Transient adenylate cyclase activation accompanies differentiation of Trypanosoma brucei from bloodstream to procyclic forms

Right place, right time: Environmental sensing and signal transduction directs cellular differentiation and motility in Trypanosoma brucei

Trypanosoma brucei and other African trypanosomes are vector-borne parasites that cause substantial human suffering across sub-Saharan Africa. The T. brucei life cycle is punctuated by numerous developmental stages, each occurring in a specific environmental niche and characterized by a unique morphology, metabolism, surface protein coat, and gene expression profile. The environmental cues and signaling pathways that drive transitions between these stages remain incompletely understood. Recent studies have started to fill this gap in knowledge. Likewise, several new studies have expanded our understanding of parasite movement through specific tissues and the parasite's ability to alter movement in response to external cues. Life cycle stage differentiation and motility are intimately integrated phenomena, as parasites must be at the right place (i.e., within a specific environmental milieu) at the right time (i.e., when they are appropriately staged and preadapted for perceiving and responding to signals) in order to complete their life cycle. In this review, we highlight some of the recent work that has transformed our understanding of signaling events that control parasite differentiation and motility. Increased knowledge of T. brucei environmental sensing and signal transduction advances our understanding of parasite biology and may direct prospective chemotherapeutic and transmission blockade strategies that are critical to eradication efforts.

Developmental regulation of edited CYb and COIII mitochondrial mRNAs is achieved by distinct mechanisms in Trypanosoma brucei

Trypanosoma brucei is a parasitic protozoan that undergoes a complex life cycle involving insect and mammalian hosts that present dramatically different nutritional environments. Mitochondrial metabolism and gene expression are highly regulated to accommodate these environmental changes, including regulation of mRNAs that require extensive uridine insertion/deletion (U-indel) editing for their maturation. Here, we use high throughput sequencing and a method for promoting life cycle changes in vitro to assess the mechanisms and timing of developmentally regulated edited mRNA expression. We show that edited CYb mRNA is downregulated in mammalian bloodstream forms (BSF) at the level of editing initiation and/or edited mRNA stability. In contrast, edited COIII mRNAs are depleted in BSF by inhibition of editing progression. We identify cell line-specific differences in the mechanisms abrogating COIII mRNA editing, including the possible utilization of terminator gRNAs that preclude the 3' to 5' progression of editing. By examining the developmental timing of altered mitochondrial mRNA levels, we also reveal transcript-specific developmental checkpoints in epimastigote (EMF), metacyclic (MCF), and BSF. These studies represent the first analysis of the mechanisms governing edited mRNA levels during T. brucei development and the first to interrogate U-indel editing in EMF and MCF life cycle stages.

Culturing and Transfection of Pleomorphic Trypanosoma brucei

Cultivation of pleomorphic Trypanosoma brucei strains was introduced in 1996 when matrix dependence of growth of natural isolates was recognized. Semisolid agarose or liquid methylcellulose are currently used and here we provide optimized protocols for these culture methods and for transfection of pleomorphic strains. Although more laborious than standard liquid culture, culture of native pleomorphic strains is important for a number of research questions including differentiation, virulence, tissue tropism, and regulated metabolism. Some subclones of pleomorphic strains have acquired matrix independence upon passage in culture but maintained a pleomorphic phenotype. It appears that matrix dependence and pleomorphism are not tightly linked traits, yet phenotypes have to be verified before choosing one of these subclones for given experiments. Based on direct comparisons, we give recommendations for pleomorphic strain selection and culture conditions that guarantee truly pleomorphic and differentiation competent Trypanosoma brucei.

Signal Transduction Pathways as Therapeutic Target for Chagas Disease

Trypanosomatids are a group of flagellated unicellular eukaryotes, causing serious human diseases including Chagas disease (Trypanosoma cruzi), sleeping sickness (Trypanosoma brucei spp.) and Leishmaniasis (Leishmania spp.). The second messenger cAMP is involved in numerous and fundamental processes in these parasites including differentiation between stages, proliferation, osmoregulation, oxidative stress and quorum sensing. Interestingly, its signaling pathway is quite different from that of mammals, including structurally different adenylyl cyclases, the shortage of orthologous effector proteins and the absence of G-protein-coupled-receptors, among others. These characteristics make the proteins involved in these transduction pathways good candidates for therapeutic targets. However, the identification of new unknown druggable targets involves extensive research time and is economically very expensive, making difficult the transition from basic research to the clinical phase. Trypanosomatid PDEs have characteristic binding pockets that allow for a differential inhibition from their human orthologs. Modification in the approved drugs for human to convert them into trypanocidal treatments could lead to more effective therapies, shorter lab time and lower costs. In view of the fact that kinetoplastid PDEs are highly conserved with their mammalian counterparts, and since there are already numerous drugs on the market against human PDEs, the drug repositioning approach is highly promising. The development of new technologies, higher government and industrial involvement and more scientists committed to basic investigation, are the key to ultimately find an effective treatment and cure for the neglected tropical diseases.

Flagellar cAMP signaling controls trypanosome progression through host tissues

The unicellular parasite Trypanosoma brucei is transmitted between mammals by tsetse flies. Following the discovery that flagellar phosphodiesterase PDEB1 is required for trypanosomes to move in response to signals in vitro (social motility), we investigated its role in tsetse flies. Here we show that PDEB1 knockout parasites exhibit subtle changes in movement, reminiscent of bacterial chemotaxis mutants. Infecting flies with the knockout, followed by live confocal microscopy of fluorescent parasites within dual-labelled insect tissues, shows that PDEB1 is important for traversal of the peritrophic matrix, which separates the midgut lumen from the ectoperitrophic space. Without PDEB1, parasites are trapped in the lumen and cannot progress through the cycle. This demonstrates that the peritrophic matrix is a barrier that must be actively overcome and that the parasite’s flagellar cAMP signaling pathway facilitates this. Migration may depend on perception of chemotactic cues, which could stem from co-infecting parasites and/or the insect host.

Trypanosoma brucei probably relies on chemotactic signals for movement through tsetse fly tissues, but the molecular basis is unknown. Here, the authors show that flagellar cAMP signaling is required for traversal of the peritrophic matrix and that, without it, parasites are trapped in the midgut lumen.

