Commitment to differentiation and cell cycle re-entry are coincident but separable events in the transformation of African trypanosomes from their bloodstream to their insect form

The Cytological Events and Molecular Control of Life Cycle Development of Trypanosoma brucei in the Mammalian Bloodstream

African trypanosomes cause devastating disease in sub-Saharan Africa in humans and livestock. The parasite lives extracellularly within the bloodstream of mammalian hosts and is transmitted by blood-feeding tsetse flies. In the blood, trypanosomes exhibit two developmental forms: the slender form and the stumpy form. The slender form proliferates in the bloodstream, establishes the parasite numbers and avoids host immunity through antigenic variation. The stumpy form, in contrast, is non-proliferative and is adapted for transmission. Here, we overview the features of slender and stumpy form parasites in terms of their cytological and molecular characteristics and discuss how these contribute to their distinct biological functions. Thereafter, we describe the technical developments that have enabled recent discoveries that uncover how the slender to stumpy transition is enacted in molecular terms. Finally, we highlight new understanding of how control of the balance between slender and stumpy form parasites interfaces with other components of the infection dynamic of trypanosomes in their mammalian hosts. This interplay between the host environment and the parasite’s developmental biology may expose new vulnerabilities to therapeutic attack or reveal where drug control may be thwarted by the biological complexity of the parasite’s lifestyle.

A new approach to chemotherapy: drug-induced differentiation kills African trypanosomes

Human African trypanosomiasis (sleeping sickness) is a neglected tropical disease caused by Trypanosoma brucei spp. The parasites are transmitted by tsetse flies and adapt to their different hosts and environments by undergoing a series of developmental changes. During differentiation, the trypanosome alters its protein coat. Bloodstream form trypanosomes in humans have a coat of variant surface glycoprotein (VSG) that shields them from the immune system. The procyclic form, the first life-cycle stage to develop in the tsetse fly, replaces the VSG coat by procyclins; these proteins do not protect the parasite from lysis by serum components. Our study exploits the parasite-specific process of differentiation from bloodstream to procyclic forms to screen for potential drug candidates. Using transgenic trypanosomes with a reporter gene in a procyclin locus, we established a whole-cell assay for differentiation in a medium-throughput format. We screened 7,495 drug-like compounds and identified 28 hits that induced expression of the reporter and loss of VSG at concentrations in the low micromolar range. Small molecules that induce differentiation to procyclic forms could facilitate studies on the regulation of differentiation as well as serving as scaffolds for medicinal chemistry for new treatments for sleeping sickness.

Molecular control of irreversible bistability during trypanosome developmental commitment

Phosphoproteomic and functional analysis of the developmental progression of Trypanosomes demonstrates that this transition shows bistability, with commitment to differentiation requiring new protein synthesis, and that the protein kinase NRK is a key regulator.

The life cycle of Trypanosoma brucei involves developmental transitions that allow survival, proliferation, and transmission of these parasites. One of these, the differentiation of growth-arrested stumpy forms in the mammalian blood into insect-stage procyclic forms, can be induced synchronously in vitro with cis-aconitate. Here, we show that this transition is an irreversible bistable switch, and we map the point of commitment to differentiation after exposure to cis-aconitate. This irreversibility implies that positive feedback mechanisms operate to allow commitment (i.e., the establishment of “memory” of exposure to the differentiation signal). Using the reversible translational inhibitor cycloheximide, we show that this signal memory requires new protein synthesis. We further performed stable isotope labeling by amino acids in cell culture to analyze synchronized parasite populations, establishing the protein and phosphorylation profile of parasites pre- and postcommitment, thereby defining the “commitment proteome.” Functional interrogation of this data set identified Nek-related kinase as the first-discovered protein kinase controlling the initiation of differentiation to procyclic forms.

