-specific protein phosphorylation pathways, and thus as likely effectors of the signal transduction pathways in Leishmania spp. Future efforts should be directed towards the identification of the protein kinases and the critical phosphorylation sites as targets for novel therapeutic approaches.

Novel role of calmodulin in regulating protein transport to mitochondria in a unicellular eukaryote

Lower eukaryotes like the kinetoplastid parasites are good models to study evolution of cellular pathways during steps to eukaryogenesis. In this study, a kinetoplastid parasite, Leishmania donovani, was used to understand the process of mitochondrial translocation of a nucleus-encoded mitochondrial protein, the mitochondrial tryparedoxin peroxidase (mTXNPx). We report the presence of an N-terminal cleavable mitochondrial targeting signal (MTS) validated through deletion and grafting experiments. We also establish a novel finding of calmodulin (CaM) binding to the MTS of mTXNPx through specific residues. Mutation of CaM binding residues, keeping intact the residues involved in mitochondrial targeting and biochemical inhibition of CaM activity both in vitro and in vivo, prevented mitochondrial translocation. Through reconstituted import assays, we demonstrate obstruction of mitochondrial translocation either in the absence of CaM or Ca(2+) or in the presence of CaM inhibitors. We also demonstrate the prevention of temperature-driven mTXNPx aggregation in the presence of CaM. These findings establish the idea that CaM is required for the transport of the protein to mitochondria through maintenance of translocation competence posttranslation.

Cyclophilin-mediated reactivation pathway of inactive adenosine kinase aggregates

Monomeric adenosine kinase (AdK), a pivotal salvage enzyme of the purine auxotrophic parasite, Leishmania donovani, tends to aggregate naturally or selectively in presence of ADP, leading to inactivation. A cyclophilin (LdCyP) from the parasite reactivated the enzyme by disaggregating it. We studied the aggregation pathway of AdK with or without ADP. Transmission electron microscopy revealed that ADP-induced aggregates, as opposed to annular or torus-shaped natural aggregates, were mostly amorphous with protofibril-like structures. Interestingly, only the natural aggregates bound thioflavin T with a KD of 3.33 μM, indicating cross β-sheet structure. Dynamic light scattering experiments indicated that monomers formed aggregates either upon prolonged storage or ADP exposure. ADP-aggregates were disaggregated by LdCyP with concomitant reactivation of the enzyme. The activity revived with decrease in the aggregate size. Displacement of ADP from the ADP-aggregated enzyme by LdCyP resulted in reactivation. CD-spectral studies suggested that, like the natural aggregates, ADP induced formation of β-sheet structure in the ADP-aggregates. However, unlike the natural aggregate, it could be reconverted to α-helical conformation upon addition of LdCyP. Based on the results, a regulatory mechanism through interplay of ADP and/or LdCyP interaction with the enzyme is envisaged and a pathway of AdK reactivation by LdCyP-chaperone is proposed.

Evolutionary conservation and diversification of the translation initiation apparatus in trypanosomatids

Trypanosomatids are ancient eukaryotic parasites that migrate between insect vectors and mammalian hosts, causing a range of diseases in humans and domestic animals. Trypanosomatids feature a multitude of unusual molecular features, including polycistronic transcription and subsequent processing by trans-splicing and polyadenylation. Regulation of protein coding genes is posttranscriptional and thus, translation regulation is fundamental for activating the developmental program of gene expression. The spliced-leader RNA is attached to all mRNAs. It contains an unusual hypermethylated cap-4 structure in its 5′ end. The cap-binding complex, eIF4F, has gone through evolutionary changes in accordance with the requirement to bind cap-4. The eIF4F components in trypanosomatids are highly diverged from their orthologs in higher eukaryotes, and their potential functions are discussed. The cap-binding activity in all eukaryotes is a target for regulation and plays a similar role in trypanosomatids. Recent studies revealed a novel eIF4E-interacting protein, involved in directing stage-specific and stress-induced translation pathways. Translation regulation during stress also follows unusual regulatory cues, as the increased translation of Hsp83 following heat stress is driven by a defined element in the 3′ UTR, unlike higher eukaryotes. Overall, the environmental switches experienced by trypanosomatids during their life cycle seem to affect their translational machinery in unique ways.

Trypanosoma cruzi: modulation of HSP70 mRNA stability by untranslated regions during heat shock

Gene regulation in trypanosomatids occurs mainly by post-transcriptional mechanisms modulating mRNA stability and translation. We have investigated heat shock protein (HSP) 70 gene regulation in Trypanosoma cruzi, the causal agent of Chagas' disease. The HSP70 mRNA's half-life increases after heat shock, and the stabilization is dependent on protein synthesis. In a cell-free RNA decay assay, a U-rich region in the 3' untranslated region (UTR) is a target for degradation, which is reduced when in the presence of protein extracts from heat shocked cells. In a transfected reporter gene assay, both the 5'- and 3'-UTRs confer temperature-dependent regulation. Both UTRs must be present to increase mRNA stability at 37 degrees C, indicating that the 5'- and 3'-UTRs act cooperatively to stabilize HSP70 mRNA during heat shock. We conclude that HSP70 5'- and 3'-UTRs regulate mRNA stability during heat shock in T. cruzi.

Phosphoproteome dynamics reveal heat-shock protein complexes specific to the Leishmania donovani infectious stage

Leishmania is exposed to a sudden increase in environmental temperature during the infectious cycle that triggers stage differentiation and adapts the parasite phenotype to intracellular survival in the mammalian host. The absence of classical promoter-dependent mechanisms of gene regulation and constitutive expression of most of the heat-shock proteins (HSPs) in these human pathogens raise important unresolved questions as to regulation of the heat-shock response and stage-specific functions of Leishmania HSPs. Here we used a gel-based quantitative approach to assess the Leishmania donovani phosphoproteome and revealed that 38% of the proteins showed significant stage-specific differences, with a strong focus of amastigote-specific phosphoproteins on chaperone function. We identified STI1/HOP-containing chaperone complexes that interact with ribosomal client proteins in an amastigote-specific manner. Genetic analysis of STI1/HOP phosphorylation sites in conditional sti1(-/-) null mutant parasites revealed two phosphoserine residues essential for parasite viability. Phosphorylation of the major Leishmania chaperones at the pathogenic stage suggests that these proteins may be promising drug targets via inhibition of their respective protein kinases.

