Trypanosoma brucei brucei: endocytic recycling is important for mouse infectivity

Endocytosis in the African trypanosome, Trypanosoma brucei, is intimately involved in maintaining homeostasis of the cell surface proteome, morphology of the flagellar pocket and has recently been demonstrated as a bona fide drug target. RNAi-mediated knockdown of many factors required for endocytic transport, including several small GTPases, the major coat protein clathrin and a clathrin-associated receptor, epsinR, results in rapid cell death in vitro. Rapid loss of viability in vitro precludes meaningful investigation by RNAi of the roles of trypanosome endocytosis in vivo. Here we have sought to address this issue using strategies designed to produce milder effects on the endocytic system than complete functional ablation. We created a trypanosome clathrin heavy chain hemizygote and several lines expressing mutant forms of Rab5 and Rab11, described previously. All are viable in in vitro culture, with negligible impact to proliferative rates or cell cycle. Clathrin hemizygotes express clathrin heavy chain at ∼50% of wild type levels, but despite this demonstrate no defect to growth in mice, while none of the Rab5 mutants affected proliferation in vivo, despite clear evidence for effects on endocytosis. By contrast we find that expressing a dominantly active Rab11 mutant led to compromised growth in mice. These data indicate that trypanosomes likely tolerate the effects of partly decreased clathrin expression and alterations in early endocytosis, but are more sensitive to alterations in the recycling arm of the pathway.

Functional characterisation and drug target validation of a mitotic kinesin-13 in Trypanosoma brucei

Mitotic kinesins are essential for faithful chromosome segregation and cell proliferation. Therefore, in humans, kinesin motor proteins have been identified as anti-cancer drug targets and small molecule inhibitors are now tested in clinical studies. Phylogenetic analyses have assigned five of the approximately fifty kinesin motor proteins coded by Trypanosoma brucei genome to the Kinesin-13 family. Kinesins of this family have unusual biochemical properties because they do not transport cargo along microtubules but are able to depolymerise microtubules at their ends, therefore contributing to the regulation of microtubule length. In other eukaryotic genomes sequenced to date, only between one and three Kinesin-13s are present. We have used immunolocalisation, RNAi-mediated protein depletion, biochemical in vitro assays and a mouse model of infection to study the single mitotic Kinesin-13 in T. brucei. Subcellular localisation of all five T. brucei Kinesin-13s revealed distinct distributions, indicating that the expansion of this kinesin family in kinetoplastids is accompanied by functional diversification. Only a single kinesin (TbKif13-1) has a nuclear localisation. Using active, recombinant TbKif13-1 in in vitro assays we experimentally confirm the depolymerising properties of this kinesin. We analyse the biological function of TbKif13-1 by RNAi-mediated protein depletion and show its central role in regulating spindle assembly during mitosis. Absence of the protein leads to abnormally long and bent mitotic spindles, causing chromosome mis-segregation and cell death. RNAi-depletion in a mouse model of infection completely prevents infection with the parasite. Given its essential role in mitosis, proliferation and survival of the parasite and the availability of a simple in vitro activity assay, TbKif13-1 has been identified as an excellent potential drug target.

A novel phosphatase cascade regulates differentiation in Trypanosoma brucei via a glycosomal signaling pathway

In the mammalian bloodstream, the sleeping sickness parasite Trypanosoma brucei is held poised for transmission by the activity of a tyrosine phosphatase, TbPTP1. This prevents differentiation of the transmissible "stumpy forms" until entry into the tsetse fly, whereupon TbPTP1 is inactivated and major changes in parasite physiology are initiated to allow colonization of the arthropod vector. Using a substrate-trapping approach, we identified the downstream step in this developmental signaling pathway as a DxDxT phosphatase, TbPIP39, which is activated upon tyrosine phosphorylation, and hence is negatively regulated by TbPTP1. In vitro, TbPIP39 promotes the activity of TbPTP1, thereby reinforcing its own repression, this being alleviated by the trypanosome differentiation triggers citrate and cis-aconitate, generating a potentially bistable regulatory switch. Supporting a role in signal transduction, TbPIP39 becomes rapidly tyrosine-phosphorylated during differentiation, and RNAi-mediated transcript ablation in stumpy forms inhibits parasite development. Interestingly, TbPIP39 localizes in glycosomes, peroxisome-like organelles that compartmentalize the trypanosome glycolytic reactions among other enzymatic activities. Our results invoke a phosphatase signaling cascade in which the developmental signal is trafficked to a unique metabolic organelle in the parasite: the glycosome. This is the first characterized environmental signaling pathway targeted directly to a peroxisome-like organelle in any eukaryotic cell.

