Identification of stage-regulated and differentiation-enriched transcripts during transformation of the African trypanosome from its bloodstream to procyclic form

An Alba-domain protein required for proteome remodelling during trypanosome differentiation and host transition

The transition between hosts is a challenge for digenetic parasites as it is unpredictable. For Trypanosoma brucei subspecies, which are disseminated by tsetse flies, adaptation to the new host requires differentiation of stumpy forms picked up from mammals to procyclic forms in the fly midgut. Here we show that the Alba-domain protein Alba3 is not essential for mammalian slender forms, nor is it required for differentiation of slender to stumpy forms in culture or in mice. It is crucial, however, for the development of T. brucei procyclic forms during the host transition. While steady state levels of mRNAs in differentiating cells are barely affected by the loss of Alba3, there are major repercussions for the proteome. Mechanistically, Alba3 aids differentiation by rapidly releasing stumpy forms from translational repression and stimulating polysome formation. In its absence, parasites fail to remodel their proteome appropriately, lack components of the mitochondrial respiratory chain and show reduced infection of tsetse. Interestingly, Alba3 and the closely related Alba4 are functionally redundant in slender forms, but Alba4 cannot compensate for the lack of Alba3 during differentiation from the stumpy to the procyclic form. We postulate that Alba-domain proteins play similar roles in regulating translation in other protozoan parasites, in particular during life-cycle and host transitions.

Trypanosoma brucei is a unicellular eukaryotic parasite that is responsible for African trypanosomiasis. The parasite needs two hosts, mammals and tsetse flies, in order to complete its life cycle. Throughout its developmental cycle, T. brucei encounters diverse environments to which it has to adapt in order to maintain its transmission and infectivity. Successful adaptation to the new environment and transition to different life-cycle stages are the general challenges faced by many digenetic parasites. In this study we show that the Alba-domain protein Alba3 is essential for differentiation of the mammalian stumpy form (transition form) to the procyclic form in the tsetse host. An Alba3 deletion mutant infects mice and shows characteristic waves of parasitaemia, but is severely compromised in its ability to infect tsetse flies. Stumpy forms are translationally repressed, but are poised to resume protein synthesis during differentiation. We show that Alba3 is key to efficient escape from translation repression; in its absence, there is a delay in the formation of polysomes and resumption of protein synthesis. This impacts the formation of procyclic-specific mitochondrial respiratory complex proteins as well as the repression of some bloodstream-specific proteins. This is the first time that a single protein has been shown to have a major influence on translation as an adaptive response to changing hosts. It is also the first time that a mechanism has been established for Alba-domain proteins in parasites.

Messenger RNA levels of the Polo-like kinase gene (PLK) correlate with cytokinesis in the Trypanosoma rangeli cell cycle

BACKGROUND

Trypanosoma rangeli is a protozoan parasite that is non-virulent to the mammalian host and is morphologically and genomically related to Trypanosoma cruzi, whose proliferation within the mammalian host is controversially discussed.

OBJECTIVES

We aimed to investigate the T. rangeli cell cycle in vitro and in vivo by characterizing the timespan of the parasite life cycle and by proposing a molecular marker to assess cytokinesis.

METHODOLOGY

The morphological events and their timing during the cell cycle of T. rangeli epimastigotes were assessed using DNA staining, flagellum labelling and bromodeoxyuridine incorporation. Messenger RNA levels of four genes previously associated with the cell cycle of trypanosomatids (AUK1, PLK, MOB1 and TRACK) were evaluated in the different T. rangeli forms.

FINDINGS

T. rangeli epimastigotes completed the cell cycle in vitro in 20.8 h. PLK emerged as a potential molecular marker for cell division, as its mRNA levels were significantly increased in exponentially growing epimastigotes compared with growth-arrested parasites or in vitro-differentiated trypomastigotes. PLK expression in T. rangeli can be detected near the flagellum protrusion site, reinforcing its role in the cell cycle. Interestingly, T. rangeli bloodstream trypomastigotes exhibited very low mRNA levels of PLK and were almost entirely composed of parasites in G1 phase.