Preferential translation of Hsp83 in Leishmania requires a thermosensitive polypyrimidine-rich element in the 3' UTR and involves scanning of the 5' UTR

Heat shock proteins (HSPs) provide a useful system for studying developmental patterns in the digenetic Leishmania parasites, since their expression is induced in the mammalian life form. Translation regulation plays a key role in control of protein coding genes in trypanosomatids, and is directed exclusively by elements in the 3' untranslated region (UTR). Using sequential deletions of the Leishmania Hsp83 3' UTR (888 nucleotides [nt]), we mapped a region of 150 nt that was required, but not sufficient for preferential translation of a reporter gene at mammalian-like temperatures, suggesting that changes in RNA structure could be involved. An advanced bioinformatics package for prediction of RNA folding (UNAfold) marked the regulatory region on a highly probable structural arm that includes a polypyrimidine tract (PPT). Mutagenesis of this PPT abrogated completely preferential translation of the fused reporter gene. Furthermore, temperature elevation caused the regulatory region to melt more extensively than the same region that lacked the PPT. We propose that at elevated temperatures the regulatory element in the 3' UTR is more accessible to mediators that promote its interaction with the basal translation components at the 5' end during mRNA circularization. Translation initiation of Hsp83 at all temperatures appears to proceed via scanning of the 5' UTR, since a hairpin structure abolishes expression of a fused reporter gene.

Chemical Stress does not Induce Heat Shock Protein Synthesis in Leishmania donovani

The cellular heat shock response in kinetoplastid protozoa is regulated exclusively at a post-transcriptional level. The heat-inducibility of heat shock protein synthesis is retained under actinomycin C(1) which indicates an inducible translation of heat shock mRNAs. We have also assessed the ability of various chemicals known to be effective triggers of the heat shock response in higher eukaryotes to induce heat shock protein synthesis in Leishmania donovani. None of the tested chemicals elicited a stress response. We propose that the lack of transcription regulation in the kinetoplastida precludes a stress response under chemical stress.

Genome-wide gene expression profiling analysis of Leishmania major and Leishmania infantum developmental stages reveals substantial differences between the two species

BACKGROUND

Leishmania parasites cause a diverse spectrum of diseases in humans ranging from spontaneously healing skin lesions (e.g., L. major) to life-threatening visceral diseases (e.g., L. infantum). The high conservation in gene content and genome organization between Leishmania major and Leishmania infantum contrasts their distinct pathophysiologies, suggesting that highly regulated hierarchical and temporal changes in gene expression may be involved.

RESULTS

We used a multispecies DNA oligonucleotide microarray to compare whole-genome expression patterns of promastigote (sandfly vector) and amastigote (mammalian macrophages) developmental stages between L. major and L. infantum. Seven per cent of the total L. infantum genome and 9.3% of the L. major genome were differentially expressed at the RNA level throughout development. The main variations were found in genes involved in metabolism, cellular organization and biogenesis, transport and genes encoding unknown function. Remarkably, this comparative global interspecies analysis demonstrated that only 10-12% of the differentially expressed genes were common to L. major and L. infantum. Differentially expressed genes are randomly distributed across chromosomes further supporting a posttranscriptional control, which is likely to involve a variety of 3'UTR elements.

CONCLUSION

This study highlighted substantial differences in gene expression patterns between L. major and L. infantum. These important species-specific differences in stage-regulated gene expression may contribute to the disease tropism that distinguishes L. major from L. infantum.

Coordinate regulation of a family of promastigote-enriched mRNAs by the 3'UTR PRE element in Leishmania mexicana

Post-transcriptional regulation is a key feature controlling gene expression in the protozoan parasite Leishmania. The nine-nucleotide paraflagellar rod regulatory element (PRE) in the 3'UTR of Leishmania mexicana PFR2 is both necessary and sufficient for the observed 10-fold higher level of PFR2 mRNA in promastigotes compared to amastigotes. It is also found in the 3'UTRs of all known PFR genes. A search of the Leishmania major Friedlin genomic database revealed several genes that share this cis element including a homolog of a heterotrimeric kinesin II subunit, and a gene that shares identity to a homolog of a Plasmodium antigen. In this study, we show that genes that harbor the PRE display promastigote-enriched transcript accumulation ranging from 4- to 15-fold. Northern analysis on episomal block substitution constructs revealed that the regulatory element is necessary for the proper steady-state accumulation of mRNA in L. mexicana paraflagellar rod gene 4 (PFR4). Also we show that the PRE plays a major role in the proper steady-state mRNA accumulation of PFR1, but may not account for the full regulatory mechanism acting on this mRNA. Our evidence suggests that the PRE coordinately regulates the mRNA abundance of not only the PFR family of genes, but also in a larger group of genes that have unrelated functions. Although the PRE alone can regulate some mRNAs, it may also act in concert with additional elements to control other RNA transcripts.

Members of a large retroposon family are determinants of post-transcriptional gene expression in Leishmania

Trypanosomatids are unicellular protists that include the human pathogens Leishmania spp. (leishmaniasis), Trypanosoma brucei (sleeping sickness), and Trypanosoma cruzi (Chagas disease). Analysis of their recently completed genomes confirmed the presence of non-long-terminal repeat retrotransposons, also called retroposons. Using the 79-bp signature sequence common to all trypanosomatid retroposons as bait, we identified in the Leishmania major genome two new large families of small elements--LmSIDER1 (785 copies) and LmSIDER2 (1,073 copies)--that fulfill all the characteristics of extinct trypanosomatid retroposons. LmSIDERs are approximately 70 times more abundant in L. major compared to T. brucei and are found almost exclusively within the 3'-untranslated regions (3'UTRs) of L. major mRNAs. We provide experimental evidence that LmSIDER2 act as mRNA instability elements and that LmSIDER2-containing mRNAs are generally expressed at lower levels compared to the non-LmSIDER2 mRNAs. The considerable expansion of LmSIDERs within 3'UTRs in an organism lacking transcriptional control and their role in regulating mRNA stability indicate that Leishmania have probably recycled these short retroposons to globally modulate the expression of a number of genes. To our knowledge, this is the first example in eukaryotes of the domestication and expansion of a family of mobile elements that have evolved to fulfill a critical cellular function.

