Differentiation of Trypanosoma brucei may be stage non-specific and does not require progression of cell cycle

Identification of Resistance Determinants for a Promising Antileishmanial Oxaborole Series

Current treatment options for visceral leishmaniasis have several drawbacks, and clinicians are confronted with an increasing number of treatment failures. To overcome this, the Drugs for Neglected Diseases initiative (DNDi) has invested in the development of novel antileishmanial leads, including a very promising class of oxaboroles. The mode of action/resistance of this series to Leishmania is still unknown and may be important for its further development and implementation. Repeated in vivo drug exposure and an in vitro selection procedure on both extracellular promastigote and intracellular amastigote stages were both unable to select for resistance. The use of specific inhibitors for ABC-transporters could not demonstrate the putative involvement of efflux pumps. Selection experiments and inhibitor studies, therefore, suggest that resistance to oxaboroles may not emerge readily in the field. The selection of a genome-wide cosmid library coupled to next-generation sequencing (Cos-seq) was used to identify resistance determinants and putative targets. This resulted in the identification of a highly enriched cosmid, harboring genes of chromosome 2 that confer a subtly increased resistance to the oxaboroles tested. Moderately enriched cosmids encompassing a region of chromosome 34 contained the cleavage and polyadenylation specificity factor (cpsf) gene, encoding the molecular target of several related benzoxaboroles in other organisms.

The ubiquitin-conjugating enzyme CDC34 is essential for cytokinesis in contrast to putative subunits of a SCF complex in Trypanosoma brucei

The ubiquitin-proteasome system is a post-translational regulatory pathway for controlling protein stability and activity that underlies many fundamental cellular processes, including cell cycle progression. Target proteins are tagged with ubiquitin molecules through the action of an enzymatic cascade composed of E1 ubiquitin activating enzymes, E2 ubiquitin conjugating enzymes, and E3 ubiquitin ligases. One of the E3 ligases known to be responsible for the ubiquitination of cell cycle regulators in eukaryotes is the SKP1-CUL1-F-box complex (SCFC). In this work, we identified and studied the function of homologue proteins of the SCFC in the life cycle of Trypanosoma brucei, the causal agent of the African sleeping sickness. Depletion of trypanosomal SCFC components TbRBX1, TbSKP1, and TbCDC34 by RNAi resulted in decreased growth rate and contrasting cell cycle abnormalities for both procyclic (PCF) and bloodstream (BSF) forms. Depletion of TbRBX1 in PCF cells interfered with kinetoplast replication, whilst depletion of TbSKP1 arrested PCF and BSF cells in the G1/S transition. Silencing of TbCDC34 in BSF cells resulted in a block in cytokinesis and caused rapid clearance of parasites from infected mice. We also show that TbCDC34 is able to conjugate ubiquitin in vitro and in vivo, and that its activity is necessary for T. brucei infection progression in mice. This study reveals that different components of a putative SCFC have contrasting phenotypes once depleted from the cells, and that TbCDC34 is essential for trypanosome replication, making it a potential target for therapeutic intervention.

African sleeping sickness is a neglected tropical disease caused by infection with the protozoan parasite Trypanosoma brucei, which is transmitted to humans by tsetse flies (Glossina genus). Treatment of the disease is complex and relies on limited pharmaceutical options. Understanding how T. brucei regulates cell cycle progression at a molecular level when alternating between the mammalian host and the insect vector could lead to better therapies. In this study, we examined different T. brucei proteins with homology to components of the SKP1-CUL1-F-box ubiquitin ligase complex (SCFC), previously characterized in other eukaryotes as a regulator of cell cycle progression. We found that depletion of the homologues of a putative SCFC cause T. brucei to develop abnormally, generating different phenotypes of the mammalian and insect stages. Interestingly, depletion of the ubiquitin conjugating enzyme TbCDC34 arrest cells in a pre-cytokinesis stage, indicating that this protein is essential for cytokinesis. In addition to improving our fundamental understanding of the molecular regulation underlying the sophisticated life cycle of T. brucei, this work pinpoints a potential target for drug development against trypanosomiasis.