MAIN CONCLUSIONS

Our work is the first to describe the T. rangeli cell cycle in vitro and proposes that PLK mRNA levels could be a useful tool to investigate the T. rangeli ability to proliferate within the mammalian host bloodstream.

Endogenous overexpression of an active phosphorylated form of DNA polymerase β under oxidative stress in Trypanosoma cruzi

Trypanosoma cruzi is exposed during its life to exogenous and endogenous oxidative stress, leading to damage of several macromolecules such as DNA. There are many DNA repair pathways in the nucleus and mitochondria (kinetoplast), where specific protein complexes detect and eliminate damage to DNA. One group of these proteins is the DNA polymerases. In particular, Tc DNA polymerase β participates in kinetoplast DNA replication and repair. However, the mechanisms which control its expression under oxidative stress are still unknown. Here we describe the effect of oxidative stress on the expression and function of Tc DNA polymerase β To this end parasite cells (epimastigotes and trypomastigotes) were exposed to peroxide during short periods of time. Tc DNA polymerase β which was associated physically with kinetoplast DNA, showed increased protein levels in response to peroxide damage in both parasite forms analyzed. Two forms of DNA polymerase β were identified and overexpressed after peroxide treatment. One of them was phosphorylated and active in DNA synthesis after renaturation on polyacrylamide electrophoresis gel. This phosphorylated form showed 3-4-fold increase in both parasite forms. Our findings indicate that these increments in protein levels are not under transcriptional control because the level of Tc DNA polymerase β mRNA is maintained or slightly decreased during the exposure to oxidative stress. We propose a mechanism where a DNA repair pathway activates a cascade leading to the increment of expression and phosphorylation of Tc DNA polymerase β in response to oxidative damage, which is discussed in the context of what is known in other trypanosomes which lack transcriptional control.

Exposure of Trypanosome cruzi to oxidative stress leads to damage of several macromolecules such as DNA. DNA polymerases play a very important role in DNA repair after oxidative damage. One of them is Tc DNA polymerase β. In this work, two form of this DNA polymerase were identified and overexpressed in T. cruzi cells after hydrogen peroxide treatment been one of them a phosphorylated and highly active form. The increment of Tc DNA polymerase β was not correlated with changes in mRNA levels, indicating absence of transcriptional control. We propose a mechanism where hydrogen peroxide treatment activates a pathway leading to expression and phosphorylation of Tc DNA polymerase β in response to oxidative damage.

Transcriptomics and proteomics in human African trypanosomiasis: current status and perspectives

Human African trypanosomiasis, or sleeping sickness, is a neglected vector-borne parasitic disease caused by protozoa of the species Trypanosoma brucei sensu lato. Within this complex species, T. b. gambiense is responsible for the chronic form of sleeping sickness in Western and Central Africa, whereas T. b. rhodesiense causes the acute form of the disease in East Africa. Presently, 1.5 million disability-adjusted life years (DALYs) per year are lost due to sleeping sickness. In addition, on the basis of the mortality, the disease is ranked ninth out of 25 human infectious and parasitic diseases in Africa. Diagnosis is complex and needs the intervention of a specialized skilled staff; treatment is difficult and expensive and has potentially life-threatening side effects. The use of transcriptomic and proteomic technologies, currently in rapid development and increasing in sensitivity and discriminating power, is already generating a large panel of promising results. The objective of these technologies is to significantly increase our knowledge of the molecular mechanisms governing the parasite establishment in its vector, the development cycle of the parasite during the parasite's intra-vector life, its interactions with the fly and the other microbial inhabitants of the gut, and finally human host-trypanosome interactions. Such fundamental investigations are expected to provide opportunities to identify key molecular events that would constitute accurate targets for further development of tools dedicated to field work for early, sensitive, and stage-discriminant diagnosis, epidemiology, new chemotherapy, and potentially vaccine development, all of which will contribute to fighting the disease. The present review highlights the contributions of the transcriptomic and proteomic analyses developed thus far in order to identify potential targets (genes or proteins) and biological pathways that may constitute a critical step in the identification of new targets for the development of new tools for diagnostic and therapeutic purposes.