Amino acid residues of Leishmania donovani cyclophilin key to interaction with its adenosine kinase: biological implications

Cyclophilins (CyPs), by interacting with a variety of proteins, often modulate their biological activities and thus have been implicated in several cellular functions. However, mechanisms that determine such interactions are poorly understood. We earlier reported that an endoplasmic reticulum (ER)-located cyclophilin (LdCyP) from the purine auxotrophic parasitic protozoan Leishmania donovani reactivated its adenosine kinase (AdK). The AdK-reactivating property of LdCyP was however abolished at high ionic strength but not by nonionic detergents. Modeling of LdCyP, based on its crystal structure solved at 1.97 A resolution, revealed several solvent-exposed hydrophobic and charged residues. Mutagenesis of several of such solvent-exposed residues was performed and their corresponding activities with regard to their (i) AdK reactivation property, (ii) ability to form complex with the enzyme, (iii) capacity to induce red shift in the intrinsic tryptophan fluorescence maxima of AdK, and (iv) efficiency to withdraw the ADP inhibition from the AdK-mediated reaction were compared to the wild-type protein. Results indicated that while the replacement of R147 with either A or D severely impaired all of the above characteristics displayed by the wild-type LdCyP, the effect of mutating K114 and K153 was although relatively less but nevertheless noticeable. Alteration of other exposed hydrophobic and charged residues apparently did not have any discernible effect. Under the condition of cellular stress, the ER-located LdCyP is released into the cytoplasm with concomitant increase both in the specific activity of the cytosol-resident AdK and the uptake of radiolabeled Ado into the cells. These experiments, besides demonstrating the importance of the positive charge, identified R147 as the most crucial residue in the LdCyP-AdK interaction and provide evidence for the stress-induced retrograde translocation of LdCyP from the ER to the cytoplasm. A possible implication of this interaction in the life cycle of the parasite is proposed.

Reversal of ADP-mediated aggregation of adenosine kinase by cyclophilin leads to its reactivation

Cyclophilins have been implicated in several important cellular functions. Our earlier results showed that reactivation of adenosine kinase (AdK) by CyP (LdCyP) from the parasitic protozoa Leishmania donovani is accompanied with disaggregation of the enzyme [Chakraborty, A., et al. (2002) J. Biol. Chem. 277, 47451-47460; Chakraborty, A., et al. (2004) Biochemistry 43, 11862-11872]. However, it remained to be known why the enzyme displayed progressive inhibition during the time-dependent reaction and what LdCyP does to prevent and/or reverse the inhibition. Herein, we demonstrate that one of its reaction products, ADP but not AMP, facilitates the formation of AdK aggregates, leading to its inactivation. Further studies revealed that LdCyP reactivates the enzyme by withdrawing the ADP inhibition. To investigate the molecular mechanism, the intrinsic tryptophan fluorescence and polarization of AdK were monitored in the presence of either LdCyP or ADP and in combination thereof. Whereas in the presence of LdCyP the tryptophan fluorescence emission maxima of AdK exhibited a red shift, ADP had a quenching effect. However, both the red shift and quenching became less noticeable when one (W234) of the two tryptophan residues of AdK was altered, indicating W234 fluorescence is relatively more sensitive to both LdCyP and ADP binding. Kinetic measurements indicated that LdCyP-facilitated reactivation of AdK is accompanied with a concomitant increase in the KD of ADP but not of AMP. Interestingly, addition of myokinase (MK) and pyruvate kinase (PK) along with phosphoenolpyruvate, either singly or in conjunction, to the AdK reaction mixture led to its reactivation. The effect of PK but not of MK could be substituted by CyP and vice versa. Taken together, the results suggest that LdCyP-induced reactivation occurs due to conformational reorientation of AdK in a manner that decreases the affinity of the enzyme for ADP with consequent relief from the ADP-mediated aggregation.

Identification of developmentally-regulated proteins in Leishmania panamensis by proteome profiling of promastigotes and axenic amastigotes

We have employed proteomics to identify proteins upregulated in the amastigote life-stage of Leishmaniapanamensis, using axenically-differentiated forms as models of authentic intracellular parasites. Resolution of the soluble proteomes of axenic amastigotes and promastigotes by two-dimensional electrophoresis (2DE) in the neutral pI range (5-7) revealed equivalent numbers of protein spots in both life-stages (644-682 using Coomassie Blue and 851-863 by silver staining). Although representing a relatively low proportion (8.1-10.8%) of the predicted 8000 gene products of Leishmania, these proteome maps enabled the reproducible detection of 75 differentially-regulated protein spots in amastigotes, comprising 24 spots "uniquely" expressed in this life-stage and 51 over-expressed by 1.2-5.7-fold compared to promastigotes. Of the 11 amastigote-specific spots analysed by mass spectrometry (MS), 5 yielded peptide sequences with no orthologues in Leishmania major, and the remaining 6 were identified as 7 distinct proteins (some of which were truncated isoforms) representing several functional classes: carbohydrate/energy metabolism (fructose 1,6-bisphosphate aldolase, glucose 6-phosphate dehydrogenase, pyruvate dehydrogenase), stress response (heat shock protein [HSP] 83), cell membrane/cytoskeleton (beta-tubulin), amino acid metabolism (cysteine synthase) and cell-cycle (ran-binding protein). Four additional over-expressed spots were tentatively identified as HSPs 60 and 70 and HSP 70-related proteins -1 and -4 by positional analogy with these landmark proteins in the Leishmania guyanensis proteome. Our data demonstrate the feasibility of proteomics as an approach to identify novel developmentally-regulated proteins linked to Leishmania differentiation and intracellular survival, while simultaneously pinpointing therapeutic targets. In particular, the amastigote-specific expression of cysteine synthase underlines the importance of de novo cysteine synthesis both as a potential parasite virulence factor and as a major metabolic difference from mammalian host cells.