Seasonal variation of tsetse fly species abundance and prevalence of trypanosomes in the Maasai Steppe, Tanzania

Tsetse flies, the vectors of trypanosomiasis, represent a threat to public health and economy in sub-Saharan Africa. Despite these concerns, information on temporal and spatial dynamics of tsetse and trypanosomes remain limited and may be a reason that control strategies are less effective. The current study assessed the temporal variation of the relative abundance of tsetse fly species and trypanosome prevalence in relation to climate in the Maasai Steppe of Tanzania in 2014-2015. Tsetse flies were captured using odor-baited Epsilon traps deployed in ten sites selected through random subsampling of the major vegetation types in the area. Fly species were identified morphologically and trypanosome species classified using PCR. The climate dataset was acquired from the African Flood and Drought Monitor repository. Three species of tsetse flies were identified: G. swynnertoni (70.8%), G. m. morsitans (23.4%), and G.pallidipes (5.8%). All species showed monthly changes in abundance with most of the flies collected in July. The relative abundance of G. m. morsitans and G. swynnertoni was negatively correlated with maximum and minimum temperature, respectively. Three trypanosome species were recorded: T. vivax (82.1%), T. brucei (8.93%), and T. congolense (3.57%). The peak of trypanosome infections in the flies was found in October and was three months after the tsetse abundance peak; prevalence was negatively correlated with tsetse abundance. A strong positive relationship was found between trypanosome prevalence and temperature. In conclusion, we find that trypanosome prevalence is dependent on fly availability, and temperature drives both tsetse fly relative abundance and trypanosome prevalence.

Distinct roles of a mitogen-activated protein kinase in cytokinesis between different life cycle forms of Trypanosoma brucei

Mitogen-activated protein kinase (MAPK) modules are evolutionarily conserved signaling cascades that function in response to the environment and play crucial roles in intracellular signal transduction in eukaryotes. The involvement of a MAP kinase in regulating cytokinesis in yeast, animals, and plants has been reported, but the requirement for a MAP kinase for cytokinesis in the early-branching protozoa is not documented. Here, we show that a MAP kinase homolog (TbMAPK6) from Trypanosoma brucei plays distinct roles in cytokinesis in two life cycle forms of T. brucei. TbMAPK6 is distributed throughout the cytosol in the procyclic form but is localized in both the cytosol and the nucleus in the bloodstream form. RNA interference (RNAi) of TbMAPK6 results in moderate growth inhibition in the procyclic form but severe growth defects and rapid cell death in the bloodstream form. Moreover, TbMAPK6 appears to be implicated in furrow ingression and cytokinesis completion in the procyclic form but is essential for cytokinesis initiation in the bloodstream form. Despite the distinct defects in cytokinesis in the two forms, RNAi of TbMAPK6 also caused defective basal body duplication/segregation in a small cell population in both life cycle forms. Altogether, our results demonstrate the involvement of the TbMAPK6-mediated pathway in regulating cytokinesis in trypanosomes and suggest distinct roles of TbMAPK6 in cytokinesis between different life cycle stages of T. brucei.

Identification of a Wee1-like kinase gene essential for procyclic Trypanosoma brucei survival

Regulation of eukaryotic cell cycle progression requires sequential activation and inactivation of cyclin-dependent kinases (CDKs). Activation of the cyclin B-cdc2 kinase complex is a pivotal step in mitotic initiation and the tyrosine kinase Wee1 is a key regulator of cell cycle sequence during G2/M transition and inhibits mitotic entry by phosphorylating the inhibitory tyrosine 15 on the cdc2 M-phase-inducing kinase. Wee1 degradation is essential for the exit from the G2 phase. In trypanosomatids, little is known about the genes that regulate cyclin B-cdc2 complexes at the G2/M transition of their cell cycle. Although canonical tyrosine kinases are absent in the genome of trypanosomatids, phosphorylation on protein tyrosine residues has been reported in Trypanosoma brucei. Here, we characterized a Wee1-like protein kinase gene from T. brucei. Expression of TbWee1 in a Schizosaccharomyces pombe strain null for Wee1 inhibited cell division and caused cell elongation. This demonstrates the lengthening of G2, which provided cells with extra time to grow before dividing. The Wee1-like protein kinase was expressed in the procyclic and bloodstream proliferative slender forms of T. brucei and the role of Wee1 in cell cycle progression was analyzed by generating RNA interference cell lines. In the procyclic form of T. brucei, the knock-down of TbWee1 expression by RNAi led to inhibition of parasite growth. Abnormal phenotypes showing an increase in the percentage of cells with 1N0K, 0N1K and 2N1K were observed in these RNAi cell lines. Using parasites with a synchronized cell cycle, we demonstrated that TbWee1 is linked to the G2/M phase. We also showed that TbWee1 is an essential gene necessary for proper cell cycle progression and parasite growth in T. brucei. Our results provide evidence for the existence of a functional Wee1 in T. brucei with a potential role in cell division at G2/M.