Transcriptome analysis of differentiating trypanosomes reveals the existence of multiple post-transcriptional regulons

Background

Trypanosome gene expression is regulated almost exclusively at the post-transcriptional level, with mRNA degradation playing a decisive role. When trypanosomes are transferred from the blood of a mammal to the midgut of a Tsetse fly, they transform to procyclic forms: gene expression is reprogrammed, changing the cell surface and switching the mode of energy metabolism. Within the blood, trypanosomes can pre-adapt for Tsetse transmission, becoming growth-arrested stumpy forms. We describe here the transitions in gene expression that occur during differentiation of in-vitro cultured bloodstream forms to procyclic forms.

Results

Some mRNAs showed changes within 30 min of cis-aconitate addition, whereas others responded 12-24 hours later. For the first 12 h after addition of cis-aconitate, cells accumulated at the G1 phase of the cell cycle, and showed decreases in mRNAs required for proliferation, mimicking the changes seen in stumpy forms: many mRNAs needed for ribosomal and flagellar biogenesis showed striking co-regulation. Other mRNAs encoding components of signal transduction pathways and potential regulators were specifically induced only during differentiation. Messenger RNAs encoding proteins required for individual metabolic pathways were often co-regulated.

Conclusion

Trypanosome genes form post-transcriptional regulons in which mRNAs with functions in particular pathways, or encoding components of protein complexes, show almost identical patterns of regulation.

The RACK1 signal anchor protein from Trypanosoma brucei associates with eukaryotic elongation factor 1A: a role for translational control in cytokinesis

RACK1 is a WD-repeat protein that forms signal complexes at appropriate locations in the cell. RACK1 homologues are core components of ribosomes from yeast, plants and mammals. In contrast, a cryo-EM analysis of trypanosome ribosomes failed to detect RACK1, thus eliminating an important translational regulatory mechanism. Here we report that TbRACK1 from Trypanosoma brucei associates with eukaryotic translation elongation factor-1a (eEF1A) as determined by tandem MS of TAP-TbRACK1 affinity eluates, co-sedimentation in a sucrose gradient, and co-precipitation assays. Consistent with these observations, sucrose gradient purified 80S monosomes and translating polysomes each contained TbRACK1. When RNAi was used to deplete cells of TbRACK1, a shift in the polysome profile was observed, while the phosphorylation of a ribosomal protein increased. Under these conditions, cell growth became hypersensitive to the translational inhibitor anisomycin. The kinetoplasts and nuclei were misaligned in the postmitotic cells, resulting in partial cleavage furrow ingression during cytokinesis. Overall, these findings identify eEF1A as a novel TbRACK1 binding partner and establish TbRACK1 as a component of the trypanosome translational apparatus. The synergy between anisomycin and TbRACK1 RNAi suggests that continued translation is required for complete ingression of the cleavage furrow.

cDNA expression library screening and identification of two novel antigens: ubiquitin and receptor for activated C kinase (RACK) homologue, of the fish parasite Trypanosoma carassii

Trypanosoma carassii is a kinetoplastid parasite infecting cyprinid fish with a high prevalence in nature. Antibodies have been shown to play a protective role in the immune response against this parasite in common carp, Cyprinus carpio. To identify immunogenic and putative protective T. carassii antigens we constructed a lambdaTriplEx2 expression library of the parasite and screened this with pooled carp immune serum collected 6 weeks post-infection. Screening of the library not only revealed ribosomal proteins but identified ubiquitin and a homologue of the receptor for activated C kinase (RACK) as immunogenic proteins. Equivalents of all these proteins have been identified as immunogenic in expression library screenings of other Trypanosomatida, suggesting an evolutionary conservation of their immunogenicity. The possibility that ubiquitin and/or the homologue of RACK could represent protective antigens and be targets for the design of novel therapies is discussed.