The transcriptomes of Trypanosoma brucei Lister 427 and TREU927 bloodstream and procyclic trypomastigotes

We describe developmentally regulated genes in two strains of Trypanosoma brucei: the monomorphic strain Lister 427 and the pleomorphic strain TREU927. Expression patterns were obtained using an array of 24,567 genomic fragments. Probes were prepared from bloodstream-form or procyclic-form trypanosomes. Fourteen procyclic-specific and 77 bloodstream-specific signals were obtained from sequences matching variant surface glycoprotein or associated genes, and a further 17 regulated sequences were repetitive or transposable-element-related. Two hundred and eighty-six regulated spots corresponded to mRNAs from other protein-coding genes; these spots represent 191 different proteins. Regulation of 113 different genes (79 from procyclic forms, 34 from bloodstream-forms) was supported by at least two independent experiments or criteria; of these, about 60 were novel. Only two genes -- encoding HSP83 and an importin-related protein -- appeared to be regulated in the TREU927 strain only. Our results confirmed previous estimates that 2% of trypanosome genes show developmental regulation at the mRNA level.

Characterization and developmental gene regulation of a large gene family encoding amastin surface proteins in Leishmania spp

The ability of Leishmania amastigotes to survive within the drastic environmental changes encountered in the phagolysosomes of mammalian macrophages is heavily dependent on the developmental regulation of a variety of genes. The identification of genes that are expressed preferentially in the mammalian stage of the parasite should increase our understanding of the molecular mechanisms regulating stage-specific gene expression and of the determinants that control its intracellular survival and contribute to its pathogenesis. We report here detailed sequence characterization and structural organization of the amastin gene family in Leishmania major and Leishmania infantum and the study of their developmental gene regulation throughout the parasite's life cycle. Amastin surface proteins represent the largest developmentally regulated gene family reported so far in Leishmania comprising up to 45 members. All the members of the amastin gene family in both Leishmania and Trypanosoma species share a similar structural organization and contain a highly conserved 11 amino acid extracellular domain, which is unique to amastin proteins. The majority of the amastin gene homologs are specifically expressed in the amastigote stage of the parasite. Three distinct RNA elements were identified in the 3'-untranslated regions (3'UTR) of the amastin transcripts. The majority of these transcripts contain a conserved 450 nt cis-acting 3'UTR element shown previously to regulate stage-specific gene expression at the level of translation, which suggests that several amastin homologs may be regulated by a similar mechanism of translational control inside the macrophage. These findings further highlight the unique features of gene expression control in Leishmania.

The expression of HSP83 genes in Leishmania infantum is affected by temperature and by stage-differentiation and is regulated at the levels of mRNA stability and translation

Background

Exposure of Leishmania promastigotes to the temperature of their mammalian hosts results in the induction of a typical heat shock response. It has been suggested that heat shock proteins play an important role in parasite survival and differentiation.

Results

Here we report the studies on the expression of the heat shock protein 83 (HSP83) genes of Leishmania infantum. Confirming previous observations for other Leishmania species, we found that the L. infantum HSP83 transcripts also show a temperature-dependent accumulation that is controlled by a post-transcriptional mechanism involving sequences located in the 3'-untranslated region (3'-UTR). However, contrary to that described for L. amazonensis, the accumulation of the HSP83 transcripts in L. infantum is dependent on active protein synthesis. The translation of HSP83 transcripts is enhanced during heat shock and, as first described in L. amazonensis, we show that the 3'-UTR of the L. infantum HSP83 gene is essential for this translational control. Measurement of the steady-state levels of HSP83 transcripts along the promastigote-to-amastigote differentiation evidenced a specific profile of HSP83 RNAs: after an initial accumulation of HSP83 transcripts observed short after (2 h) incubation in the differentiation conditions, the amount of HSP83 RNA decreased to a steady-state level lower than in undifferentiated promastigotes. We show that this transient accumulation is linked to the presence of the 3'-UTR and flanking regions. Again, an 8-fold increase in translation of the HSP83 transcripts is observed short after the initiation of the axenic differentiation, but it is not sustained after 9 h.

Conclusions

This transient expression of HSP83 genes could be relevant for the differentiation of Leishmania, and the underlying regulatory mechanism may be part of the developmental program of this parasite.

The heat shock protein HSP70 and heat shock cognate protein HSC70 contribute to antimony tolerance in the protozoan parasite leishmania

Antimony-containing drugs are still the drugs of choice in the treatment of infections caused by the parasite Leishmania. Resistance to antimony is now common in some parts of the world, and several mechanisms of resistance have been described. By transfecting cosmid banks and selecting with potassium antimonyl tartrate (SbIII), we have isolated a cosmid associated with resistance. This cosmid contains 2 copies of the heat shock protein 70 (HSP70) and 1 copy of the heat shock cognate protein 70 (HSC70). Several data linked HSP70 to antimony response and resistance. First, several Leishmania species, both as promastigotes and amastigotes, increased the expression of their HSP70 proteins when grown in the presence of 1 or 2 times the Effect Concentration 50% of SbIII. In several mutants selected for resistance to either SbIII or to the related metal arsenite, the HSP70 proteins were found to be overexpressed. This increase was also observed in revertant cells grown for several passages in the absence of SbIII, suggesting that this increased production of HSP70 is stable. Transfection of HSP70 or HSC70 in Leishmania cells does not confer resistance directly, though these transfectants were better able to tolerate a shock with SbIII. Our results are consistent with HSP70 and HSC70 being a first line of defense against SbIII until more specific and efficient resistance mechanisms take over.