Third target of rapamycin complex negatively regulates development of quiescence in Trypanosoma brucei

African trypanosomes are protozoan parasites transmitted by a tsetse fly vector to a mammalian host. The life cycle includes highly proliferative forms and quiescent forms, the latter being adapted to host transmission. The signaling pathways controlling the developmental switch between the two forms remain unknown. Trypanosoma brucei contains two target of rapamycin (TOR) kinases, TbTOR1 and TbTOR2, and two TOR complexes, TbTORC1 and TbTORC2. Surprisingly, two additional TOR kinases are encoded in the T. brucei genome. We report that TbTOR4 associates with an Armadillo domain-containing protein (TbArmtor), a major vault protein, and LST8 to form a unique TOR complex, TbTORC4. Depletion of TbTOR4 caused irreversible differentiation of the parasite into the quiescent form. AMP and hydrolysable analogs of cAMP inhibited TbTOR4 expression and induced the stumpy quiescent form. Our results reveal unexpected complexity in TOR signaling and show that TbTORC4 negatively regulates differentiation of the proliferative form into the quiescent form.

The life cycle of Trypanosoma (Nannomonas) congolense in the tsetse fly

Background

The tsetse-transmitted African trypanosomes cause diseases of importance to the health of both humans and livestock. The life cycles of these trypanosomes in the fly were described in the last century, but comparatively few details are available for Trypanosoma (Nannomonas) congolense, despite the fact that it is probably the most prevalent and widespread pathogenic species for livestock in tropical Africa. When the fly takes up bloodstream form trypanosomes, the initial establishment of midgut infection and invasion of the proventriculus is much the same in T. congolense and T. brucei. However, the developmental pathways subsequently diverge, with production of infective metacyclics in the proboscis for T. congolense and in the salivary glands for T. brucei. Whereas events during migration from the proventriculus are understood for T. brucei, knowledge of the corresponding developmental pathway in T. congolense is rudimentary. The recent publication of the genome sequence makes it timely to re-investigate the life cycle of T. congolense.

Methods

Experimental tsetse flies were fed an initial bloodmeal containing T. congolense strain 1/148 and dissected 2 to 78 days later. Trypanosomes recovered from the midgut, proventriculus, proboscis and cibarium were fixed and stained for digital image analysis. Trypanosomes contained in spit samples from individually caged flies were analysed similarly. Mensural data from individual trypanosomes were subjected to principal components analysis.

Results

Flies were more susceptible to infection with T. congolense than T. brucei; a high proportion of flies infected with T. congolense established a midgut and subsequent proboscis infection, whereas many T. brucei infections were lost in the migration from foregut to salivary glands. In T. congolense, trypomastigotes ceased division in the proventriculus and became uniform in size. The trypanosomes retained trypomastigote morphology during migration via the foregut to the mouthparts and we confirmed that the trypomastigote-epimastigote transition occurred in the proboscis. We found no equivalent to the asymmetric division stage in T. brucei that mediates transition of proventricular trypomastigotes to epimastigotes. In T. congolense extremely long epimastigotes with remarkably elongated posterior ends were observed in both the proboscis and cibarium; no difference was found in the developmental stages in these two organs. Dividing trypomastigotes and epimastigotes were recovered from the proboscis, some of which were in transition from trypomastigote to epimastigote and vice versa. It remains uncertain whether these morphological transitions are mediated by cell division, since we also found non-dividing cells with a variously positioned, juxta-nuclear kinetoplast.

Conclusions

We have presented a detailed description of the life cycle of T. congolense in its tsetse fly vector. During development in the fly T. congolense shares a common migratory pathway with its close relative T. brucei, culminating in the production of small metacyclic trypanosomes that can be inoculated with the saliva. Despite this outward similarity in life cycle, the transitional developmental stages in the foregut and mouthparts are remarkably different in the two trypanosome species.

Trypanosomal immune evasion, chronicity and transmission: an elegant balancing act

During their life cycle, trypanosomes must overcome conflicting demands to ensure their survival and transmission. First, they must evade immunity without overwhelming the host. Second, they must generate and maintain transmission stages at sufficient levels to allow passage into their tsetse vector. Finally, they must rapidly commit to onward development when they enter the tsetse fly. On the basis of recent quantification and modelling of Trypanosoma brucei infection dynamics, we propose that the interplay between immune evasion and development achieves both infection chronicity and transmissibility. Moreover, we suggest that a novel form of bistable regulation ensures developmental commitment on entry into the tsetse fly midgut.