Adenylate Cyclases of Trypanosoma brucei, Environmental Sensors and Controllers of Host Innate Immune Response

Trypanosoma brucei, etiological agent of Sleeping Sickness in Africa, is the prototype of African trypanosomes, protozoan extracellular flagellate parasites transmitted by saliva (Salivaria). In these parasites the molecular controls of the cell cycle and environmental sensing are elaborate and concentrated at the flagellum. Genomic analyses suggest that these parasites appear to differ considerably from the host in signaling mechanisms, with the exception of receptor-type adenylate cyclases (AC) that are topologically similar to receptor-type guanylate cyclase (GC) of higher eukaryotes but control a new class of cAMP targets of unknown function, the cAMP response proteins (CARPs), rather than the classical protein kinase A cAMP effector (PKA). T. brucei possesses a large polymorphic family of ACs, mainly associated with the flagellar membrane, and these are involved in inhibition of the innate immune response of the host prior to the massive release of immunomodulatory factors at the first peak of parasitemia. Recent evidence suggests that in T. brucei several insect-specific AC isoforms are involved in social motility, whereas only a few AC isoforms are involved in cytokinesis control of bloodstream forms, attesting that a complex signaling pathway is required for environmental sensing. In this review, after a general update on cAMP signaling pathway and the multiple roles of cAMP, I summarize the existing knowledge of the mechanisms by which pathogenic microorganisms modulate cAMP levels to escape immune defense.

Transcriptional Regulation of Telomeric Expression Sites and Antigenic Variation in Trypanosomes

INTRODUCTION

Trypanosoma brucei uses antigenic variation to evade the host antibody clearance by periodically changing its Variant Surface Glycoprotein (VSGs) coat. T. brucei encode over 2,500 VSG genes and pseudogenes, however they transcribe only one VSG gene at time from one of the 20 telomeric Expression Sites (ESs). VSGs are transcribed in a monoallelic fashion by RNA polymerase I from an extranucleolar site named ES body. VSG antigenic switching occurs by transcriptional switching between telomeric ESs or by recombination of the VSG gene expressed. VSG expression is developmentally regulated and its transcription is controlled by epigenetic mechanisms and influenced by a telomere position effect.

CONCLUSION

Here, we discuss 1) the molecular basis underlying transcription of telomeric ESs and VSG antigenic switching; 2) the current knowledge of VSG monoallelic expression; 3) the role of inositol phosphate pathway in the regulation of VSG expression and switching; and 4) the developmental regulation of Pol I transcription of procyclin and VSG genes.

The ever unfolding story of cAMP signaling in trypanosomatids: vive la difference!

Kinetoplastids are unicellular, eukaryotic, flagellated protozoans containing the eponymous kinetoplast. Within this order, the family of trypanosomatids are responsible for some of the most serious human diseases, including Chagas disease (Trypanosoma cruzi), sleeping sickness (Trypanosoma brucei spp.), and leishmaniasis (Leishmania spp). Although cAMP is produced during the life cycle stages of these parasites, its signaling pathways are very different from those of mammals. The absence of G-protein-coupled receptors, the presence of structurally different adenylyl cyclases, the paucity of known cAMP effector proteins and the stringent need for regulation of cAMP in the small kinetoplastid cells all suggest a significantly different biochemical pathway and likely cell biology. However, each of the main kinetoplastid parasites express four class 1-type cyclic nucleotide-specific phosphodiesterases (PDEA-D), which have highly similar catalytic domains to that of human PDEs. To date, only TbrPDEB, expressed as two slightly different isoforms TbrPDEB1 and B2, has been found to be essential when ablated. Although the genomes contain reasonably well conserved genes for catalytic and regulatory domains of protein kinase A, these have been shown to have varied structural and functional roles in the different species. Recent discovery of a role of cAMP/AMP metabolism in a quorum-sensing signaling pathway in T. brucei, and the identification of downstream cAMP Response Proteins (CARPs) whose expression levels correlate with sensitivity to PDE inhibitors, suggests a complex signaling cascade. The interplay between the roles of these novel CARPs and the quorum-sensing signaling pathway on cell division and differentiation makes for intriguing cell biology and a new paradigm in cAMP signal transduction, as well as potential targets for trypanosomatid-specific cAMP pathway-based therapeutics.

Cyclic AMP Regulates Social Behavior in African Trypanosomes

The protozoan parasite Trypanosoma brucei engages in surface-induced social behavior, termed social motility, characterized by single cells assembling into multicellular groups that coordinate their movements in response to extracellular signals. Social motility requires sensing and responding to extracellular signals, but the underlying mechanisms are unknown. Here we report that T. brucei social motility depends on cyclic AMP (cAMP) signaling systems in the parasite's flagellum (synonymous with cilium). Pharmacological inhibition of cAMP-specific phosphodiesterase (PDE) completely blocks social motility without impacting the viability or motility of individual cells. Using a fluorescence resonance energy transfer (FRET)-based sensor to monitor cAMP dynamics in live cells, we demonstrate that this block in social motility correlates with an increase in intracellular cAMP levels. RNA interference (RNAi) knockdown of the flagellar PDEB1 phenocopies pharmacological PDE inhibition, demonstrating that PDEB1 is required for social motility. Using parasites expressing distinct fluorescent proteins to monitor individuals in a genetically heterogeneous community, we found that the social motility defect of PDEB1 knockdowns is complemented by wild-type parasites in trans. Therefore, PDEB1 knockdown cells are competent for social motility but appear to lack a necessary factor that can be provided by wild-type cells. The combined data demonstrate that the role of cyclic nucleotides in regulating microbial social behavior extends to African trypanosomes and provide an example of transcomplementation in parasitic protozoa.

IMPORTANCE

In bacteria, studies of cell-cell communication and social behavior have profoundly influenced our understanding of microbial physiology, signaling, and pathogenesis. In contrast, mechanisms underlying social behavior in protozoan parasites are mostly unknown. Here we show that social behavior in the protozoan parasite Trypanosoma brucei is governed by cyclic-AMP signaling systems in the flagellum, with intriguing parallels to signaling systems that control bacterial social behavior. We also generated a T. brucei social behavior mutant and found that the mutant phenotype is complemented by wild-type cells grown in the same culture. Our findings open new avenues for dissecting social behavior and signaling in protozoan parasites and illustrate the capacity of these organisms to influence each other's behavior in mixed communities.