The transcriptome and its translation during recovery from cell cycle arrest in Saccharomyces cerevisiae

Complete genome sequences together with high throughput technologies have made comprehensive characterizations of gene expression patterns possible. While genome-wide measurement of mRNA levels was one of the first applications of these advances, other important aspects of gene expression are also amenable to a genomic approach, for example, the translation of message into protein. Earlier we reported a high throughput technology for simultaneously studying mRNA level and translation, which we termed translation state array analysis, or TSAA. The current studies test the proposition that TSAA can identify novel instances of translation regulation at the genome-wide level. As a biological model, cultures of Saccharomyces cerevisiae were cell cycle-arrested using either alpha-factor or the temperature-sensitive cdc15-2 allele. Forty-eight mRNAs were found to change significantly in translation state following release from alpha-factor arrest, including genes involved in pheromone response and cell cycle arrest such as BAR1, SST2, and FAR1. After the shift of the cdc15-2 strain from 37 degrees C to 25 degrees C, 54 mRNAs were altered in translation state, including the products of the stress genes HSP82, HSC82, and SSA2. Thus, regulation at the translational level seems to play a significant role in the response of yeast cells to external physical or biological cues. In contrast, surprisingly few genes were found to be translationally controlled as cells progressed through the cell cycle. Additional refinements of TSAA should allow characterization of both transcriptional and translational regulatory networks on a genomic scale, providing an additional layer of information that can be integrated into models of system biology and function.

A Single-domain Cyclophilin from Leishmania donovaniReactivates Soluble Aggregates of Adenosine Kinase by Isomerase-independent Chaperone Function

Disaggregation and reactivation of aggregated proteins by chaperones is well established. However, little is known regarding such kind of function of single-domain small cyclophilins (CyPs). Here we demonstrate that, with increasing concentrations, fully active adenosine kinase (AdK) of Leishmania donovani tends to form soluble aggregates, resulting in inactivation. Using this inactive enzyme as the substrate, it is shown that a CyP from L. donovani (LdCyP) alone can cause complete disaggregation, leading to reactivation of the enzyme. The reactivating ability of LdCyP remains unaffected even in the presence of cyclosporin A and macromolecular crowding agents. The reactivation occurs noncatalytically and is reversible. A truncated LdCyP, devoid of 88 amino acids from the N terminus, is found to be required in near stoichiometric proportion to reactivate AdK, suggesting essentiality of the C-terminal region. Gel filtration and light-scattering experiments together with protein cross-linking studies revealed that both full-length LdCyP and the truncated form directly interact with AdK and convert oligomeric forms of the enzyme to monomeric state. Homology modeling studies suggest that the exposed hydrophobic residues of LdCyP, by interacting with solvent-accessible hydrophobic surface of AdK, pull apart its aggregated inactive oligomers to functional monomers. Clearly, the results are consistent with the interpretation that the higher efficiency of the truncated LdCyP is most likely due to increased exposure of the hydrophobic residues on its surface. These observations, besides establishing L. donovani AdK as one of the model enzymes to study aggregation-disaggregation of proteins, raise the possibility that single-domain small CyPs, under physiological conditions, may regulate the activity of aggregation-prone proteins by ensuring their disaggregation.

A common mechanism of stage-regulated gene expression in Leishmania mediated by a conserved 3'-untranslated region element

Developmental regulation of mRNA levels in trypanosomatid protozoa is determined post-transcriptionally and often involves sequences located in the 3'-untranslated regions (3'-UTR) of the mRNAs. We have previously identified a developmentally regulated gene family in Leishmania encoding the amastin surface proteins and showed that stage-specific accumulation of the amastin mRNA is mediated by sequences within the 3'-UTR. Here we identified a 450-nt region within the amastin 3'-UTR that can confer amastigote-specific gene expression by a novel mechanism that increases mRNA translation without an increase in mRNA stability. Remarkably, this 450-nt 3'-UTR element is highly conserved among a large number of Leishmania mRNAs in several Leishmania species. Here we show that several of these mRNAs are differentially expressed in the intracellular amastigote stage of the parasite and that the 450-nt conserved element in their 3'-UTRs is responsible for stage-specific gene regulation. We propose that the 450-nt conserved element, which is unlike any other regulatory element identified thus far, is part of a common mechanism of stage-regulated gene expression in Leishmania that regulates mRNA translation in response to intracellular stresses.

Developmental regulation of heat shock protein 83 in Leishmania. 3' processing and mRNA stability control transcript abundance, and translation id directed by a determinant in the 3'-untranslated region

Developmental gene regulation in trypanosomatids proceeds exclusively by post-transcriptional mechanisms. Stability and abundance of heat shock protein (HSP)70 and HSP83 transcripts in Leishmania increase at mammalian-like temperatures, and their translation is enhanced. Here we report that the 3'-untranslated region (UTR) of HSP83 (886 nucleotides) confers the temperature-dependent pattern of regulation on a chloramphenicol acetyltransferase (CAT) reporter transcript. We also show that the majority of the 3'-UTR sequences are required for increasing mRNA stability during heat shock. Processing of the HSP70 and HSP83 primary transcripts to poly(A)(+) mRNA was more efficient during heat shock; therefore, even when stability at 33 degrees C was reduced by deletions in the 3'-UTR, transcripts still accumulated to comparable and even higher levels. Translation of heat shock transcripts in Leishmania increases dramatically upon temperature elevation. Unlike in other eukaryotes in which the 5'-UTR confers preferential translation on heat shock transcripts, we show that translational control of HSP83 in Leishmania originates from its 3'-UTR. The 5'-UTR alone cannot induce translation during heat shock, but it has a minor contribution when combined with the HSP83 3'-UTR. We identified an element located between positions 201 and 472 of the 3'-UTR which is essential for increasing translation of the CAT-HSP83 reporter RNA at 33-37 degrees C. This region confers preferential translation during heat shock even in transcripts that were less stable. Thus, investigating the traditionally conserved heat shock response reveals that Leishmania parasites use unique pathways for translational control.