Detailed interrogation of trypanosome cell biology via differential organelle staining and automated image analysis

BACKGROUND

Many trypanosomatid protozoa are important human or animal pathogens. The well defined morphology and precisely choreographed division of trypanosomatid cells makes morphological analysis a powerful tool for analyzing the effect of mutations, chemical insults and changes between lifecycle stages. High-throughput image analysis of micrographs has the potential to accelerate collection of quantitative morphological data. Trypanosomatid cells have two large DNA-containing organelles, the kinetoplast (mitochondrial DNA) and nucleus, which provide useful markers for morphometric analysis; however they need to be accurately identified and often lie in close proximity. This presents a technical challenge. Accurate identification and quantitation of the DNA content of these organelles is a central requirement of any automated analysis method.

RESULTS

We have developed a technique based on double staining of the DNA with a minor groove binding (4'', 6-diamidino-2-phenylindole (DAPI)) and a base pair intercalating (propidium iodide (PI) or SYBR green) fluorescent stain and color deconvolution. This allows the identification of kinetoplast and nuclear DNA in the micrograph based on whether the organelle has DNA with a more A-T or G-C rich composition. Following unambiguous identification of the kinetoplasts and nuclei the resulting images are amenable to quantitative automated analysis of kinetoplast and nucleus number and DNA content. On this foundation we have developed a demonstrative analysis tool capable of measuring kinetoplast and nucleus DNA content, size and position and cell body shape, length and width automatically.

CONCLUSIONS

Our approach to DNA staining and automated quantitative analysis of trypanosomatid morphology accelerated analysis of trypanosomatid protozoa. We have validated this approach using Leishmania mexicana, Crithidia fasciculata and wild-type and mutant Trypanosoma brucei. Automated analysis of T. brucei morphology was of comparable quality to manual analysis while being faster and less susceptible to experimentalist bias. The complete data set from each cell and all analysis parameters used can be recorded ensuring repeatability and allowing complete data archiving and reanalysis.

Assembly of the flagellum and its role in cell morphogenesis in Trypanosoma brucei

Eukaryotic flagella are microtubule-based structures required for a variety of functions including cell motility and sensory perception. Most eukaryotic flagella grow out from a cell into the surrounding medium, but when the flagellum of the protozoan parasite Trypanosoma brucei exits the cell via the flagellar pocket, it is attached along the length of the cell body by a cytoskeletal structure called the flagellum attachment zone (FAZ). The exact reasons for flagellum attachment have remained elusive, but evidence is emerging that the attached flagellum plays a major role in cell morphogenesis in this organism. In this review we discuss evidence published in the past four years that is unravelling the role of the flagellum in organelle segregation, inheritance of cell shape and cytokinesis.

Genome-wide expression profiling of in vivo-derived bloodstream parasite stages and dynamic analysis of mRNA alterations during synchronous differentiation in Trypanosoma brucei

Background

Trypanosomes undergo extensive developmental changes during their complex life cycle. Crucial among these is the transition between slender and stumpy bloodstream forms and, thereafter, the differentiation from stumpy to tsetse-midgut procyclic forms. These developmental events are highly regulated, temporally reproducible and accompanied by expression changes mediated almost exclusively at the post-transcriptional level.

Results

In this study we have examined, by whole-genome microarray analysis, the mRNA abundance of genes in slender and stumpy forms of T.brucei AnTat1.1 cells, and also during their synchronous differentiation to procyclic forms. In total, five biological replicates representing the differentiation of matched parasite populations derived from five individual mouse infections were assayed, with RNAs being derived at key biological time points during the time course of their synchronous differentiation to procyclic forms. Importantly, the biological context of these mRNA profiles was established by assaying the coincident cellular events in each population (surface antigen exchange, morphological restructuring, cell cycle re-entry), thereby linking the observed gene expression changes to the well-established framework of trypanosome differentiation.

Conclusion

Using stringent statistical analysis and validation of the derived profiles against experimentally-predicted gene expression and phenotypic changes, we have established the profile of regulated gene expression during these important life-cycle transitions. The highly synchronous nature of differentiation between stumpy and procyclic forms also means that these studies of mRNA profiles are directly relevant to the changes in mRNA abundance within individual cells during this well-characterised developmental transition.