New discoveries in the transmission biology of sleeping sickness parasites: applying the basics

The sleeping sickness parasite, Trypanosoma brucei, must differentiate in response to the changing environments that it encounters during its complex life cycle. One developmental form, the bloodstream stumpy stage, plays an important role in infection dynamics and transmission of the parasite. Recent advances have shed light on the molecular mechanisms by which these stumpy forms differentiate as they are transmitted from the mammalian host to the insect vector of sleeping sickness, tsetse flies. These molecular advances now provide improved experimental tools for the study of stumpy formation and function within the mammalian bloodstream. They also offer new routes to therapy via high-throughput screens for agents that accelerate parasite development. Here, we shall discuss the recent advances that have been made and the prospects for future research now available.

The trypanosome Rab-related proteins RabX1 and RabX2 play no role in intracellular trafficking but may be involved in fly infectivity

Background

Rab GTPases constitute the largest subgroup of the Ras superfamily and are primarily involved in vesicle targeting. The full extent of Rab family function is unexplored. Several divergent Rab-like proteins are known but few have been characterized. In Trypanosoma brucei there are sixteen Rab genes, but RabX1, RabX2 and RabX3 are divergent within canonical sequence regions. Where known, trypanosome Rab functions are broadly conserved when orthologous relationships may be robustly established, but specific functions for RabX1, X2 and X3 have yet to be determined. RabX1 and RabX2 originated via tandem duplication and subcellular localization places RabX1 at the endoplasmic reticulum, while RabX2 is at the Golgi complex, suggesting distinct functions. We wished to determine whether RabX1 and RabX2 are involved in vesicle transport or other cellular processes.

Methodology/Principal Findings

Using comparative genomics we find that RabX1 and RabX2 are restricted to trypanosomatids. Gene knockout indicates that RabX1 and RabX2 are non-essential. Simultaneous RNAi knockdown of both RabX1 and RabX2, while partial, was also non-lethal and may suggest non-redundant function, consistent with the distinct locations of the proteins. Analysis of the knockout cell lines unexpectedly failed to uncover a defect in exocytosis, endocytosis or in the morphology or location of multiple markers for the endomembrane system, suggesting that neither RabX1 nor RabX2 has a major role in intracellular transport. However, it was apparent that RabX1 and RabX2 knockout cells displayed somewhat enhanced survival within flies.

Conclusions/Significance

RabX1 and RabX2, two members of the trypanosome Rab subfamily, were shown to have no major detectable role in intracellular transport, despite the localization of each gene product to highly specific endomembrane compartments. These data extend the functional scope of Rab proteins in trypanosomes to include non-canonical roles in differentiation-associated processes in protozoa.

Cell-cycle synchronisation of bloodstream forms of Trypanosoma brucei using Vybrant DyeCycle Violet-based sorting

Studies on the cell-cycle of Trypanosoma brucei have revealed several unusual characteristics that differ from the model eukaryotic organisms. However, the inability to isolate homogenous populations of parasites in distinct cell-cycle stages has limited the analysis of trypanosome cell division and complicated the understanding of mutant phenotypes with possible impact on cell-cycle related events. Although hydroxyurea-induced cell-cycle arrest in procyclic and bloodstream forms has been applied recently with success, such block-release protocols can complicate the analysis of cell-cycle regulated events and have the potential to disrupt important cell-cycle checkpoints. An alternative approach based on flow cytometry of parasites stained with Vybrant DyeCycle Orange circumvents this problem, but is restricted to procyclic form parasites. Here, we apply Vybrant Dyecycle Violet staining coupled with flow cytometry to effectively select different cell-cycle stages of bloodstream form trypanosomes. Moreover, the sorted parasites remain viable, although synchrony is rapidly lost. This method enables cell-cycle enrichment of populations of trypanosomes in their mammal infective stage, particularly at the G1 phase.