Post-transcriptional control of nuclear-encoded cytochrome oxidase subunits in Trypanosoma brucei: evidence for genome-wide conservation of life-cycle stage-specific regulatory elements

Trypanosomes represent an excellent model for the post-transcriptional regulation of gene expression because their genome is organized into polycistronic transcription units. However, few signals governing developmental stage-specific expression have been identified, with there being no compelling evidence for widespread conservation of regulatory motifs. As a tool to search for common regulatory sequences we have used the nuclear-encoded components of the cytochrome oxidase (COX) complex of the trypanosome respiratory chain. Components of this complex represent a form of post-transcriptional operon because trypanosome mitochondrial activity is unusual in being developmentally programmed. By genome analysis we identified the genes for seven components of the COX complex. Each mRNA exhibits bloodstream stage-specific instability, which is not mediated by the RNA silencing pathway but which is alleviated by cycloheximide. Reporter assays have identified regulatory regions within the 3′-untranslated regions of three COX mRNAs operating principally at the translational level, but also via mRNA stability. Interrogation of the mapped regions via oligonucleotide frequency scoring provides evidence for genome-wide conservation of regulatory sequences among a large cohort of procyclic-enriched transcripts. Analysis of the co-regulated subunits of a stage-specific enzyme is therefore a novel approach to uncover cryptic regulatory sequences controlling gene expression at the post-transcriptional level.

The procyclin-associated genes of Trypanosoma brucei are not essential for cyclical transmission by tsetse

EP and GPEET procyclins are the major surface glycoproteins of Trypanosoma brucei in the midgut of tsetse flies (Glossina spp.). The procyclin genes are located at the beginning of polycistronic transcription units and are followed by at least one procyclin-associated gene (PAG). The EP/PAG1 locus on one copy of chromosome X begins with the three genes EP1, EP2 and PAG1; the end of this unit has not been characterized previously. The EP/PAG2 locus on the other copy of chromosome X contains the same procyclin genes followed by PAG2 and PAG4. Here we show that the EP/PAG1 locus in AnTat1.1 has to be extended by three more PAGs, which we named PAG5, PAG2* and PAG4. The EP/PAG2 locus most likely evolved from the EP/PAG1 locus by deletion of a fragment from within PAG1 to PAG2*. The procyclin loci on the two copies of chromosome VI are indistinguishable, and contain the genes GPEET, EP3, PAG3 and GRESAG2.1. The mRNA levels of PAG1, PAG2 and PAG3 are transiently increased during differentiation of bloodstream forms to procyclic forms. Unexpectedly, procyclic forms of a PAG knockout clone lacking all eight PAGs in the procyclin loci were transmissible by Glossina morsitans. Furthermore, the deletion mutant could still establish midgut infections when competing with a tagged clone with the full complement of PAGs. Cyclical transmission was also possible when tsetse flies were infected with bloodstream forms of the deletion mutant, demonstrating that the PAGs are not essential for the differentiation of bloodstream to procyclic forms in vivo.