Troglitazone induces differentiation in Trypanosoma brucei

Trypanosoma brucei, a protozoan parasite causing sleeping sickness, is transmitted by the tsetse fly and undergoes a complex lifecycle including several defined stages within the insect vector and its mammalian host. In the latter, differentiation from the long slender to the short stumpy form is induced by a yet unknown factor of trypanosomal origin. Here we describe that some thiazolidinediones are also able to induce differentiation. In higher eukaryotes, thiazolidinediones are involved in metabolism and differentiation processes mainly by binding to the intracellular receptor peroxisome proliferator activated receptor gamma. Our studies focus on the effects of troglitazone on bloodstream form trypanosomes. Differentiation was monitored using mitochondrial markers (membrane potential, succinate dehydrogenase activity, inhibition of oxygen uptake by KCN, amount of cytochrome transcripts), morphological changes (Transmission EM and light microscopy), and transformation experiments (loss of the Variant Surface Glycoprotein coat and increase of dihydroliponamide dehydrogenase activity). To further investigate the mechanisms responsible for these changes, microarray analyses were performed, showing an upregulation of expression site associated gene 8 (ESAG8), a potential differentiation regulator.

Protein tyrosine phosphatase TbPTP1: A molecular switch controlling life cycle differentiation in trypanosomes

Differentiation in African trypanosomes (Trypanosoma brucei) entails passage between a mammalian host, where parasites exist as a proliferative slender form or a G0-arrested stumpy form, and the tsetse fly. Stumpy forms arise at the peak of each parasitaemia and are committed to differentiation to procyclic forms that inhabit the tsetse midgut. We have identified a protein tyrosine phosphatase (TbPTP1) that inhibits trypanosome differentiation. Consistent with a tyrosine phosphatase, recombinant TbPTP1 exhibits the anticipated substrate and inhibitor profile, and its activity is impaired by reversible oxidation. TbPTP1 inactivation in monomorphic bloodstream trypanosomes by RNA interference or pharmacological inhibition triggers spontaneous differentiation to procyclic forms in a subset of committed cells. Consistent with this observation, homogeneous populations of stumpy forms synchronously differentiate to procyclic forms when tyrosine phosphatase activity is inhibited. Our data invoke a new model for trypanosome development in which differentiation to procyclic forms is prevented in the bloodstream by tyrosine dephosphorylation. It may be possible to use PTP1B inhibitors to block trypanosomatid transmission.

Energy metabolism and its compartmentation in Trypanosoma brucei

African trypanosomes are parasitic protozoa of the order of Kinetoplastida, which cause sleeping sickness and nagana. Trypanosomes are not only of scientific interest because of their clinical importance, but also because these protozoa contain several very unusual biological features, such as their special energy metabolism. The energy metabolism of Trypanosoma brucei differs significantly from that of its host, not only because it comprises distinct enzymes and metabolic pathways, but also because some of the glycolytic enzymes are localized in organelles called glycosomes. Furthermore, the energy metabolism changes drastically during the complex life cycle of this parasite. This review will focus on the recent advances made in understanding the process of ATP production in T. brucei during its life cycle and the consequences of the special subcellular compartmentation.

The extraordinary mitochondrion and unusual citric acid cycle in Trypanosoma brucei

African trypanosomes are parasitic protozoa that cause sleeping sickness and nagana. Trypanosomes are not only of scientific interest because of their clinical importance, but also because these protozoa contain several very unusual biological features, such as their specially adapted mitochondrion and the compartmentalization of glycolytic enzymes in glycosomes. The energy metabolism of Trypanosoma brucei differs significantly from that of their hosts and changes drastically during the life cycle. Despite the presence of all citric acid cycle enzymes in procyclic insect-stage T. brucei, citric acid cycle activity is not used for energy generation. Recent investigations on the influence of substrate availability on the type of energy metabolism showed that absence of glycolytic substrates did not induce a shift from a fermentative metabolism to complete oxidation of substrates. Apparently, insect-stage T. brucei use parts of the citric acid cycle for other purposes than for complete degradation of mitochondrial substrates. Parts of the cycle are suggested to be used for (i) transport of acetyl-CoA units from the mitochondrion to the cytosol for the biosynthesis of fatty acids, (ii) degradation of proline and glutamate to succinate, (iii) generation of malate, which can then be used for gluconeogenesis. Therefore the citric acid cycle in trypanosomes does not function as a cycle.