A DnaJ-like protein homologous to the yeast co-chaperone Sis1 (TcJ6p) is involved in initiation of translation in Trypanosoma cruzi

In eukaryotes, proteins homologous to the bacterial DnaJ protein are involved in regulation of the Hsp70 molecular chaperones, which are implicated in a variety of protein biogenesis pathways. We report herewith the molecular characterization of a T. cruzi DnaJ gene, termed TcJ6, encoding a protein that displays high sequence homology with the Saccharomyces cerevisiae Sis1 co-chaperone required for the initiation of translation. TcJ6 protein was expressed as a polypeptide of 36.5 kDa at a constant level during parasite differentiation and was associated to the cytoplasmic fraction. We showed that overexpression of TcJ6 complemented a temperature-sensitive yeast sis1 mutant. In addition, sucrose gradient sedimentation analysis of polysomes from T. cruzi and a yeast mutant overexpressing TcJ6p showed that the trypanosomal co-chaperone was closely associated with ribosomal subunits, 80 S monosomes and the smaller polysomes, as observed for Sis1p. Furthermore, in T. cruzi TcJ6p was also found to be preferentially concentrated around the nucleus, giving a speckled staining pattern. This suggests that TcJ6p is associated with the endoplasmic reticulum. Taken together, these data suggest that the trypanosomal DnaJ is involved in initiation of translation.

Degradation of the unstable EP1 mRNA in Trypanosoma brucei involves initial destruction of the 3'-untranslated region

Kinetoplastid protozoa regulate their gene expression primarily through control of mRNA degradation and translation. We describe here the degradation of three reporter mRNAs in Trypanosoma brucei. One mRNA had the 3'-untranslated region (3'-UTR) from the developmentally regulated EP1 mRNA, which is abundant in the procyclic (tsetse fly) form of the parasite but is almost undetectable in the bloodstream form. This untranslated region includes a 26 nt U-rich sequence that causes extreme RNA instability in the bloodstream form. The two other RNAs, which are not developmentally regulated, had either the actin 3'-UTR, or a version of the EP1 sequence lacking the 26 nt bloodstream-form instability element. All RNAs had poly(A) tails approximately 200 nt long, in both bloodstream and procyclic forms. Degradation of the two constitutively expressed mRNAs involved deadenylation and degradation by both 5'-->3' and 3'-->5' exonucleases. In contrast, in bloodstream forms, the 3'-end of the RNA bearing the bloodstream-form instability element disappeared very rapidly after transcription inhibition and partially deadenylated intermediates were not seen. The instability element may cause extremely rapid deadenylation, or it may be targeted by an endonuclease.

Heat shock protein 90 homeostasis controls stage differentiation in Leishmania donovani

The differentiation of Leishmania parasites from the insect stage, the promastigote, toward the pathogenic mammalian stage, the amastigote, is triggered primarily by the rise in ambient temperature encountered during the insect-to-mammal transmission. We show here that inactivation of heat shock protein (Hsp) 90, with the use of the drugs geldanamycin or radicicol, mimics transmission and induces the differentiation from the promastigote to the amastigote stage. Geldanamycin also induces a growth arrest of cultured promastigotes that can be forestalled by overexpression of the cytoplasmic Hsp90. Moreover, we demonstrate that Hsp90 serves as a feedback inhibitor of the cellular heat shock response in Leishmania. Our results are consistent with Hsp90 homeostasis serving as cellular thermometer for these primitive eukaryotes, controlling both the heat shock response and morphological differentiation.

Lack of abundance of cytoplasmic cyclosporin A-binding protein renders free-living Leishmania donovani resistant to cyclosporin A

The majority of the effects of cyclosporin A (CsA) on cells is caused by the inhibition of phosphatase activity of calcineurin (CN) by the cyclophilin A (CyPA)-CsA complex formed in the cytoplasm. Although CsA inhibits the proliferation of a large number of parasites, not all are susceptible. The presence of structurally altered CyPA with lower affinity for CsA had been suggested to be the cause of resistance. We report here the identification and cloning of a high affinity CsA-binding protein (LdCyP) from Leishmania donovani, a trypanosomatid parasite that is naturally resistant to CsA. The translated LdCyP consists of 187 amino acids with a cleavable 21-amino acid hydrophobic NH(2)-terminal extension. Modeling studies confirmed that all the residues of human CyPs responsible for interaction with CsA are sequentially and conformationally conserved in LdCyP. The purified recombinant protein displayed biochemical parameters comparable to human CyPs. Reverse transcription-polymerase chain reaction analysis confirmed that LdCyP was abundantly expressed. Immunoblot experiments and direct CsA binding studies revealed that LdCyP located in the subcellular organelles constituted the bulk of the CsA binding activity present in L. donovani, whereas the level of binding activity in the cytosol was conspicuously low. CsA selectively facilitated the secretion of LdCyP in the culture medium. Based on these results, it is concluded that the insensitivity of L. donovani to CsA is probably due to the paucity of CsA binding activity in the cytoplasm of the parasite. We suggest that LdCyP, located in the secretory pathway, may function as a chaperone by binding to membrane proteins rather than as the mediator of CN inhibition.

Regulation of GP63 mRNA stability in promastigotes of virulent and attenuated Leishmania chagasi

GP63 is a 63-kDa glycoprotein that is abundantly expressed on the surface of all Leishmania species and is involved in several steps of promastigote infection of host cells. Leishmania chagasi has at least 18 haploid msp (major surface protease) genes encoding GP63 that are divided into three classes, mspS, mspL or mspC, according to their unique 3' UTR sequences and differential expression. All three msp classes are constitutively transcribed during virulent promastigote growth in vitro, although mspL mRNA is most abundant during logarithmic phase and mspS mRNA predominates in stationary phase. Thus, the steady state levels of the mspL and mspS mRNAs are post-transcriptionally regulated. Using Actinomycin D to arrest transcription, we found that in virulent promastigotes the half-life (t(1/2)) of mspL mRNA is coordinately modulated with growth phase, decreasing from a mean of 84 min during early logarithmic growth to a mean of 17 min at a stage intermediate between logarithmic and stationary phase. However, in attenuated promastigotes, the t(1/2) of mspL RNA remains the same throughout parasite growth. In contrast to mspL RNA, the t(1/2) of mspS and mspC RNA is constant throughout all growth phases of both virulent and attenuated promastigote growth. The presence of the translation inhibitor cycloheximide increases the t(1/2) of mspL RNA 4-6-fold in both virulent and attenuated promastigotes at all growth phases. These results indicate that the t(1/2) of mspL RNA is maintained by at least two distinct mechanisms - one activated during growth to stationary phase and the other dependent on a labile negative regulatory protein factor(s).