The Giardia cell cycle progresses independently of the anaphase-promoting complex

Most cell cycle regulation research has been conducted in model organisms representing a very small part of the eukaryotic domain. The highly divergent human pathogen Giardia intestinalis is ideal for studying the conservation of eukaryotic pathways. Although Giardia has many cell cycle regulatory components, its genome lacks all anaphase-promoting complex (APC) components. In the present study, we show that a single mitotic cyclin in Giardia is essential for progression into mitosis. Strikingly, Giardia cyclin B lacks the conserved N-terminal motif required for timely degradation mediated by the APC and ubiquitin conjugation. Expression of Giardia cyclin B in fission yeast is toxic, leading to a prophase arrest, and this toxicity is suppressed by the addition of a fission yeast degradation motif. Cyclin B is degraded during mitosis in Giardia cells, but this degradation appears to be independent of the ubiquitination pathway. Other putative APC substrates, aurora and polo-like kinases, also show no evidence of ubiquitination. This is the first example of mitosis not regulated by the APC and might reflect an evolutionary ancient form of cell cycle regulation.

An orphan kinesin in trypanosomes cooperates with a kinetoplastid-specific kinesin to maintain cell morphology by regulating subpellicular microtubules

Microtubules are a vital part of the cytoskeleton of eukaryotic cells and are involved in various cellular processes. The cytoskeleton of Trypanosoma brucei is characterized by an array of subpellicular microtubules and is essential for maintenance of cell shape and polarity, but little is known about the regulation of the assembly and organization of the subpellicular microtubule corset. Here, we report that the orphan kinesin TbKIN-D regulates the organization of subpellicular microtubules and is required for maintaining cell morphology. TbKIN-D possesses in vitro ATPase activity, associates with cytoskeletal microtubules and is distributed throughout the cytoskeleton at all cell cycle stages. RNAi of TbKIN-D disrupts the organization of the subpellicular microtubule corset and distorts cell morphology, resulting in round cells with an elongated posterior filled with newly assembled microtubules. Depletion of TbKIN-D also abolishes the segregation of organelles and cytoskeletal structures, suggesting that cellular morphogenesis is essential for proper organelle segregation. Moreover, TbKIN-D deficiency impairs the attachment of the new flagellum without compromising the formation of the flagellum attachment zone. Finally, we identified TbKIN-C, a kinetoplastid-specific kinesin known to regulate subpellicular microtubules and cell morphogenesis in T. brucei, as a partner of TbKIN-D. Further, we demonstrate that interaction between TbKIN-C and TbKIN-D requires the coiled-coil motifs in the C-termini of both proteins. Altogether, our results suggest that TbKIN-D cooperates with TbKIN-C to maintain cell morphology by regulating the organization of the subpellicular microtubule corset.