Disruption of the developmental programme of Trypanosoma brucei by genetic ablation of TbZFP1, a differentiation-enriched CCCH protein

The regulation of differentiation is particularly important in microbial eukaryotes that inhabit multiple environments. The parasite Trypanosoma brucei is an extreme example of this, requiring exquisite gene regulation during transmission from mammals to the tsetse fly vector. Unusually, trypanosomes rely almost exclusively on post-transcriptional mechanisms for regulated gene expression. Hence, RNA binding proteins are potentially of great significance in controlling stage-regulated processes. We have previously identified TbZFP1 as a trypanosome molecule transiently enriched during differentiation to tsetse midgut procyclic forms. This small protein (101 amino acids) contains the unusual CCCH zinc finger, an RNA binding motif. Here, we show that genetic ablation of TbZFP1 compromises repositioning of the mitochondrial genome, a specific event in the strictly regulated differentiation programme. Despite this, other events that occur both before and after this remain intact. Significantly, this phenotype correlates with the TbZFP1 expression profile during differentiation. This is the first genetic disruption of a developmental regulator in T. brucei. It demonstrates that programmed events in parasite development can be uncoupled at the molecular level. It also further supports the importance of CCCH proteins in key aspects of trypanosome cell function.

Interactions between tsetse and trypanosomes with implications for the control of trypanosomiasis

Tsetse flies (Diptera: Glossinidae) are vectors of several species of pathogenic trypanosomes in tropical Africa. Human African trypanosomiasis (HAT) is a zoonosis caused by Trypanosoma brucei rhodesiense in East Africa and T. b. gambiense in West and Central Africa. About 100000 new cases are reported per year, with many more probably remaining undetected. Sixty million people living in 36 countries are at risk of infection. Recently, T. b. gambiense trypanosomiasis has emerged as a major public health problem in Central Africa, especially in the Democratic Republic of Congo, Angola and southern Sudan where civil war has hampered control efforts. African trypanosomes also cause nagana in livestock. T. vivax and T. congolense are major pathogens of cattle and other ruminants, while T. simiae causes high mortality in domestic pigs; T. brucei affects all livestock, with particularly severe effects in equines and dogs. Central to the control of these diseases is control of the tsetse vector, which should be very effective since trypanosomes rely on this single insect for transmission. However, the area infested by tsetse has increased in the past century. Recent advances in molecular technologies and their application to insects have revolutionized the field of vector biology, and there is hope that such new approaches may form the basis for future tsetse control strategies. This article reviews the known biology of trypanosome development in the fly in the context of the physiology of the digestive system and interactions of the immune defences and symbiotic flora.

Surface coat remodeling during differentiation of Trypanosoma brucei

African trypanosomes (Trypanosoma brucei) are digenetic parasites whose lifecycle alternates between the mammalian bloodstream and the midgut of the tsetse fly vector. In mammals, proliferating long slender parasites transform into non-diving short stumpy forms, which differentiate into procyclic forms when ingested by the tsetse fly. A hallmark of differentiation is the replacement of the bloodstream stage surface coat composed of variant surface glycoprotein (VSG) with a new coat composed of procylin. An undefined endoprotease and endogenous glycosylphosphatidylinositol-specific phospholipase C (GPI-PLC) have been implicated in releasing the old VSG coat. However, GPI hydrolysis has been considered unimportant because (i) GPI-PLC null mutants are fully viable and (ii) cytosolic GPI-PLC is localized away from cell surface VSG. Utilizing an in vitro differentiation assay with pleomorphic strains we have investigated these modes of VSG release. Shedding is initially by GPI hydrolysis, which ultimately accounts for a substantial portion of total release. Surface biotinylation assays indicate that GPI-PLC does gain access to extracellular VSG, suggesting that this mode is primed in the starting short stumpy population. Proteolytic release is up-regulated during differentiation and is stereoselectively inhibited by peptidomimetic collagenase inhibitors, implicating a zinc metalloprotease. This protease may be related to TbMSP-B, a trypanosomal homologue of Leishmania major surface protease (MSP) described in the accompanying paper (LaCount, D. J., Gruszynski, A. E., Grandgenett, P. M., Bangs, J. D., and Donelson, J. E. (2003) J. Biol. Chem. 278, 24658-24664). Overall, our results demonstrate that surface coat remodeling during differentiation has multiple mechanisms and that GPI-PLC plays a more significant role in VSG release than previously thought.