Regulation of surface coat exchange by differentiating African trypanosomes

African trypanosomes (Trypanosoma brucei) have a digenetic lifecycle that alternates between the mammalian bloodstream and the tsetse fly vector. In the bloodstream, replicating long slender parasites transform into non-dividing short stumpy forms. Upon transmission into the fly midgut, short stumpy cells differentiate into actively dividing procyclics. A hallmark of this process is the replacement of the bloodstream-stage surface coat composed of variant surface glycoprotein (VSG) with a new coat composed of procyclin. Pre-existing VSG is shed by a zinc metalloprotease activity (MSP-B) and glycosylphosphatidylinositol-specific phospholipase C (GPI-PLC). We now provide a detailed analysis of the coordinate and inverse regulation of these activities during synchronous differentiation. MSP-B mRNA and protein levels are upregulated during differentiation at the same time as proteolysis whereas GPI-PLC levels decrease. When transcription or translation is inhibited, VSG release is incomplete and a substantial amount of protein stays cell-associated. Both modes of release are still evident under these conditions, but GPI hydrolysis plays a quantitatively minor role during normal differentiation. Nevertheless, GPI biosynthesis shifts early in differentiation from a GPI-PLC sensitive structure to a resistant procyclic-type anchor. Translation inhibition also results in a marked increase in the mRNA levels of both MSP-B and GPI-PLC, consistent with negative regulation by labile protein factors. The relegation of short stumpy surface GPI-PLC to a secondary role in differentiation suggests that it may play a more important role as a virulence factor within the mammalian host.

The RACK1 homologue from Trypanosoma brucei is required for the onset and progression of cytokinesis

The receptor for activated C kinase 1 (RACK1) is a conserved scaffold protein that helps regulate a range of cell activities including cell growth, shape, and protein translation. We report that a homologue of RACK1 is required for cytokinesis in pathogenic Trypanosoma brucei. The protein, referred to as TRACK, is comprised of WD repeat elements and can complement cpc2 null mutants of Schizosaccharomyces pombe. TRACK is expressed throughout the trypanosome life cycle and is distributed predominantly in a perinuclear region and the cytoplasm but not along the endoplasmic reticulum, mitochondrion, or cleavage furrow of dividing cells. When tetracycline-inducible RNA interference (RNAi) is used to deplete the cellular content of TRACK, the cells remain metabolically active, but growth is inhibited. In bloodstream forms, growth arrest is due to a delay in the onset of cytokinesis. By contrast, procyclic forms are able to initiate cytokinesis in the absence of TRACK but arrest midway through cell cleavage. The RNAi cells undergo multiple rounds of partial cytokinesis and accumulate nuclei and cytoplasmic extensions with attached flagella. The TRACK RNAi construct is also inducible within infected mice. Under these conditions parasites are eliminated from peripheral blood within 3 days post-infection. Taken as a whole, these data indicate that trypanosomes utilize a RACK1 homologue to regulate the final stages of mitosis. Moreover, disrupting the interaction between TRACK and its partners might be targeted in the design of novel therapies.

Downregulation of the nuclear-encoded subunits of the complexes III and IV disrupts their respective complexes but not complex I in procyclic Trypanosoma brucei

The function, stability and mutual interactions of selected nuclear-encoded subunits of respiratory complexes III and IV were studied in the Trypanosoma brucei procyclics using RNA interference (RNAi). The growth rates and oxygen consumption of clonal cell lines of knock-downs for apocytochrome c1 (apoc1) and the Rieske Fe-S protein (Rieske) of complex III, and cytochrome c oxidase subunit 6 (cox6) of complex IV were markedly decreased after RNAi induction. Western analysis of mitochondrial lysates using specific antibodies confirmed complete elimination of the targeted proteins 4-6 days after induction. The Rieske protein was reduced in the apoc1 knock-down and vice versa, indicating a mutual interdependence of these components of complex III. However, another subunit of complex IV remained at the wild-type level in the cox6 knock-down. As revealed by two-dimensional blue native/SDS-PAGE electrophoresis, silencing of a single subunit resulted in the disruption of the respective complex, while the other complex remained unaffected. Membrane potential was reproducibly decreased in the knock-downs and the activities of complex III and/or IV, but not complex I, were drastically reduced, as measured by activity assays and histochemical staining. Using specific inhibitors, we have shown that in procyclics with depleted subunits of the respiratory complexes the flow of electrons was partially re-directed to the alternative oxidase. The apparent absence in T. brucei procyclics of a supercomplex composed of complexes I and III may represent an ancestral state of the respiratory chain.