Differential trypanosome surface coat regulation by a CCCH protein that co-associates with procyclin mRNA cis-elements

The genome of Trypanosoma brucei is unusual in being regulated almost entirely at the post-transcriptional level. In terms of regulation, the best-studied genes are procyclins, which encode a family of major surface GPI-anchored glycoproteins (EP1, EP2, EP3, GPEET) that show differential expression in the parasite's tsetse-fly vector. Although procyclin mRNA cis-regulatory sequences have provided the paradigm for post-transcriptional control in kinetoplastid parasites, trans-acting regulators of procyclin mRNAs are unidentified, despite intensive effort over 15 years. Here we identify the developmental regulator, TbZFP3, a CCCH-class predicted RNA binding protein, as an isoform-specific regulator of Procyclin surface coat expression in trypanosomes. We demonstrate (i) that endogenous TbZFP3 shows sequence-specific co-precipitation of EP1 and GPEET, but not EP2 and EP3, procyclin mRNA isoforms, (ii) that ectopic overexpression of TbZFP3 does not perturb the mRNA abundance of procyclin transcripts, but rather that (iii) their protein expression is regulated in an isoform-specific manner, as evidenced by mass spectrometric analysis of the Procyclin expression signature in the transgenic cell lines. The TbZFP3 mRNA–protein complex (TbZFP3mRNP) is identified as a trans-regulator of differential surface protein expression in trypanosomes. Moreover, its sequence-specific interactions with procyclin mRNAs are compatible with long-established predictions for Procyclin regulation. Combined with the known association of TbZFP3 with the translational apparatus, this study provides a long-sought missing link between surface protein cis-regulatory signals and the gene expression machinery in trypanosomes.

Trypanosomes, the tropical parasites that cause African sleeping sickness, show a number of biological peculiarities that distinguish them from other eukaryotes. One is the unusual way in which they regulate gene expression. Unlike most eukaryotes, trypanosomes do not regulate gene expression by controlling the rate of messenger RNA synthesis, but, instead, control the stability of messenger mRNAs (and, hence, their abundance) and also their rate of translation into protein. The best-studied model for this “post-transcriptional” gene expression control in trypanosomes is the procyclin mRNAs, which encode the major surface proteins of the parasite in the tsetse fly. In this study we demonstrate that a small kinetoplastid-specific protein (TbZFP3) co-associates with the mRNAs for some procyclin isoforms (EP1, GPEET procyclin) but not others (EP2, EP3 procyclin). Furthermore, we show that this is dependent upon sequences in the EP1 procyclin 3′untranslated region known to govern its mRNA turnover and protein synthesis. Finally, we demonstrate that limited over-expression of TbZFP3 causes a change in the surface protein expression profile on cultured parasites from GPEET to EP1 Procyclin. Our data identify TbZFP3 as an important post-transcriptional regulator of Procyclin expression, the first such protein factor identified.

Genetic parameters for carcass composition and performance data in crossbred lambs measured by Video Image Analysis

A total of 7074 crossbred lambs, produced by mating crossbred Mule ewes with terminal sire rams were used in this study. Of these, 630 were scanned using a Video Image Analysis (VIA) to estimate carcass quality traits. Genetic parameters for average daily gain (ADG), scanning live weight (SW), ultrasonic measures of muscle (UMD) and fat (UFD) depths, cold carcass weight (CCW) and VIA measurements of primal carcass joint weights (LEG, CHUMP, LOIN, BREAST and SHOULDER) were estimated using multivariate animal models. Additionally, VIA traits were evaluated under a repeatability model, considering the primal joints as repeated measures of the same trait. Direct heritability estimates were low to moderate (0.08-0.26) for VIA measurements of primal joints. Repeatability estimates for VIA traits were high (>0.90). Moderate to high heritability estimates (0.25-0.55) were found for performance traits (ADG, SW, UMD and UFD) and CCW. Genetic correlations between VIA traits and ADG were strong (0.75-0.93). Most of the VIA traits were highly correlated to SW (0.60-0.97). UFD was significantly negatively correlated with UMD (-0.22), ADG (-0.18) and CCW (-0.18). The results of this study suggest that selection on performance and carcass traits, measured by VIA, could possibly improve primal meat yield of carcass cuts without increasing the overall carcass fatness. High repeatability estimates of VIA traits and moderate heritabilities of the most valuable carcass joints suggests that including VIA information in breeding programs would be useful in order to improve carcass quality.