Structure-function relationship of the Polo-like kinase in Trypanosoma brucei

Polo-like kinases (Plks) play multiple roles in mitosis and cytokinesis in eukaryotes and are characterized by the C-terminal Polo-box domain (PBD), which is implicated in binding to Plk substrates, targeting Plk and regulating Plk activity. The Plk homolog in Trypanosoma brucei (TbPLK) possesses a similar architecture, but it lacks the crucial residues involved in substrate binding and regulates cytokinesis but not mitosis. Little is known about the regulation of TbPLK and the role of the PBD in TbPLK localization and function. Here, we addressed the requirement of the kinase activity and the PBD for TbPLK localization and function through coupling RNAi of endogenous TbPLK with ectopic expression of TbPLK mutants. We demonstrate that the kinase activity and phosphorylation of two threonine residues, Thr198 and Thr202, in the activation loop (T-loop) of the kinase domain are essential for TbPLK function but not for TbPLK localization. Deletion of the PBD abolishes TbPLK localization, but the PBD itself is not correctly targeted, indicating that TbPLK localization requires both the PBD and the kinase domain. Surprisingly, the kinase domain of TbPLK, but not the PBD, binds to its substrates TbCentrin2 and p110, suggesting that TbPLK might interact with its substrate through different mechanisms. Finally, the PBD interacts with the kinase domain of TbPLK and inhibits its activity, and this inhibition is relieved when Thr198 is phosphorylated. Together, these results suggest an essential role of T-loop phosphorylation in TbPLK activation and crucial roles of the PBD in regulating TbPLK activity and localization.

A kinetoplastid-specific kinesin is required for cytokinesis and for maintenance of cell morphology in Trypanosoma brucei

Kinesins are motor-based transport proteins that play diverse roles in various cellular processes. The trypanosome genome lacks the homologues of many conserved mitotic kinesins, but encodes a number of trypanosome-specific kinesins with unknown function. Here, we report the biochemical and functional characterization of TbKIN-C, a trypanosome-specific kinesin, which was initially identified through an RNAi screen for cytokinesis genes in T. brucei. TbKIN-C possesses in vitro ATPase activity and associates with cytoskeletal tubulin microtubules in vivo. It is distributed throughout the cytoskeleton with a focal enrichment at the posterior end of the cell during early cell cycle stages. RNAi of TbKIN-C resulted in distorted cell shape with an elongated posterior filled with tyrosinated tubulin microtubules. Silencing of TbKIN-C impaired the segregation of organelles and cytoskeletal structures and led to detachment of the new flagellum and a small portion of the cytoplasm. We also show that RNAi of TbKIN-C compromised cytokinesis and abolished the trans-localization of TbCPC1, a subunit of the chromosomal passenger complex, from the central spindle to the initiation site of cytokinesis. Our results suggest an essential role of TbKIN-C in maintaining cell morphology, likely through regulating microtubule dynamics at the posterior end of the cell.

ALBA proteins are stage regulated during trypanosome development in the tsetse fly and participate in differentiation

The protozoan parasite Trypanosoma brucei is responsible for sleeping sickness and alternates between mammal and tsetse fly hosts. Two proteins of the ALBA family associate to mRNA in cytoplasmic granules during starvation stress, are stage regulated, and contribute to trypanosome development in the tsetse fly.

The protozoan parasite Trypanosoma brucei is responsible for sleeping sickness and alternates between mammal and tsetse fly hosts, where it has to adapt to different environments. We investigated the role of two members of the ALBA family, which encodes hypothetical RNA-binding proteins conserved in most eukaryotes. We show that ALBA3/4 proteins colocalize with the DHH1 RNA-binding protein and with a subset of poly(A+) RNA in stress granules upon starvation. Depletion of ALBA3/4 proteins by RNA interference in the cultured procyclic stage produces cell modifications mimicking several morphogenetic aspects of trypanosome differentiation that usually take place in the fly midgut. A combination of immunofluorescence data and videomicroscopy analysis of live trypanosomes expressing endogenously ALBA fused with fluorescent proteins revealed that ALBA3/4 are present throughout the development of the parasite in the tsetse fly, with the striking exception of the transition stages found in the proventriculus region. This involves migration of the nucleus toward the posterior end of the cell, a phenomenon that is perturbed upon forced expression of ALBA3 during the differentiation process, showing for the first time the involvement of an RNA-binding protein in trypanosome development in vivo.