A novel ERK-like, CRK-like protein kinase that modulates growth in Trypanosoma brucei via an autoregulatory C-terminal extension

The protozoan parasite Trypanosoma brucei undergoes a complex developmental cycle coordinated with cell cycle control. These processes in eukaryotes are frequently regulated through mitogen-activated protein kinases (MAPKs) and cyclin-dependent protein kinases (CDKs), respectively. We have discovered a novel protein kinase which shares features of both ERK-type MAPKs and CDKs (T. brucei ERK-like, CDK-like protein kinase). This molecule, named TbECK1, is similar to the unusual mammalian KKIAMRE protein kinase family. Moreover, TbECK1 possesses a long C-terminal extension reminiscent of those found in mammalian ERK5, ERK7 and ERK8. Expression analyses demonstrate that TbECK1 is constitutively expressed during the trypanosome life cycle at both RNA and protein level. In transgenic parasites we demonstrate that expression of a mutant of TbECK1 that lacks the C-terminal extension produces a slow growth phenotype, associated with the appearance of cells with aberrant karyotypes. Using this as an assay we further demonstrate that the phenotype is dependent upon the potential for catalytic activity of TbECK1 and on the integrity of at least one of the phosphorylable amino acids in its phosphorylation lip. C-terminal extensions are a common feature of kinetoplastid protein kinases. Our results demonstrate for the first time that this domain has a regulatory function.

Mitochondrial differentiation in kinetoplastid protozoa: a plethora of RNA controls

Differentiation of kinetoplastid protozoa during their complex life cycles is accompanied by stepwise changes in mitochondrial functions. Recent studies have begun to reveal multilevel post-transcriptional regulatory mechanisms by which the expression of the nuclear and mitochondrially encoded components of respiratory enzymes is coordinated, as well as the identities of some general and gene-specific factors controlling mitochondrial differentiation.

A novel purine nucleoside transporter whose expression is up-regulated in the short stumpy form of the Trypanosoma brucei life cycle

Purine nucleoside and nucleobase transporters play a vital role in the metabolism and survival of Trypanosoma brucei because this parasitic protozoan is unable to synthesize purines de novo and thus must acquire preformed purines from its hosts. These parasites express a variety of nucleoside and nucleobase permeases with diverse substrate specificities and distinct patterns of expression during the trypanosome life cycle. We report here that expression of the newly characterized T. brucei nucleoside transporter 10 gene (TbNT10) is up-regulated in the short stumpy form of the life cycle, the bloodstream form of the parasite that is pre-adapted for infection of the tsetse fly vector. Functional expression of TbNT10 in Saccharomyces cerevisiae reveals that the TbNT10 gene encodes an adenosine/guanosine/inosine transporter with apparent Km values of approximately 1 microM and hence is a high affinity purine nucleoside transporter. The restricted expression of TbNT10 during the life cycle suggests that the functional properties of this permease may be specialized to support development and growth of the differentiated short stumpy form or to promote the transformation of short stumpy to procyclic forms within the insect vector.

Trypanosomatid histones

The histones are responsible for packaging and regulating access to eukaryotic genomes. Trypanosomatids are flagellated protists that diverged early from the eukaryotic lineage and include parasites that cause disease in humans and other mammals. Here, we review the properties of histones in parasitic trypanosomatids, from gene organization and sequence to expression, post-translational modification and function within chromatin. Phylogenetic and experimental analysis indicates that certain specifically conserved histone sequence motifs, particularly within the N-terminal 'tail' domains, possibly represent functionally important modification substrates conserved throughout the eukaryotic lineage. For example, histone H3 contains a highly conserved methylation substrate. Trypanosomatids also possess at least three variant histones. Among these is an orthologue of H2A.Z, a histone involved in protecting 'active' chromatin from silencing in yeast. Histones provide docking platforms for a variety of regulatory factors. The presence of histone modification and variant histones in trypanosomatids therefore represents evidence for a network that provides the discrimination required to regulate transcription, recombination, repair and chromosome replication and segregation.