Modelling trypanosome chronicity: VSG dynasties and parasite density

A new mathematical model developed by Lythgoe et al. shows that the semi-predictable order of trypanosome antigenic variation can be generated by two parasite-intrinsic factors. The first is the different probabilities of antigen-gene activation that result from the different molecular mechanisms by which the genes become expressed. The second is the density-dependent differentiation of slender to stumpy cells. The study has important implications for understanding the dynamics of antigenic variation and for modelling the consequences of therapeutic strategies directed against trypanosomes.

Restless gossamers: antibody clearance by hydrodynamic flow forces generated at the surface of motile trypanosome parasites

Trypanosomes evade antibody-mediated lysis via antigenic variation and rapid antibody removal from their cell surface. Recently, in Cell, Engstler et al. (2007) have discovered the mechanism for antibody clearance. Hydrodynamic forces generated by trypanosome swimming create a current, causing surface-bound antibodies to act as "molecular sails." Consequently, they are swept to the cell posterior, internalized via the flagellar-pocket, and degraded. Hydrodynamic sorting is a novel biological process, possibly applicable in other contexts.

The synergistic effect of nisin and lactoferrin on the inhibition of Listeria monocytogenes and Escherichia coli O157:H7

AIMS

The goal of this study was to determine whether nisin and lactoferrin would act synergistically to inhibit the growth of Listeria monocytogenes and Escherichia coli O157:H7.

METHODS AND RESULTS

Lactoferrin and nisin separately or in combination were suspended in peptone yeast glucose broth and following inoculation with L. monocytogenes or E. coli O157:H7 growth inhibition of each pathogen was determined. At 1000 microg ml(-1) lactoferrin L. monocytogenes was effectively inhibited. However, E. coli O157:H7 initially was inhibited and then grew to cell density similar to the control. A combination of 500 microg ml(-1) of lactoferrin and 250 IU ml(-1) of nisin effectively inhibited the growth of E. coli O157:H7, whereas, 250 microg ml(-1) of lactoferrin and 10 IU ml(-1) of nisin were inhibitory to L. monocytogenes.

CONCLUSIONS

The results suggest that lactoferrin and nisin act synergistically to inhibit the growth of L. monocytogenes and E. coli O157:H7.

SIGNIFICANCE AND IMPACT OF THE STUDY

Natural preservatives that are active against gram-positive and gram-negative pathogens are desirable to the food industry and consumers. This study demonstrates that lactoferrin and nisin work synergistically reducing the levels required independently inhibiting growth of two major foodborne pathogens. Previous reported results indicated a low level of antimicrobial activity; however, this work was not performed in low divalent cation concentration media. It has been suggested that nondivalent cation-limiting medium such as trypticase soy broth (TSB), can reduce or completely eliminate the inhibitory activity. Further knowledge of these interactions can increase the understanding of the antimicrobial activity of lactoferrin. This should make the use of these compounds by industry more attractive.

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

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

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.

Interaction of live and dead Escherichia coli O157:H7 and fluorescent microspheres with lettuce tissue suggests bacterial processes do not mediate adherence

AIMS

The goal of this study was to determine whether any specific bacterial processes (biochemical or genetic) or cell surface moieties were required for the interaction between Escherichia coli O157:H7 and lettuce plant tissue.

METHODS AND RESULTS

Escherichia coli O157:H7 and Fluospheres (fluorescent polystyrene microspheres) were used in experiments to investigate interactions with lettuce. Fluospheres were used as they are a non-biological material, of similar size and shape to a bacterial cell, but lack bacterial cell surface moieties and the ability to respond genetically. Live and glutaraldehyde-killed E. coli O157:H7 attached at levels of c. 5.8 log(10) cells per cm(2) following immersion of lettuce pieces into a suspension containing c. 8 log(10) CFU ml(-1). In a separate experiment, numbers of bacteria or Fluospheres associated with lettuce decreased by c. 1.5 log cm(-2) following a 1-min wash. Exposure times of 1 min, 1 h, or 6 h had little effect on the level of attachment for Fluospheres, and live or killed cells of E. coli O157:H7 to lettuce tissue.