Inducible resistance to oxidant stress in the protozoan Leishmania chagasi

Leishmania sp. protozoa are introduced into a mammalian skin by a sandfly vector, whereupon they encounter increased temperature and toxic oxidants generated during phagocytosis. We studied the effects of 37 degrees C "heat shock" or sublethal menadione, which generates superoxide and hydrogen peroxide, on Leishmania chagasi virulence. Both heat and menadione caused parasites to become more resistant to H(2)O(2)-mediated toxicity. Peroxide resistance was also induced as promastigotes developed in culture from logarithmic to their virulent stationary phase form. Peroxide resistance was not associated with an increase in reduced thiols (trypanothione and glutathione) or increased activity of ornithine decarboxylase, which is rate-limiting in trypanothione synthesis. Membrane lipophosphoglycan increased in size as parasites developed to stationary phase but not after environmental exposures. Instead, parasites underwent a heat shock response upon exposure to heat or sublethal menadione, detected by increased levels of HSP70. Transfection of promastigotes with L. chagasi HSP70 caused a heat-inducible increase in resistance to peroxide, implying it is involved in antioxidant defense. We conclude that leishmania have redundant mechanisms for resisting toxic oxidants. Some are induced during developmental change and others are induced in response to environmental stress.

A new developmentally regulated gene family in Leishmania amastigotes encoding a homolog of amastin surface proteins

The ability of Leishmania to survive within the phagolysosomes of mammalian macrophages is heavily dependent on the developmental regulation of a number of genes. Characterization of genes preferentially expressed during the parasite's intracellular growth would help to elucidate the mechanisms controlling stage-specific gene regulation and the intracellular life of the parasite in general. Using a genomic approach based on the differential hybridization screening of high-density filters, we have identified a new developmentally regulated gene in Leishmania, which is part of a multigene family and encodes a highly hydrophobic protein that shares homology with the Trypanosoma cruzi amastin proteins. The fusion of the Leishmania amastin gene homolog with the green fluorescent protein and analysis by confocal microscopy suggested a surface localization for this protein. The amastin gene homolog is expressed predominantly in the amastigote form of several Leishmania species and is strictly regulated by acidic pH at the post-transcriptional level. Its developmental expression involves sequences within the 3'-untranslated region.

Identification of a putative regulatory element in the 3'-untranslated region that controls expression of HSP70 in Leishmania infantum

The regulation of HSP70 gene expression in Leishmania infantum, in contrast to most eukaryotes, occurs by mechanisms that operate exclusively at the post-transcriptional level. During the normal growth of L. infantum promastigotes at 26 degrees C the mRNAs derived from the sixth gene of the HSP70 locus are more abundant than the mRNAs derived from the other five HSP70 genes, but only the latter transcripts accumulate after incubation at 37 degrees C. Here, it was found that the full-length 3'untranslated region (UTR) and downstream sequences of the HSP70 genes are necessary for a correct polyadenylation of both types of transcripts and responsible for the differences in the steady-state levels of the transcripts. Also, it was found that the addition of the 3'-UTR-I (common to the first five genes of the L. infantum HSP70 gene cluster) to a reporter gene is sufficient to achieve an accumulation of the corresponding transcripts at 37 degrees C. This effect was, furthermore, found to be strand dependent. A progressive shortening of the 1063-base 3'-UTR-I has shown that the temperature-dependent accumulation was lost after deletion of 364-nucleotides from the 3' end. In addition, the accumulation of reporter transcripts at 37 degrees C was not observed in a plasmid construct containing an internal deletion (region 699-816) of the 3'-UTR-I. Thus, our data suggest that RNAs derived from L. infantum HSP70 genes 1-5 contain a cis-acting sequence that functions as a positive element during heat shock.

Effect of acidic pH on heat shock gene expression in Leishmania

Temperature and pH shifts trigger differential gene expression and stage transformation in Leishmania. The parasites encounter dramatic fluctuations in the extra-cellular pH between the mid-gut of the sand fly (pH>8) and the phagolysosomal vacuole of mammalian macrophages (pH<6). The authors examined the effect of pH shifts on heat shock gene expression in Leishmania amazonensis and Leishmania donovani promastigotes. Acidic pH resulted in preferential stability of the hsp83 transcripts at 26 degrees C, but hsp transcripts were not preferentially translated as observed during heat shock. Pre-conditioning of promastigotes to acidic pH did not alter the temperature threshold for hsp synthesis but lead to an increase in hsp synthesis mainly in L. donovani at 37 degrees C, and to a slight decrease in the arrest of tubulin synthesis in L. amazonensis. The stage specific morphological alterations that take place in vitro correlated with the arrest in tubulin synthesis and occurred at different temperatures in L. donovani and L. amazonensis.

Developmental regulation of spliced leader RNA gene in Leishmania donovani amastigotes is mediated by specific polyadenylation

Leishmania cycles between the insect vector and its mammalian host undergoing several important changes mediated by the stage-specific expression of a number of genes. Using a genomic differential screening approach, we isolated differentially expressed cosmid clones carrying several copies of the mini-exon gene. We report that the spliced leader (SL) RNA, essential for the maturation of all pre-mRNAs by trans-splicing, is developmentally regulated in Leishmania donovani amastigotes and that this regulation is rapidly induced upon parasite growth under acidic conditions. Stage-specific regulation of the SL RNA is associated with the expression of a larger approximately 170-nucleotide transcript that bears an additional 15-nucleotide sequence at its 3'-end and is polyadenylated in contrast to the mature SL RNA. The poly(A)+ SL RNA represents 12-16% of the total SL transcript synthesized in amastigotes and is 2.5-3-fold more stable than the poly(A)- transcript. The poly(A)+ SL transcript is synthesized specifically from one class of the genomic mini-exon copies. Polyadenylation of the SL RNA may control the levels of the SL mature transcript under amastigote growth and may represent an additional step in the gene regulation process during parasite differentiation.