In vitro metacyclogenesis of Trypanosoma cruzi induced by starvation correlates with a transient adenylyl cyclase stimulation as well as with a constitutive upregulation of adenylyl cyclase expression

The Trypanosoma cruzi adenylyl cyclase (AC) multigene family encodes different isoforms (around 15) sharing a variable large N-terminal domain, which is extracellular and receptor-like, followed by a transmembrane helix and a conserved C-terminal catalytic domain. It was proposed that these key enzymes in the cAMP signalling pathway allow the parasite to sense its changing extracellular milieu in order to rapidly adapt to its new environment, which is generally achieved through a differentiation process. One of the critical differentiation events the parasitic protozoan T. cruzi undergoes during its life cycle, known as metacyclogenesis, occurs in the digestive tract of the insect and corresponds to the differentiation from noninfective epimastigotes to infective metacyclic trypomastigote forms. By in vitro monitoring the activity of AC during metacyclogenesis, we showed that both the activity of AC and the intracellular cAMP content follow a similar pattern of transient stimulation in a two-step process, with a first activation peak occurring during the first hours of nutritional stress and a second peak between 6 and 48 h, corresponding to the cellular adhesion. During this differentiation process, a general mechanism of upregulation of AC expression of both mRNA and protein is triggered and in particular for a major subclass of these enzymes that are present in various gene copies commonly associated to the THT gene clusters. Although the scattered genome distribution of these gene copies is rather unusual in trypanosomatids and seems to be a recent acquisition in the evolution of the T. cruzi clade, their encoded product redistributed on the flagellum of the parasite upon differentiation could be important to sense the extracellular milieu.

Insect stage-specific receptor adenylate cyclases are localized to distinct subdomains of the Trypanosoma brucei Flagellar membrane

Increasing evidence indicates that the Trypanosoma brucei flagellum (synonymous with cilium) plays important roles in host-parasite interactions. Several studies have identified virulence factors and signaling proteins in the flagellar membrane of bloodstream-stage T. brucei, but less is known about flagellar membrane proteins in procyclic, insect-stage parasites. Here we report on the identification of several receptor-type flagellar adenylate cyclases (ACs) that are specifically upregulated in procyclic T. brucei parasites. Identification of insect stage-specific ACs is novel, as previously studied ACs were constitutively expressed or confined to bloodstream-stage parasites. We show that procyclic stage-specific ACs are glycosylated, surface-exposed proteins that dimerize and possess catalytic activity. We used gene-specific tags to examine the distribution of individual AC isoforms. All ACs examined localized to the flagellum. Notably, however, while some ACs were distributed along the length of the flagellum, others specifically localized to the flagellum tip. These are the first transmembrane domain proteins to be localized specifically at the flagellum tip in T. brucei, emphasizing that the flagellum membrane is organized into specific subdomains. Deletion analysis reveals that C-terminal sequences are critical for targeting ACs to the flagellum, and sequence comparisons suggest that differential subflagellar localization might be specified by isoform-specific C termini. Our combined results suggest insect stage-specific roles for a subset of flagellar adenylate cyclases and support a microdomain model for flagellar cyclic AMP (cAMP) signaling in T. brucei. In this model, cAMP production is compartmentalized through differential localization of individual ACs, thereby allowing diverse cellular responses to be controlled by a common signaling molecule.

Cytokinesis of Trypanosoma brucei bloodstream forms depends on expression of adenylyl cyclases of the ESAG4 or ESAG4-like subfamily

Antigenic variation of the parasite Trypanosoma brucei operates by monoallelic expression of a variant surface glycoprotein (VSG) from a collection of multiple telomeric expression sites (ESs). Each of these ESs harbours a long polycistronic transcription unit containing several expression site-associated genes (ESAGs). ESAG4 copies encode bloodstream stage-specific adenylyl cyclases (AC) and belong to a larger gene family of around 80 members, the majority of which, termed genes related to ESAG4 (GRESAG4s), are not encoded in ESs and are expressed constitutively in the life cycle. Here we report that ablation of ESAG4 from the active ES did not affect parasite growth, neither in culture nor upon rodent infection, and did not significantly change total AC activity. In contrast, inducible RNAi-mediated knock-down of an AC subfamily that includes ESAG4 and two ESAG4-like GRESAG4 (ESAG4L) genes, decreased total AC activity and induced a lethal phenotype linked to impaired cytokinesis. In the Δesag4 line compensatory upregulation of apparently functionally redundant ESAG4L genes was observed, suggesting that the ESAG4/ESAG4L-subfamily ACs are involved in the control of cell division. How deregulated adenylyl cyclases or cAMP might impair cytokinesis is discussed.

Cyclic-nucleotide signalling in protozoa

Compared with the impressive progress in understanding signal transduction pathways and mechanisms in mammalian systems, advances in protozoan signalling processes, including cyclic nucleotide metabolism, have been very slow. This is in large part connected to the fact that the components of these pathways are very different in the protozoan parasites, as confirmed by the recently completed genome. For instance, kinetoplastids have no equivalents to the mammalian Class I adenylyl cyclases (ACs) in their genomes nor any of the subunits of the associated G-proteins. The cyclases in kinetoplastid parasites contain a single transmembrane domain, a conserved intracellular catalytic domain and a highly variable extracellular domain - consistent with the expression of multiple receptor-activated cyclases - but no receptor ligands, agonists or antagonists have been identified. Apicomplexan AC and guanylyl cyclase (GC) are even more unusual, potentially being bifunctional, harbouring either a putative ion channel (AC) or a P-type ATPase-like domain (GC) alongside the catalytic region. Phosphodiesterases (PDEs) and cyclic-nucleotide-activated protein kinases are essentially conserved in protozoa, although mostly insensitive to inhibitors of the mammalian proteins. Some of the PDEs have now been validated as promising drug targets. In the following manuscript, we will summarize the existing literature on cAMP and cGMP in protozoa: cyclases, PDEs and cyclic-nucleotide-dependent kinases.