SIGNIFICANCE

These results indicate that bacterial processes and cell surface moieties are not required for the initial interaction of E. coli O157:H7 to lettuce plant tissue.

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

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

The developmental cell biology of Trypanosoma brucei

Trypanosoma brucei provides an excellent system for studies of many aspects of cell biology, including cell structure and morphology, organelle positioning, cell division and protein trafficking. However, the trypanosome has a complex life cycle in which it must adapt either to the mammalian bloodstream or to different compartments within the tsetse fly. These differentiation events require stage-specific changes to basic cell biological processes and reflect responses to environmental stimuli and programmed differentiation events that must occur within a single cell. The organization of cell structure is fundamental to the trypanosome throughout its life cycle. Modulations of the overall cell morphology and positioning of the specialized mitochondrial genome, flagellum and associated basal body provide the classical descriptions of the different life cycle stages of the parasite. The dependency relationships that govern these morphological changes are now beginning to be understood and their molecular basis identified. The overall picture emerging is of a highly organized cell in which the rules established for cell division and morphogenesis in organisms such as yeast and mammalian cells do not necessarily apply. Therefore, understanding the developmental cell biology of the African trypanosome is providing insight into both fundamentally conserved and fundamentally different aspects of the organization of the eukaryotic cell.

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.

Isolates of Trichuris muris elicit different adaptive immune responses in their murine host

The J and S isolates of Trichuris muris have different infection profiles in C57BL/6 mice; J worms are expelled, S worms survive to chronicity. Building on this, the ability of the J and S isolates to survive, and the quality of the immune response induced was explored in three different strains of mouse. The resistant BALB/c mouse mounted a strong Th2 response against both isolates, which were quickly expelled. The susceptible AKR host mounted a Th1 response and retained both isolates. Despite equivalent worm exposure, mesenteric lymph node cells from AKR mice infected with the S isolate produced significantly higher levels of IL-12 and the intestinal mastocytosis was reduced. IgG1 and IgG2a from S-infected AKR mice recognized low molecular weight antigens not recognized by J-infected mice. Differential expulsion kinetics was observed in the slower-responding C57BL/6 strain; J worms were expelled but S isolate worms were retained. Survival of the S isolate was again associated with elevated IL-12 and decreased Th2 responses. In resistant mouse strains, the outcome of infection is thus dominantly influenced by host genetics. However, in the slower-responding host, isolate-derived factors may play a role in shaping the quality of the adaptive immune response, thus influencing parasite survival.

tbCPSF30 depletion by RNA interference disrupts polycistronic RNA processing in Trypanosoma brucei

Gene expression in eukaryotes requires the post-transcriptional cleavage of mRNA precursors into mature mRNAs. In Trypanosoma brucei, mRNA processing is of particular importance, since most transcripts are derived from polycistronic transcription units. This organization dictates that regulated gene expression is promoter-independent and governed at the posttranscriptional level. We have identified tbCPSF30, a protein containing five CCCH zinc finger motifs, which is a homologue of the cleavage and polyadenylation specificity factor (CPSF) 30-kDa subunit, a component of the machinery required for 3'-end formation in yeast and mammals. Using gene silencing of tbCPSF30 by RNA interference, we demonstrate that this gene is essential in bloodstream and procyclic forms of T. brucei. Interestingly, tbCPSF30-specific RNA interference results in the accumulation of an aberrant tbCPSF30 mRNA species concomitant with depletion of tbCPSF30 protein. tbCPSF30 protein depletion is accompanied by the accumulation of unprocessed tubulin RNAs, implicating tbCPSF30 in polycistronic RNA processing. By genome data base mining, we also identify several other putative components of the T. brucei cleavage and polyadenylation machinery, indicating their conservation throughout eukaryotic evolution. This study is the first to identify and characterize a core component of the T. brucei CPSF and show its involvement in polycistronic RNA processing.