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.

Human malaria parasites display a receptor for activated C kinase ortholog

Receptors for activated C kinases (RACKs) are scaffold proteins that anchor diverse signaling proteins and are involved in modulating cell cycle. We report the cloning and cellular localization of a RACK ortholog (PfRACK) in the human malaria parasite Plasmodium falciparum. The full-length transcript obtained by 3(') and 5(') RACE has 1.4 kbp with a predicted ORF of 972 bp, coding for a protein with 323 residues of 35.8 kDa molecular weight and pI 6.38. PfRACK has 59% and 60% identity at the amino acid level to Chlamydomonas reinhardtii and Danio rerio RACKs, respectively, presenting seven WD40 motifs and retaining the conserved domains in repeats III (DVFSVSF) and VI (STINSLCF) that are important for PKC binding. Semi-quantitative RT-PCR revealed that PfRACK is constitutively expressed in the intraerythrocytic stages of P. falciparum. Using confocal microscopy, PfRACK was immunolocalized in all parasite stages, being conspicuously spread throughout the schizont. The high similarity of PfRACK to those previously described in other organisms, as well as its constitutive expression in Plasmodium asexual stages, suggests that it might play a key role in the regulatory processes of malaria parasite life cycle.

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.

Mitochondrial development during life cycle differentiation of African trypanosomes: evidence for a kinetoplast-dependent differentiation control point

Life cycle differentiation of African trypanosomes entails developmental regulation of mitochondrial activity. This requires regulation of the nuclear genome and the kinetoplast, the trypanosome's unusual mitochondrial genome. To investigate the potential cross talk between the nuclear and mitochondrial genome during the events of differentiation, we have 1) disrupted expression of a nuclear-encoded component of the cytochrome oxidase (COX) complex; and 2) generated dyskinetoplastid cells, which lack a mitochondrial genome. Using RNA interference (RNAi) and by disrupting the nuclear COX VI gene, we demonstrate independent regulation of COX component mRNAs encoded in the nucleus and kinetoplast. However, two independent approaches (acriflavine treatment and RNA interference ablation of mitochondrial topoisomerase II) failed to establish clonal lines of dyskinetoplastid bloodstream forms. Nevertheless, dyskinetoplastid forms generated in vivo could undergo two life cycle differentiation events: transition from bloodstream slender to stumpy forms and the initiation of transformation to procyclic forms. However, they subsequently arrested at a specific point in this developmental program before cell cycle reentry. These results provide strong evidence for a requirement for kinetoplast DNA in the bloodstream and for a kinetoplast-dependent control point during differentiation to procyclic forms.

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.

A novel CCCH protein which modulates differentiation of Trypanosoma brucei to its procyclic form

Cell differentiation in Trypanosoma brucei involves highly regulated changes in morphology, proliferation and metabolism. However, the controls of these developmental processes are unknown. We have identified two novel proteins from the rare CCCH zinc finger family, each <140 amino acids in length and implicated in life cycle regulation. TbZFP1 is transiently enriched during differentiation from the bloodstream to procyclic form, whereas tbZFP2, when ablated in bloodstream forms by RNA interference, inhibits this developmental step. Moreover, expressing an ectopic copy of tbZFP2 results in a dramatic procyclic stage-specific remodelling of the trypanosome cytoskeleton similar to the morphogenic events of differentiation. This phenotype, we term 'nozzle', involves polar extension of microtubules at the posterior end of the cell and is dependent upon a motif hitherto restricted to E3 ubiquitin ligases. TbZFP1 and tbZFP2 represent the first molecules implicated in the control of trypanosome differentiation to the procyclic form.