A survey of nucleotide cyclases in actinobacteria: unique domain organization and expansion of the class III cyclase family in Mycobacterium tuberculosis

Cyclic nucleotides are well-known second messengers involved in the regulation of important metabolic pathways or virulence factors. There are six different classes of nucleotide cyclases that can accomplish the task of generating cAMP, and four of these are restricted to the prokaryotes. The role of cAMP has been implicated in the virulence and regulation of secondary metabolites in the phylum Actinobacteria, which contains important pathogens, such as Mycobacterium tuberculosis, M. leprae, M. bovis and Corynebacterium, and industrial organisms from the genus Streptomyces. We have analysed the actinobacterial genome sequences found in current databases for the presence of different classes of nucleotide cyclases, and find that only class III cyclases are present in these organisms. Importantly, prominent members such as M. tuberculosis and M. leprae have 17 and 4 class III cyclases, respectively, encoded in their genomes, some of which display interesting domain fusions seen for the first time. In addition, a pseudogene corresponding to a cyclase from M. avium has been identified as the only cyclase pseudogene in M. tuberculosis and M. bovis. The Corynebacterium and Streptomyces genomes encode only a single adenylyl cyclase each, both of which have corresponding orthologues in M. tuberculosis. A clustering of the cyclase domains in Actinobacteria reveals the presence of typical eukaryote-like, fungi-like and other bacteria-like class III cyclase sequences within this phylum, suggesting that these proteins may have significant roles to play in this important group of organisms.

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.

Hydrolysis products of cAMP analogs cause transformation of Trypanosoma brucei from slender to stumpy-like forms

African sleeping sickness is a disease caused by Trypanosoma brucei. T. brucei proliferate rapidly in the mammalian bloodstream as long, slender forms, but at higher population densities they transform into nondividing, short, stumpy forms. This is thought to be a mechanism adopted by T. brucei to establish a stable host-parasite relationship and to allow a transition into the insect stage of its life cycle. Earlier studies have suggested a role for cAMP in mediating this transformation. In this study, using membrane-permeable nucleotide analogs, we show that it is not the cAMP analogs themselves but rather the hydrolyzed products of membrane-permeable cAMP analogs that prevent proliferation of T. brucei. The metabolic products are more potent than the cAMP analogs, and hydrolysis-resistant cAMP analogs are not antiproliferative. We further show that the antiproliferative effect of these membrane-permeable adenosine analogs is caused by transformation into forms resembling short, stumpy bloodstream forms. These data suggest that the slender-to-stumpy transformation of T. brucei may not be mediated directly by cAMP and also raise the possibility of using such adenosine analogs as antitrypanosomal drugs.

Cycles within cycles: the interplay between differentiation and cell division in Trypanosoma brucei

The life cycle o f the African trypanosome is divided between the mammal and the tsetse. Those life cycle stages which traverse between these two hosts appear to be pre-adopted for survival in their new habitat They are also non-dividing. Here, Keith Matthews and Keith Gull discuss how and why trypanosomes might enmesh the control o f their cell cycle with their regulation o f the transition between different life cycle forms.

Cold shock and regulation of surface protein trafficking convey sensitization to inducers of stage differentiation in Trypanosoma brucei

Transmission of a protozoan parasite from a vertebrate to invertebrate host is accompanied by cellular differentiation. The signals from the environment that trigger the process are poorly understood. The model parasite Trypanosoma brucei proliferates in the mammalian bloodstream and in the tsetse fly. On ingestion by the tsetse, the trypanosome undergoes a rapid differentiation that is marked by replacement of the variant surface glycoprotein (VSG) coat with GPI-anchored EP and GPEET procyclins. Here we show that a cold shock of DeltaT > 15 degrees C is sufficient to reversibly induce high-level expression of the insect stage-specific EP gene in the mammalian bloodstream stages of T. brucei. The 3'-UTR of the EP mRNA is necessary and sufficient for the increased expression. During cold shock, EP protein accumulates in the endosomal compartment in the proliferating, slender, bloodstream stage, whereas the EP is present on the plasma membrane in the quiescent, stumpy, bloodstream stage. Thus, there is a novel developmentally regulated cell surface access control mechanism for a GPI-anchored protein. In addition to inducing EP expression, cold shock results in the acquisition of sensitivity to micromolar concentrations of cis-aconitate and citrate by stumpy but not slender bloodstream forms. The cis-aconitate and citrate commit stumpy bloodstream cells to differentiation to the procyclic stage along with rapid initial proliferation. We propose a hierarchical model of three events that regulate differentiation after transmission to the tsetse: sensing the temperature change, surface access of a putative receptor, and sensing of a chemical cue.

Structure, function and evolution of microbial adenylyl and guanylyl cyclases

Cells respond to signals of both environmental and biological origin. Responses are often receptor mediated and result in the synthesis of so-called second messengers that then provide a link between extracellular signals and downstream events, including changes in gene expression. Cyclic nucleotides (cAMP and cGMP) are among the most widely studied of this class of molecule. Research on their function and mode of action has been a paradigm for signal transduction systems and has shaped our understanding of this important area of biology. Cyclic nucleotides have diverse regulatory roles in both unicellular and multicellular organisms, highlighting the utility and success of this system of molecular communication. This review will examine the structural diversity of microbial adenylyl and guanylyl cyclases, the enzymes that synthesize cAMP and cGMP respectively. We will address the relationship of structure to biological function and speculate on the complex origin of these crucial regulatory molecules. A review is timely because the explosion of data from the various genome projects is providing new and exciting insights into protein function and evolution.