A protein kinase specifically associated with proliferative forms of Trypanosoma brucei is functionally related to a yeast kinase involved in the co-ordination of cell shape and division

The life cycle of African trypanosomes is characterized by the alternation of proliferative and quiescent stages but the molecular details of this process remain unknown. Here, we describe a new cytoplasmic protein kinase from Trypanosoma brucei, termed TBPK50, that belongs to a family of protein kinases involved in the regulation of the cell cycle, cell shape and proliferation. TBPK50 is expressed only in proliferative forms but is totally absent in quiescent cells despite the fact that the gene is constitutively transcribed at the same level throughout the life cycle. It is probable that TBPK50 has very specific substrate requirements as it was unable to transphosphorylate a range of classical phosphoacceptor substrates in vitro, although an autophosphorylation activity was readily detectable in the same assays. Complementation studies using a fission yeast mutant demonstrated that TBPK50 is a functional homologue of Orb6, a protein kinase involved in the regulation of cellular morphology and cell cycle progression in yeast. These results link the expression of TBPK50 and the growth status of trypanosomes and support the view that this protein kinase is likely to be involved in the control of life cycle progression and cell division of these parasites.

Characterisation of the growth and differentiation in vivo and in vitro-of bloodstream-form Trypanosoma brucei strain TREU 927

Trypanosoma brucei TREU 927/4 has been chosen as the reference strain targeted for complete sequencing of the genome of the African trypanosome. This line is pleomorphic in mammalian hosts and is fly transmissible; however it is relatively unstable with respect to variable surface glycoprotein (VSG) expression. Therefore, we subjected TREU 927/4 to 27 rapid syringe passages through mice, and derived a cloned line which expressed Glasgow University Trypanozoon antigen type (GUTat) 10.1 with relative stability. This line also retained pleomorphism in the bloodstream, being able to generate homogeneous populations of stumpy forms in mice. Furthermore, these parasites remain able to transform to procyclic forms synchronously in vitro and can complete their life cycle in tsetse flies. The passaged cell line was also adapted to in vitro bloodstream-form culture and transfected with a construct encoding the tetracycline repressor (TETR) protein. The resulting TETR subline no longer expressed the GUTat 10.1 VSG but remained able to generate uniform populations of stumpy form cells in mice immunocompromised with cyclophosphamide. They could also differentiate to procyclic forms synchronously in vitro. The generated lines and analyses of their growth and differentiation will provide a basic resource for the analysis and interpretation of gene function in the T. brucei genome reference strain.

A novel selection regime for differentiation defects demonstrates an essential role for the stumpy form in the life cycle of the African trypanosome

A novel selection scheme has been developed to isolate bloodstream forms of Trypanosoma brucei, which are defective in their ability to differentiate to the procyclic stage. Detailed characterization of one selected cell line (defective in differentiation clone 1 [DiD-1]) has demonstrated that these cells are indistinguishable from the wild-type population in terms of their morphology, cell cycle progression, and biochemical characteristics but are defective in their ability to initiate differentiation to the procyclic form. Although a small proportion of DiD-1 cells remain able to transform, deletion of the genes for glycophosphatidyl inositol-phospholipase C demonstrated that this enzyme was not responsible for this inefficient differentiation. However, the attenuated growth of the Delta-glycophosphatidyl inositol-phospholipase C DiD-1 cells in mice permitted the expression of stumpy characteristics in this previously monomorphic cell line, and concomitantly their ability to differentiate efficiently was restored. Our results indicate that monomorphic cells retain expression of a characteristic of the stumpy form essential for differentiation, and that this is reduced in the defective cells. This approach provides a new route to dissection of the cytological and molecular basis of life cycle progression in the African trypanosome.