The Cdc45·Mcm2-7·GINS protein complex in trypanosomes regulates DNA replication and interacts with two Orc1-like proteins in the origin recognition complex

Accurate DNA replication requires a complex interplay of many regulatory proteins at replication origins. The CMG (Cdc45·Mcm2-7·GINS) complex, which is composed of Cdc45, Mcm2-7, and the GINS (Go-Ichi-Ni-San) complex consisting of Sld5 and Psf1 to Psf3, is recruited by Cdc6 and Cdt1 onto origins bound by the heterohexameric origin recognition complex (ORC) and functions as a replicative helicase. Trypanosoma brucei, an early branched microbial eukaryote, appears to express an archaea-like ORC consisting of a single Orc1/Cdc6-like protein. However, unlike archaea, trypanosomes possess components of the eukaryote-like CMG complex, but whether they form an active helicase complex, associate with the ORC, and regulate DNA replication remains unknown. Here, we demonstrated that the CMG complex is formed in vivo in trypanosomes and that Mcm2-7 helicase activity is activated by the association with Cdc45 and the GINS complex in vitro. Mcm2-7 and GINS proteins are confined to the nucleus throughout the cell cycle, whereas Cdc45 is exported out of the nucleus after DNA replication, indicating that nuclear exclusion of Cdc45 constitutes one mechanism for preventing DNA re-replication in trypanosomes. With the exception of Mcm4, Mcm6, and Psf1, knockdown of individual CMG genes inhibits DNA replication and cell proliferation. Finally, we identified a novel Orc1-like protein, Orc1b, as an additional component of the ORC and showed that both Orc1b and Orc1/Cdc6 associate with Mcm2-7 via interactions with Mcm3. All together, we identified the Cdc45·Mcm2-7·GINS complex as the replicative helicase that interacts with two Orc1-like proteins in the unusual origin recognition complex in trypanosomes.

High-throughput analysis of an RNAi library identifies novel kinase targets in Trypanosoma brucei

New drugs are needed to treat human African trypanosomiasis because the currently approved treatments are toxic or limited in efficacy. One strategy for developing new drugs involves discovering novel genes whose products can be targeted for modulation by small-molecule chemotherapeutic agents. The Trypanosoma brucei genome contains many genes with the potential to become such targets. Kinases represent one group of genes that regulate many important cell functions and can be modulated by small molecules, thus representing a promising group of enzymes to screen for potential therapeutic targets. RNAi screens could help identify the most promising kinase targets, but the lack of suitable assays represents a barrier for optimizing the use of this technology in T. brucei. Here, we describe an RNAi screen of a small RNAi library targeting 30 members of the T. brucei kinome utilizing a luciferase-based assay. This screen both validated the luciferase-based assay as a suitable method for conducting RNAi screens in T. brucei and also identified two kinases (CRK12 and ERK8) that are essential for normal proliferation by the parasite.

Trypanosoma brucei: reduction of GPI-phospholipase C protein during differentiation is dependent on replication of newly transformed cells

The protozoan parasite Trypanosoma brucei lives in the bloodstream of vertebrates or in a tsetse fly. Expression of a GPI-phospholipase C polypeptide (GPI-PLCp) in the parasite is restricted to the bloodstream form. Events controlling the amount of GPI-PLCp expressed during differentiation are not completely understood. Our metabolic "pulse-chase" analysis reveals that GPI-PLCp is stable in bloodstream form. However, during differentiation of bloodstream to insect stage (procyclic) T. brucei, translation GPI-PLC mRNA ceases within 8h of initiating transformation. GPI-PLCp is not lost precipitously from newly transformed procyclic trypanosomes. Nascent procyclics contain 400-fold more GPI-PLCp than established insect stage T. brucei. Reduction of GPI-PLCp in early-stage procyclics is linked to parasite replication. Sixteen cell divisions are required to reduce the amount of GPI-PLCp in newly differentiated procyclics to levels present in the established procyclic. GPI-PLCp is retained in strains of T. brucei that fail to replicate after differentiation of the bloodstream to the procyclic form. Thus, at least two factors control levels of GPI-PLCp during differentiation of bloodstream T. brucei; (i) repression of GPI-PLC mRNA translation, and (ii) sustained replication of newly transformed procyclic T. brucei. These studies illustrate the importance of repeated cell divisions in controlling the steady-state amount of GPI-PLCp during differentiation of the African trypanosome.