Loss of the mono-allelic control of the VSG expression sites during the development of Trypanosoma brucei in the bloodstream

Transcription of the variant surface glycoprotein (VSG) gene of Trypanosoma brucei occurs in a single of multiple polycistronic expression sites (ESs). Analysis of RNA from proliferative long slender (LS) bloodstream forms demonstrated that initiation of transcription occurs in different ESs, but inefficient RNA processing and elongation is linked to RNA polymerase arrest in all except one unit at a time. The pattern of ES transcripts was analysed during the transformation of dividing LS forms into quiescent short stumpy (SS) forms. The results demonstrated that the mono-allelic control allowing preferential RNA production from a given ES stops during this process. Accordingly, the steady-state ES transcripts, particularly the VSG mRNA, were strongly reduced. However, transcripts from the beginning of different ESs were still synthesized, and in vitro run-on transcription analysis indicated that RNA polymerase was still fully associated with the promoter-proximal half of the 'active' ES. Analysis of transcripts from two central tandem genes confirmed the existence of a residual decreasing transcriptional gradient in the 'active' ES of SS forms. Thus, in these forms the RNA polymerase of the ES is inactivated in situ. This inactivation is accompanied by a strong overall reduction of nuclear DNA transcription. Although cAMP is involved in the LS to SS transformation, no direct effect of cAMP was observed on the VSG ES control.

Identification of mitogen-activated protein kinase homologues from Leishmania mexicana

Mitogen-activated protein kinases are key-regulatory elements in the differentiation, proliferation, apoptosis and stress response of eukaryotic cells. Our recent identification of a mitogen-activated protein kinase homologue in Leishmania mexicana which is essential for the proliferation of the amastigote stage of the parasite living in the parasitophorous vacuole of the infected macrophage prompted us to screen the genome of L. mexicana for additional mitogen-activated protein kinase homologues using degenerate oligonucleotide primers in a polymerase chain reaction amplification approach. We cloned and sequenced the genes for eight new mitogen-activated protein kinase homologues which were subsequently shown to be present in one copy per haploid genome. The mRNA levels of the kinases varied significantly in pro- and amastigote life stages of the parasite. We used the structural information of the p38 stress-activated protein kinase, which belongs to the family of mitogen-activated protein kinases, for the alignment of the deduced proteins and the verification of the predicted secondary structure elements. All new mitogen-activated protein kinases reveal the typical 12 subdomain primary structure, the conserved residues characterising serine/threonine protein kinases and the characteristic TXY motif in the phosphorylation lip. Typical features of some of the molecules are amino acid insertions between the subdomains and long carboxy-terminal amino acid extensions carrying putative src-homology 3-binding motifs.

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.

Role of the long slender to short stumpy transition in the life cycle of the african trypanosomes

It is shown using mouse models that the African trypanosomes exert a significant drain upon their host's carbohydrate (energy) resources; and that the higher the parasitemia, the greater the energy demand. It is, therefore, hypothesized that the long slender (LS) to short stumpy (SS) transition evolved, in part, to help control the parasitemia and to increase host survival time. It is also suggested that the SS population is heterogeneous. One part of the population is tsetse infective, while a second older SS population is undergoing apoptotic-like events, which leads to their cell death and their stimulation of the host's immune response. This immune stimulation by the old dying SS forms would eliminate the major LS and SS variant antigen population, and produce the chronic relapsing infection. It is concluded that the SS stages during the apoptosis-like process are acting altruistically. They give their lives to insure the long-term survival of the host, and to insure renewed growth of the minor LS variants and new infective SS forms. This process is predicted to increase the probability for the successful transmission of the trypanosomes to a new host.

Procyclic Trypanosoma brucei do not use Krebs cycle activity for energy generation

The importance of a functional Krebs cycle for energy generation in the procyclic stage of Trypanosoma brucei was investigated under physiological conditions during logarithmic phase growth of a pleomorphic parasite strain. Wild type procyclic cells and mutants with targeted deletion of the gene coding for aconitase were derived by synchronous in vitro differentiation from wild type and mutant (Delta aco::NEO/Delta aco::HYG) bloodstream stage parasites, respectively, where aconitase is not expressed and is dispensable. No differences in intracellular levels of glycolytic and Krebs cycle intermediates were found in procyclic wild type and mutant cells, except for citrate that accumulated up to 90-fold in the mutants, confirming the absence of aconitase activity. Surprisingly, deletion of aconitase did not change differentiation nor the growth rate or the intracellular ATP/ADP ratio in those cells. Metabolic studies using radioactively labeled substrates and NMR analysis demonstrated that glucose and proline were not degraded via the Krebs cycle to CO(2). Instead, glucose was degraded to acetate, succinate, and alanine, whereas proline was degraded to succinate. Importantly, there was absolutely no difference in the metabolic products released by wild type and aconitase knockout parasites, and both were for survival strictly dependent on respiration via the mitochondrial electron transport chain. Hence, although the Krebs cycle enzymes are present, procyclic T. brucei do not use Krebs cycle activity for energy generation, but the mitochondrial respiratory chain is essential for survival and growth. We therefore propose a revised model of the energy metabolism of procyclic T. brucei.

A novel calcium-stimulated adenylyl cyclase from Trypanosoma cruzi, which interacts with the structural flagellar protein paraflagellar rod

Trypanosoma cruzi adenylyl cyclases are encoded by a large polymorphic gene family. Although several genes have been identified in this parasite, little is known about the properties and regulation of these enzymes. Here we report the cloning and characterization of TczAC, a novel member of T. cruzi adenylyl cyclase family. The TczAC gene is expressed in all of the parasite life forms and encodes a 1,313-amino acid protein that can complement a Saccharomyces cerevisiae mutant deficient in adenylyl cyclase activity. The recombinant enzyme expressed in yeasts is constitutively active, has a low affinity for ATP (K(m) = 406 microm), and requires a divalent cation for catalysis. TczAC is inhibited by Zn(2+) and the P-site inhibitor 2'-deoxyadenosine 3'-monophosphate, suggesting some level of conservation in the catalytic mechanism with mammalian adenylyl cyclases. It shows a dose-dependent stimulation by Ca(2+) which can be reversed by high concentrations of phenothiazinic calmodulin inhibitors. However, bovine calmodulin fails to stimulate the enzyme. Using a yeast two-hybrid screen it was found that TczAC interacts through its catalytic domain with the paraflagellar rod protein, a component of the flagellar structure. Furthermore, we demonstrate that TczAC can dimerize through the same domain. These results provide novel evidence of the possible localization and regulation of this protein.

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.