Molecular characterization of two serine proteases expressed in gut tissue of the African trypanosome vector, Glossina morsitans morsitans

Serine proteases are major insect gut enzymes involved in digestion of dietary proteins, and in addition they have been implicated in the process of pathogen establishment in several vector insects. The medically important vector, tsetse fly (Diptera:Glossinidiae), is involved in the transmission of African trypanosomes, which cause devastating diseases in animals and humans. Both the male and female tsetse can transmit trypanosomes and both are strict bloodfeeders throughout all stages of their development. Here, we describe the characterization of two putative serine protease-encoding genes, Glossina serine protease-1 (Gsp1) and Glossina serine protease-2 (Gsp2) from gut tissue. Both putative cDNA products represent prepro peptides with hydrophobic signal peptide sequences associated with their 5'-end terminus. The Gsp1 cDNA encodes a putative mature protein of 245 amino acids with a molecular mass of 26 428 Da, while the predicted size of the 228 amino acid mature peptide encoded by Gsp2 cDNA is 24 573 Da. Both deduced peptides contain the Asp/His/Ser catalytic triad and the conserved residues surrounding it which are characteristic of serine proteases. In addition, both proteins have the six-conserved cysteine residues to form the three-cysteine bonds typically present in invertebrate serine proteases. Based on the presence of substrate specific residues, the Gsp1 gene encodes a chymotrypsin-like protease while Gsp2 gene encodes for a protein with trypsin-like activity. Both proteins are encoded by few loci in tsetse genome, being present in one or two copies only. The mRNA expression levels for the genes do not vary extensively throughout the digestive cycle, and high levels of mRNAs can be readily detected in the gut tissue of newly emerged flies. The levels of trypsin and chymotrypsin activities in the gut lumen increase following blood feeding and change significantly in the gut cells throughout the digestion cycle. Hence, the regulation of expression for trypsin and chymotrypsin occurs at the post-transcriptional level in tsetse. Both the coding sequences and patterns of expression of Gsp1 and Gsp2 genes are similar to the serine proteases that have been reported from the bloodfeeding insect Stomoxys calcitrans.

Cytochrome oxidase subunit VI of Trypanosoma brucei is imported without a cleaved presequence and is developmentally regulated at both RNA and protein levels

Mitochondrial respiration in the African trypanosome undergoes dramatic developmental stage regulation. This requires co-ordinated control of components encoded by both the nuclear genome and the kinetoplast, the unusual mitochondrial genome of these parasites. As a model for understanding the co-ordination of these genomes, we have examined the regulation and mitochondrial import of a nuclear-encoded component of the cytochrome oxidase complex, cytochrome oxidase subunit VI (COXVI). By generating transgenic trypanosomes expressing intact or mutant forms of this protein, we demonstrate that COXVI is not imported using a conventional cleaved presequence and show that sequences at the N-terminus of the protein are necessary for correct mitochondrial sorting. Analyses of endogenous and transgenic COXVI mRNA and protein expression in parasites undergoing developmental stage differentiation demonstrates a temporal order of control involving regulation in the abundance of, first, mRNA and then protein. This represents the first dissection of the regulation and import of a nuclear-encoded protein into the cytochrome oxidase complex in these organisms, which were among the earliest eukaryotes to possess a mitochondrion.

The cell cycle in protozoan parasites

Research into cell cycle control in protozoan parasites, which are responsible for major public health problems in the developing world, has been hampered by the difficulties in performing classical genetic analysis with these organisms. Nevertheless, in a large part thanks to the data gathered in other eukaryotic systems and to the acquisition of the sequences of parasite genes homologous to cell cycle regulators, many molecular tools required for an in-depth study of the cell cycle in protozoan parasites have been collected over the past few years. Despite the considerable phylogenetic divergence between these organisms and other eukaryotes, and notwithstanding important specificities such as the apparent lack of checkpoints during cell cycle progression, available data indicate that the major families of cell cycle regulators appear to operate in protozoan parasites. Functional studies are now needed to define the precise role of these regulators in the life cycle of the parasites, and to possibly validate cell cycle control elements as potential targets for chemotherapy.

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.

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.

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.