Trypanosomes lacking trypanothione reductase are avirulent and show increased sensitivity to oxidative stress

In Kinetoplastida, trypanothione and trypanothione reductase (TRYR) provide an intracellular reducing environment, substituting for the glutathione-glutathione reductase system found in most other organisms. To investigate the physiological role of TRYR in Trypanosoma brucei, we generated cells containing just one trypanothione reductase gene, TRYR, which was under the control of a tetracycline-inducible promoter. This enabled us to regulate TRYR activity in the cells from less than 1% to 400% of wild-type levels by adjusting the concentration of added tetracycline. In normal growth medium (which contains reducing agents), trypanosomes containing less than 10% of wild-type enzyme activity were unable to grow, although the levels of reduced trypanothione and total thiols remained constant. In media lacking reducing agents, hypersensitivity towards hydrogen peroxide (EC50 = 3.5 microM) was observed compared with the wild type (EC50 = 223 microM). The depletion of TRYR had no effect on susceptibility to melarsen oxide. The infectivity and virulence of the parasites in mice was dependent upon tetracycline-regulated TRYR activity: if the trypanosomes were injected into mice in the absence of tetracycline, no infection was detectable; and when tetracycline was withdrawn from previously infected animals, the parasitaemia was suppressed.

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

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

The use of transgenic Trypanosoma brucei to identify compounds inducing the differentiation of bloodstream forms to procyclic forms

The expression of procyclins is the earliest known marker of differentiation of bloodstream forms of Trypanosoma brucei to procyclic forms. We have generated transgenic bloodstream and procyclic forms in which the coding region of one procyclin gene was replaced by E. coli beta-glucuronidase (GUS). GUS activity can be monitored in a simple one-step colour reaction in microtitre plates; this assay is potentially suitable for large-scale screening for compounds that influence differentiation. GUS was stage-specifically expressed in procyclic forms and its synthesis occurred in parallel with that of procyclin when bloodstream forms were triggered to differentiate by the addition of cis-aconitate. GUS could also be induced by brief treatment with the proteases trypsin, pronase or thermolysin, but not with pepsin or thrombin. Interestingly, a combination of one of the active proteases with cis-aconitate resulted in increased GUS activity relative to either trigger alone. In contrast to cis-aconitate, protease treatment resulted in considerable cell death. Experiments with the pleomorphic strain AnTat 1.1 showed that long slender bloodstream forms were rapidly killed by proteases, whereas stumpy forms were largely resistant. Stumpy forms treated with trypsin differentiated synchronously and expressed procyclin with faster kinetics than when they were triggered by cis-aconitate. As predicted by the GUS assay, differentiation was even more rapid when both inducers were used simultaneously, with all cells expressing maximal levels of procyclin within 3 h.

Regulated transcription of the histone H2B genes of Trypanosoma brucei

In Trypanosoma brucei, the genes encoding histone H2B are organized in a cluster of about 10-15 tandemly linked copies per haploid genome. The H2B transcripts are processed by trans-splicing and polyadenylation, and encode a polypeptide of 111 residues with a molecular mass of 12.5 kDa. H2B mRNAs are differentially expressed during the parasite life-cycle and are present at higher levels in dividing procyclic and bloodstream slender forms than in the nondividing bloodstream stumpy forms. Analysis of H2B mRNA levels during the synchronous differentiation from stumpy to procyclics forms revealed that the abundance of these transcripts is regulated through the cell-cycle, reaching maximum levels during S-phase. Addition of hydroxyurea to procyclic forms in culture specifically decreased H2B mRNA levels by about twofold, an effect not linked to its 3' untranslated region. Inhibition of protein synthesis prevented this decrease.