Control and function of the bloodstream variant surface glycoprotein expression sites in Trypanosoma brucei

African trypanosomes escape the host immune response through a periodical change of their surface coat made of one major type of protein, the variant surface glycoprotein. From a repertoire of a thousand variant surface glycoprotein genes available, only one is expressed at a time, and this takes place in a specialised expression site itself selected from a collection of an estimated 20-30 sites. As the specialised expression sites are long polycistronic transcription units, the variant surface glycoprotein is co-transcribed with several other genes termed expression site-associated genes. How do the trypanosomes only use a single specialised expression site at a time? Why are there two dozen specialised expression sites? What are the functions of the other genes of these transcription units? We review the currently available answers to these questions.

Transformation of Leishmania mexicana metacyclic promastigotes to amastigote-like forms mediated by binding of human C-reactive protein

Infective metacyclic promastigote forms of Leishmania mexicana are introduced by the bite of sandfly vectors into their human hosts where they transform into the amastigote form. The kinetics of this process was examined in vitro in response to different combinations of temperature (26 degrees C or 32 degrees C), pH (7.2 or 5.5), and exposure to human serum. Little transformation occurred at 26 degrees C/pH 7.2, intermediate levels at 26 degrees C/pH 5.5 and 32 degrees C/pH 7.2, and the greatest response at 32 degrees C/pH 5.5. Transformation was stimulated by exposure to normal human serum, but was markedly reduced when serum previously incubated at 56 degrees C for 1 h was used (complement heat-inactivated). This stimulatory effect was reproduced by exposure to a single purified component of human serum, C-reactive protein (CRP). Binding of CRP to the whole surface of L. mexicana metacyclic promastigotes, including the flagella, was demonstrated by an indirect fluorescent antibody test. The effect of purified CRP was dose dependent and occurred using normal serum concentrations. The stimulatory effect of whole serum was oblated by CRP depletion and restored by addition of purified CRP. The effects of cAMP analogues indicated that transformation could be mediated via an adenylate cyclase cascade.

cAMP signalling in Trypanosoma brucei

Cyclic AMP was the first second messenger to be identified. After five decades of research, much is currently known about its biological functions and clinical implications. Several components of the cAMP signalling pathways, such as the G-protein coupled receptors and the phosphodiesterases, have become sensitive and specific drug targets for a host of clinical applications. Surprisingly, very little effort has been invested so far into the study of cAMP signalling in parasites, and its significance in host/parasite interaction. Our laboratory has embarked on a study of cAMP signalling in Trypanosoma brucei. A newly identified adenylyl cyclase, GRESAG4.4B, a member of a small family of closely related genes, is being used as a model molecule for investigating the mechanisms which control cyclase activity in the T. brucei cell. On the other hand, a number of genes for different families of cAMP-specific phosphodiesterases have been identified and characterised. One enzyme, TbPDE1, is coded for by a single-copy gene. Knock-outs of this gene display an almost normal phenotype in culture, indicating that TbPDE1 is not an essential enzyme under culture conditions. A second phosphodiesterase which is being studied in detail, TbPDE2A, is clearly different from TbPDE1, and it is coded for by a member of a small gene family containing about six similar, but non-identical genes. TbPDE2A, as TbPDE1, is specific for cAMP. In its N-terminal, it contains a GAF domain which may represent an allosteric cGMP-binding site. The other members of the TbPDE2 family all exhibit strongly conserved catalytic domains, but vary widely in their N-terminal regulatory domains. With regard to downstream signalling by the cAMP generated through the interplay of adenylyl cyclases and phosphodiesterases, we have recently identified a single-copy gene (TbRSU1) which codes for a putative regulatory subunit of the cAMP-regulated protein kinase A. This protein exhibits considerable similarity with its mammalian counterparts. Immunoprecipitation co-precipitates a protein kinase activity with the characteristics of protein kinase A.

The VSG expression sites of Trypanosoma brucei: multipurpose tools for the adaptation of the parasite to mammalian hosts

The variant surface glycoprotein (VSG) genes of Trypanosoma brucei are transcribed in telomeric loci termed VSG expression sites (ESs). Despite permanent initiation of transcription in most if not all of these multiple loci, RNA elongation is abortive except in bloodstream forms where full transcription up to the VSG occurs only in a single ES at a time. The ESs active in bloodstream forms are polycistronic and contain several genes in addition to the VSG, named ES-associated genes (ESAGs). So far 12 ESAGs have been identified, some of which are present only in some ESs. Most of these genes encode surface proteins and this list includes different glycosyl phosphatidyl inositol (GPI)-anchored proteins such as the heterodimeric receptor for the host transferrin (ESAG7/6), integral membrane proteins such as the receptor-like transmembrane adenylyl cyclase (ESAG4) and a surface transporter (ESAG10). An interesting exception is ESAG8, which may encode a cell cycle regulator involved in the differentiation of long slender into short stumpy bloodstream forms. Several ESAGs belong to multigene families including pseudogenes and members transcribed out of the ESs, named genes related to ESAGs (GRESAGs). However, some ESAGs (7, 6 and 8) appear to be restricted to the ESs. Most of these genes can be deleted from the active ES without apparently affecting the phenotype of bloodstream form trypanosomes, probably either due to the expression of ESAGs from 'inactive' ESs (ESAG7/6) or due to the expression of GRESAGs (in particular, GRESAGs4 and GRESAGs1). At least three ESAGs (ESAG7, ESAG6 and SRA) share the evolutionary origin of VSGs. The presence of these latter genes in ESs may confer an increased capacity of the parasite for adaptation to various mammalian hosts, as suggested in the case of ESAG7/6 and proven for SRA, which allows T. brucei to infect humans. Similarly, the existence of a collection of slightly different ESAG4s in the multiple ESs might provide the parasite with adenylyl cyclase isoforms that may regulate growth in response to different environmental conditions. The high transcription rate and high recombination level that prevail in VSG ESs may have favored the generation and/or recruitment in these sites of genes whose hyper-evolution allows adaptation to a larger variety of hosts.