Control of gene expression in Trypanosomatidae

The study of mechanisms which control gene expression in trypanosomatids has developed at an increasing rate since 1989 when the first successful DNA transfection experiments were reported. Using primarily Trypanosoma brucei as a model, several groups have begun to elucidate the basic control mechanisms and to define the cellular factors involved in mRNA transcription, processing and translation in these parasites. This review focuses on the most recent studies regarding a subset of genes that are expressed differentially during the life cycle of three groups of parasites. In addition to T. brucei, I will address studies on gene regulation in a few species of Leishmania and the results obtained by a much more limited group of laboratories studying gene expression in Trypanosoma cruzi. It is becoming evident that the regulatory strategies chosen by different species of trypanosomatids are not similar, and that for these very successful parasites it is probably advantageous to employ multiple mechanisms simultaneously. In addition, with the increasing numbers of parasite genes that have now been submitted to molecular dissection, it is also becoming evident that, among the various strategies for gene expression control, there is a predominance of regulatory pathways acting at the post-transcriptional level.

Differential regulation of multiple glucose transporter genes in Leishmania mexicana

We have studied the structure and expression of glucose transporter genes in the parasitic protozoan Leishmania mexicana. Three distinct glucose transporter isoforms, LmGT1, LmGT2, and LmGT3, are encoded by single copy genes that are clustered together at a single locus. Quantitation of Northern blots reveals that LmGT2 mRNA is present at approximately 15-fold higher level in promastigotes, the insect stage of the parasite life cycle, compared with amastigotes, the intracellular stage of the life cycle that lives within the mammalian host. In contrast, LmGT1 and LmGT3 mRNAs are expressed at similar levels in both life cycle stages. Transcription of the LmGT genes in promastigotes and axenically cultured amastigotes occurs at similar levels, as measured by nuclear run-on transcription. Consequently, the approximately 15-fold up-regulation of LmGT2 mRNA levels in promastigotes compared with amastigotes must be controlled at the post-transcriptional level. Measurement of LmGT2 RNA decay in promastigotes and axenic amastigotes treated with actinomycin D suggests that differential mRNA stability may play a role in regulating glucose transporter mRNA levels in the two life cycle stages.

The 3' untranslated region of the hsp 70 genes maintains the level of steady state mRNA in Trypanosoma brucei upon heat shock

An increase in the transcriptional efficiency at elevated temperatures is a characteristic of transcription of heat shock protein (hsp) coding genes in most eukaryotes analyzed to date. The regulatory mechanism for hsp 70 genes expression in Trypanosoma brucei does not follow the conventional transcriptional induction mechanism. The hsp 70 locus of T.brucei appears in a permanently activated state, and transcriptional induction of hsp 70 genes by heat shock does not occur in this organism. Therefore, the differential expression of the hsp 70 genes in trypanosomes is, to a large extent, post-transcriptionally controlled. Mechanisms of post-transcriptional control of the hsp 70 gene expression were investigated. Procyclic trypanosomes were normally maintained at approximately 25 degreesC. Incubation of procyclic trypanosomes at 41 degreesC drastically reduced the steady state mRNA levels of many protein coding genes. In contrast, the expression of the hsp 70 genes is either maintained at a high level or is up-regulated. The hsp 70 intergenic region promoter together with its 3' splice acceptor sites and the 5' untranslated region (UTR) are not sufficient to maintain or up-regulate the mRNA level of a reporter gene upon heat shock. However, addition of the 3' UTR of hsp 70 genes to a reporter gene, driven by different promoters, maintained a high level expression of the mRNA during heat shock. These results suggested that the 3' UTR of the hsp 70 genes is primarily responsible for the maintenance of mRNA level during heat shock, while mRNA containing the 3' UTR from many other genes may be rapidly degraded by heat shock induced processes.

Differentially expressed genes in the Trypanosoma brucei life cycle identified by RNA fingerprinting

RNA fingerprinting by arbitrarily primed polymerase chain reaction (RAP-PCR) was used to identify genes that were differentially expressed during the life cycle of Trypanosoma brucei, as well as in response to heat shock. The standard RAP-PCR protocol was varied in two ways. First, the PCR reactions sometimes included a primer derived from the 5' mini-exon sequence, to ensure that most of the products contained the 5' end of mRNAs. Second, differentially amplified products were reamplified, isolated on single strand conformation polymorphism (SSCP) gels, cloned, and sequenced. Clones representing 32 different expressed sequence tags (ESTs) were obtained. Twenty-four ESTs were confirmed as differentially expressed by RT-PCR between different stages of the parasite cycle, or in response to temperature elevation. Nine clones had significant similarities to sequences already in the database. These transcripts included genes encoding cell surface proteins, metabolic enzymes, and heat shock proteins, either from trypanosomes or other organisms. Of particular interest, ESAG1 was shown to be heat-inducible in the procyclic stage. Most of the transcripts were unrelated to any other sequences in the database, and were deposited as new ESTs. The identification of stage-specific and heat shock-regulated transcripts will complement the growing T. brucei database. In addition, this experimental approach allows previous entries in the sequence database to be annotated with regulatory information.

Leishmania donovani heat shock protein 100. Characterization and function in amastigote stage differentiation

We report the cloning and molecular analysis of the Leishmania donovani clpB gene. The protein-coding region is highly conserved compared with its L. major homologue, while 5'- and 3'-flanking DNA sequences display considerable divergence. The encoded mRNA has an unusually long 5'-leader sequence typical for RNAs, which are translated preferentially under heat stress. The gene product, a 100-kDa heat shock protein, Hsp100, becomes abundant only during sustained heat stress, but not under common chemical stresses. Hsp100 associates into trimeric complexes and is found mostly in a cytoplasmic, possibly membrane-associated, localization as determined by immune electron microscopy. Hsp100 shows immediate early expression kinetics during axenic amastigote development. In its absence, expression of at least one amastigote stage-specific protein family is impaired.