Phosphorylation of a major GPI-anchored surface protein of Trypanosoma brucei during transport to the plasma membrane

The surface coat of procyclic forms of Trypanosoma brucei consists of related, internally repetitive glycoproteins known as EP and GPEET procyclins. Previously we showed that the extracellular domain of GPEET is phosphorylated. We now show that phosphorylation of this glycosylphosphatidylinositol-anchored surface protein can be induced in vitro using a procyclic membrane extract. Using antibodies that recognize either the phosphorylated or unphosphorylated form of GPEET, we analyzed their expression during differentiation of bloodstream forms to procyclic forms. Unphosphorylated GPEET, together with EP, was detected in cell lysates 2–4 hours after initiating differentiation whereas phosphorylated GPEET only appeared after 24 hours. Surface expression of EP and both forms of GPEET occurred after 24–48 hours and correlated with the detection of phosphorylated GPEET on immuno-blots. Electron micrographs showed that unphosphorylated GPEET was predominantly in the flagellar pocket whereas the phosphorylated form was distributed over the cell surface. In contrast, expression of a membrane-bound human placental alkaline phosphatase in procyclic forms caused the accumulation of dephosphorylated GPEET on the cell surface, while the phosphorylated form was restricted to the flagellar pocket. A GPEET-Fc fusion protein, which was retained intracellularly, was not phosphorylated. We propose that unphosphorylated GPEET procyclin is transported to a location close to or at the cell surface, most probably the flagellar pocket, where it becomes phosphorylated. To the best of our knowledge, this study represents the first localization of phosphorylated and unphosphorylated forms of a GPI-anchored protein within a cell.

Developments in the differentiation of Trypanosoma brucei

During the course of their life cycle, African trypanosomes encounter many differing environments and respond to these by dramatic changes in cell shape, metabolism and patterns of gene expression. Many of these life cycle transitions can now be carried out in vitro, allowing their underlying controls to be studied. Here, Keith Matthews presents an overview of recent advances in the understanding of the regulation of these complex differentiation events.

Transformation of monomorphic Trypanosoma brucei bloodstream form trypomastigotes into procyclic forms at 37 degrees C by removing glucose from the culture medium

African trypanosomes have been shown previously to undergo efficient transformation from bloodstream forms to procyclic (insect dwelling) forms in vitro by adding citrate and/or cis-aconitate to the culture medium and lowering incubation temperature to 27 degrees C. In this paper, it is shown that strain 427 monomorphic bloodstream forms of Trypanosoma brucei grown in axenic culture at 37 degrees C can be transformed to procyclic forms by simply replacing the glucose carbon source in the culture medium with glycerol. The removal of glucose from the medium results in the loss of the variant surface glycoprotein, the acquisition of cell surface procyclic acidic repetitive protein, the synthesis of procyclic-specific glycosylphosphatidylinositol precursors and the acquisition of substantial resistance to salicyl hydroxamic acid and glycerol within 72 h. A procyclic-specific cytoskeletal protein, known to be a marker of the late stage of transformation, is fully expressed by 96 h but full trans-sialidase activity appears only after 18-30 days. The transformation process described here is slower and less efficient than that previously described for monomorphic trypanosomes, using citrate and/or cis-aconitate and temperature shift as triggers. However, the separation of the transformation process from these stimuli is significant and the effects of glucose deprivation described here may reflect some of the events that occur in vivo in the tsetse fly midgut, where glucose levels are known to be very low.

Mild acid stress as a differentiation trigger in Trypanosoma brucei

In vitro differentiation of Trypanosoma brucei from the bloodstream to the procyclic form is efficiently induced by the combination of cold shock from 37 to 27 degrees C and the addition of citrate/cis-aconitate (CCA) to the incubation medium. Here it is reported that exposure of pleomorphic bloodstream trypanosomes to mild acidic conditions (pH 5.5 for 2 h at 37 degrees C) not only accelerated the process of morphological transformation from long slender and intermediate to short stumpy bloodstream forms but also allowed their subsequent differentiation into procyclic forms even in the absence of CCA. This process appeared to involve the glycosylphosphatidylinositol-specific phospholipase C (GPI-PLC), since null GPI-PLC mutants (PLC-) appeared to be largely refractory to acid stress-induced differentiation. However, an effective response was restored upon reintegration of the GPI-PLC gene in the genome (PLC+).