Spontaneous dimerization and leucine-zipper induced activation of the recombinant catalytic domain of a new adenylyl cyclase of Trypanosoma brucei, GRESAG4.4B

In this study, we describe the isolation and characterization of a new adenylyl cyclase from Trypanosoma brucei and its activation by dimerization of the catalytic domain. In agreement with the current nomenclature of trypanosomal adenylyl cyclases, this new gene is termed GRESAG4.4B. The complete ORF of the GRESAG4.4B gene encodes a protein of 1291 amino acids. Its predicted protein structure is consistent with the structure of other trypanosomal cyclases, and with the cyclases of L. donovani. GRESAG 4.4B is constitutively expressed during the life cycle of trypanosomes. GRESAG4.4B is a member of a gene family, which contains at least six members, which are all clustered on chromosome IV. The catalytic domain of GRESAG4.4B is able to dimerize spontaneously. However, these spontaneously formed, stable dimers only show minimal enzymatic activity. The addition of a leucine zipper (LZ) derived from the S. cerevisiae GCN 4 gene to the N-terminus of the catalytic domain of GRESAG4.4B strongly activated its enzymatic activity. The LZ appears to enforce a distinct conformation of the dimer, which leads to an increased enzymatic activity, and thus may mimic the effect of ligand-induced dimerization of adenylyl cyclase in vivo.

A major surface glycoprotein of Trypanosoma brucei is expressed transiently during development and can be regulated post-transcriptionally by glycerol or hypoxia

Differentiation is a means by which unicellular parasites adapt to different environments. In some cases, the developmental program may be modulated by interactions with the host, but the mechanisms are largely unknown. Trypanosoma brucei is transmitted between mammals by tsetse flies. The development of the procyclic form in the tsetse midgut is marked by the synthesis of a new glycoprotein coat, composed of EP and GPEET procyclins, that is important for survival. Here we demonstrate that the composition of the coat changes in response to extracellular signals in vitro and during development in vivo. EP and GPEET are coinduced when differentiation is initiated. Subsequently, EP expression is maintained, whereas GPEET is repressed after 7–9 days. The timepoint at which GPEET is repressed coincides with the appearance of parasites in a new compartment of the fly midgut. In culture, down-regulation of GPEET can be prevented by exogenous glycerol or accelerated by hypoxia. Regulation is post-transcriptional, and is conferred by the GPEET 3′ untranslated region. The same sequence also regulates expression of a reporter gene in the fly. The finding that GPEET is expressed during a defined window during the establishment of infection suggests that it has a specific function in host-parasite interactions rather than a generalized role in shielding underlying membrane molecules.

Characterisation of the loci encoding the glutamic acid and alanine rich protein of Trypanosoma congolense

We have characterised the organisation of genes encoding the glutamate and alanine rich protein (GARP) surface coat of the procyclic and epimastigote stages of Trypanosoma congolense in the tsetse fly. The GARP genes are arranged at two, possibly physically linked, loci, one of which exhibits allelic variation. One locus contains a single GARP gene, whilst both alleles of the other have a large tandem array of polycistronically transcribed GARP genes. Sequence analysis has revealed that there are very few coding differences between different GARP genes. A sequence related to the Trypanosoma brucei expression site associated gene 4 (encoding a transmembrane protein with a cytoplasmic adenylate cyclase domain) has been identified within a region at the downstream flank of one locus. There is no evidence that, within the single trypanosome, GARP genes are as diverse as the procyclin genes that encode a corresponding coat in T. brucei.

Pathways involved in environmental sensing in trypanosomatids

Digenetic parasites, such as those of the order Kinetoplastida, must respond to extracellular and intracellular signals as they adapt to new environments within their different hosts. Evidence for signal transduction has been obtained for Trypanosoma brucei, T. cruzi and Leishmania, as reviewed here by Marilyn Parsons and Larry Ruben. Although the broad picture suggests similarities with the mammalian host, there are large gaps in our understanding of these processes; this probably contributes to a perception of differences. Nonetheless, current evidence suggests that the trypanosomatids might lack certain classes of signalling molecules found in other organisms.

Cyclic AMP signaling in trypanosomatids

Curative interference with signal transduction pathways is a spectacularly successful concept in many domains of modern pharmacology; indeed, the 'wonder drug' Viagra is but a humble inhibitor of a cyclic GMP (cGMP)-specific phosphodiesterase and, thus, interferes with cGMP-signaling in a strategic organ. In fact, about half of the 100 most successful drugs currently on the market act through modulating cellular signal transduction. Despite these encouraging findings, signal transduction pathways as potential drug targets in trypanosomatids have remained largely unexplored. However, what little is known indicates that adenylyl cyclases of trypanosomatids, and probably other enzymes of the cyclic nucleotide signaling pathways, are significantly different from their mammalian counterparts. Here, Christina Naula and Thomas Seebeck summarize what is known about cAMP signal transduction in trypanosomatids.

Slender and stumpy bloodstream forms of Trypanosoma brucei display a differential response to extracellular acidic and proteolytic stress

Natural infections of mammals with African trypanosomes, such as Trypanosoma brucei, are generally pleomorphic, the population consisting of different forms, termed slender and stumpy forms, that vary in number as the parasitaemia develops. We show that the differentiation of slender into stumpy forms is characterized by the acquisition by the parasite of the ability to regulate its internal pH, even in the face of a large, inwardly directed gradient of H+, as well as a tolerance towards external proteolytic stress. These adaptations effectively abbrogate cellular stress-activated signalling pathways involving adenylate cyclase and glycosylphosphoinositol-specific phospholipase-C mediated release of the surface coat. Although in metabolic terms stumpy forms of the parasite are considered to be preadapted to life in the arthropod vector, these data clearly demonstrate that these forms also possess additional cellular adaptations designed to deal with the immediate and potentially harmful changes in the extracellular environment that occur upon ingestion of a bloodmeal by the tsetse fly vector.

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.

Expression of trans-sialidase and 85-kDa glycoprotein genes in Trypanosoma cruzi is differentially regulated at the post-transcriptional level by labile protein factors

To adapt to different environments, Trypanosoma cruzi, the protozoan parasite that causes Chagas' disease, expresses a different set of proteins during development. To begin to understand the mechanism that controls this differential gene expression, we have analyzed the levels of amastin and trans-sialidase mRNAs and the mRNAs encoding members of the 85-kDa glycoprotein gene family, which are differentially expressed in the T. cruzi stages found in the mammalian host. Amastin mRNA is expressed predominantly in intracellular and proliferative amastigotes. trans-Sialidase mRNAs are found mostly in forms undergoing transformation from amastigotes to trypomastigotes inside infected cells, whereas mRNAs encoding the 85-kDa glycoproteins appear only in the infective trypomastigotes released from the cells. The genes coding for these mRNA species are constitutively transcribed in all stages of T. cruzi cells, suggesting that expression is controlled post-transcriptionally during differentiation. Inhibition of transcription by actinomycin D revealed that each mRNA species has a relatively long half-life in stages where it accumulates. In the case of the trans-sialidase and 85-kDa glycoprotein genes, mRNA accumulation was induced by treatment with the protein synthesis inhibitor cycloheximide at the stages that preceded the normal accumulation. Therefore, mRNA stabilization may account for mRNA accumulation. mRNA degradation could be promoted by proteins with high turnover, or stabilization could be promoted by forming a complex with the translational machinery at defined times in development. Identification of the factors that induce mRNA degradation or stabilization is essential to the understanding of control of gene expression in these organisms.

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+).

Changes in expression site control and DNA modification in Trypanosoma brucei during differentiation of the bloodstream form to the procyclic form

We have adapted a system for in vitro differentiation of a monomorphic trypanosome strain to monitor changes in transcription and DNA modification in expression sites during the transition of the bloodstream-form to the procyclic trypanosome. We have used trypanosomes that have a gene for drug resistance integrated in an expression site, just downstream of either an expression site promoter, or a ribosomal promoter replacing the endogenous promoter. During the transition from bloodstream-form to procyclic, the promoters in an active expression site behave as expected on the basis of previous work on these promoters in procyclics, i.e. the ribosomal replacement promoter remains fully active, whereas the expression site promoter is (incompletely) down-regulated. A silent bloodstream-form expression site promoter does not remain tightly silenced, however. There is a transient increase of transcription of the marker gene during the transition from bloodstream-form to procyclic, indicating that the control of silent expression sites differs between the bloodstream-form and the procyclic trypanosome, and that a short time is required to reset the silencing mechanisms. One of the differences between bloodstream-form and procyclic trypanosomes is the presence of the modified base beta-D-glucosyl-hydroxymethyluracil (J) in and around bloodstream-form expression sites. We have studied loss of this DNA modification and find that the change in expression site control from bloodstream-form to procyclic does not require active removal of J. Base J is lost by synthesis of new, unmodified DNA, which happens after the major changes in expression site transcription have occurred.

Cell density triggers slender to stumpy differentiation of Trypanosoma brucei bloodstream forms in culture

Differentiation from replicating slender forms to non-dividing stumpy bloodstream forms of T. brucei limits the parasite population size in the mammalian host in addition to and independently of the antibody response. Using a culture system for pleomorphic strains of T. brucei we show that slender forms very efficiently differentiate to stumpy forms in vitro and that the induction of differentiation is correlated to cell density. Differentiation in the host and in culture were compared using a battery of markers including cell morphology and volume, cell cycle position, the kinetics of the differentiation, expression of NADH dehydrogenase (diaphorase), expression of several differentially regulated transcripts and the kinetics of transformation to replicating procyclic forms after induction with cis-aconitate. By all available criteria, differentiation in culture reflects the natural process in the mammalian host. Time course experiments reveal a very tight temporal correlation between cell cycle arrest of bloodstream forms, appearance of a stumpy differentiation marker and the competence of a bloodstream form population to initiate transformation to procyclic forms in response to cis-aconitate. Our results show that induction of bloodstream form differentiation can occur independently of host-derived cues. We suggest a density sensing mechanism which induces differentiation to the non-dividing stumpy stage and thereby enables the parasite population to autoregulate its proliferation.

The GPI-phospholipase C of Trypanosoma brucei is nonessential but influences parasitemia in mice

In the mammalian host, the cell surface of Trypanosoma brucei is protected by a variant surface glycoprotein that is anchored in the plasma membrane through covalent attachment of the COOH terminus to a glycosylphosphatidylinositol. The trypanosome also contains a phospholipase C (GPI-PLC) that cleaves this anchor and could thus potentially enable the trypanosome to shed the surface coat of VSG. Indeed, release of the surface VSG can be observed within a few minutes on lysis of trypanosomes in vitro. To investigate whether the ability to cleave the membrane anchor of the VSG is an essential function of the enzyme in vivo, a GPI-PLC null mutant trypanosome has been generated by targeted gene deletion. The mutant trypanosomes are fully viable; they can go through an entire life cycle and maintain a persistent infection in mice. Thus the GPI-PLC is not an essential activity and is not necessary for antigenic variation. However, mice infected with the mutant trypanosomes have a reduced parasitemia and survive longer than those infected with control trypanosomes. This phenotype is partially alleviated when the null mutant is modified to express low levels of GPI-PLC.

High molecular mass agarose matrix supports growth of bloodstream forms of pleomorphic Trypanosoma brucei strains in axenic culture

Primary axenic culture of Trypanosoma brucei bloodstream forms almost invariably requires a period of culture adaptation with cell death and clonal selection. This has been particularly difficult and in many cases unsuccessful for natural pleomorphic strains, which are characterized by their ability to differentiate from replicating long slender bloodstream forms into short stumpy forms. Here we show that a representative set of pleomorphic T. brucei strains can be cultured in vitro on agarose plates without any previous adaptation period and selection. The slender morphology was retained and the growth rate was identical to the growth rate in vivo. Long term in vitro culture for 3 months with this method did not affect the ability of the AnTat 1.1 strain to give rise to pleomorphic infections in mice. Stumpy populations emanating from these infections transformed rapidly and synchronously into dividing procyclic forms when triggered with cis-aconitate and a temperature shift. The growth supporting activity of agarose plates could be traced to high molecular mass polymeric agarose; beta-agarase digestion destroyed the activity. Membrane chamber experiments show that direct physical contact of trypanosomes with the agarose matrix is essential. In the absence of high molecular mass agarose, the cell division process is grossly impaired. We suggest that agarose mimics an interaction of trypanosomes with the host's extracellular matrix. Applications of the culture method are discussed.