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 F-box protein CFB2 is required for cytokinesis of bloodstream-form Trypanosoma brucei

F-box proteins serve as mediators in targeting bound target proteins for ubiquitination and destruction. We here describe the roles of two F-box proteins, CFB1 and CFB2, in the trypanosome cell cycle. Five almost identical copies of CFB1 are arranged in a direct tandem repeat on Trypanosoma brucei chromosome 1; immediately downstream is a single CFB2 gene. RNAi targeting CFB1 in bloodstream-form trypanosomes had a transient effect on growth and mitosis. Depletion of CFB2, in contrast, resulted in immediate growth arrest and rapid cell death. CFB2-depleted cells accumulated nuclei and kinetoplasts with the corresponding numbers of basal bodies and flagella. The CFB2 transcript was less abundant in procyclic-form trypanosomes, and RNAi against CFB2 in these forms had no effect on growth. These results suggest that CFB2 is required for bloodstream-form trypanosome cytokinesis.

Comparative analysis of the kinomes of three pathogenic trypanosomatids: Leishmania major, Trypanosoma brucei and Trypanosoma cruzi

Background

The trypanosomatids Leishmania major, Trypanosoma brucei and Trypanosoma cruzi cause some of the most debilitating diseases of humankind: cutaneous leishmaniasis, African sleeping sickness, and Chagas disease. These protozoa possess complex life cycles that involve development in mammalian and insect hosts, and a tightly coordinated cell cycle ensures propagation of the highly polarized cells. However, the ways in which the parasites respond to their environment and coordinate intracellular processes are poorly understood. As a part of an effort to understand parasite signaling functions, we report the results of a genome-wide analysis of protein kinases (PKs) of these three trypanosomatids.

Results

Bioinformatic searches of the trypanosomatid genomes for eukaryotic PKs (ePKs) and atypical PKs (aPKs) revealed a total of 176 PKs in T. brucei, 190 in T. cruzi and 199 in L. major, most of which are orthologous across the three species. This is approximately 30% of the number in the human host and double that of the malaria parasite, Plasmodium falciparum. The representation of various groups of ePKs differs significantly as compared to humans: trypanosomatids lack receptor-linked tyrosine and tyrosine kinase-like kinases, although they do possess dual-specificity kinases. A relative expansion of the CMGC, STE and NEK groups has occurred. A large number of unique ePKs show no strong affinity to any known group. The trypanosomatids possess few ePKs with predicted transmembrane domains, suggesting that receptor ePKs are rare. Accessory Pfam domains, which are frequently present in human ePKs, are uncommon in trypanosomatid ePKs.

Conclusion

Trypanosomatids possess a large set of PKs, comprising approximately 2% of each genome, suggesting a key role for phosphorylation in parasite biology. Whilst it was possible to place most of the trypanosomatid ePKs into the seven established groups using bioinformatic analyses, it has not been possible to ascribe function based solely on sequence similarity. Hence the connection of stimuli to protein phosphorylation networks remains enigmatic. The presence of numerous PKs with significant sequence similarity to known drug targets, as well as a large number of unusual kinases that might represent novel targets, strongly argue for functional analysis of these molecules.

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.

Distinct cytoskeletal modulation and regulation of G1-S transition in the two life stages of Trypanosoma brucei

Procyclic-form Trypanosoma brucei is arrested in G1 phase with extended and/or branched posterior morphology when expression of its cdc2-related kinases 1 and 2 (CRK1 and CRK2) is knocked down by RNA interference. Transmission electron microscopy indicated that the mitochondrion in the cell is also extended and branched and associated with cortical microtubules in each elongated/branched posterior end. This posterior extension is apparently driven by the growing microtubule corset, as it can be blocked by rhizoxin, an inhibitor of microtubule assembly. In the bloodstream form of T. brucei, however, a knockdown of CRK1 and CRK2 resulted only in an enrichment of cells in G1 phase without cessation of DNA synthesis or elongated/branched posterior ends. A triple knockdown of CRK1, CRK2 and CycE1/CYC2 in the bloodstream form resulted in 15% of the cells arrested in G1 phase, but no cells had an abnormal posterior morphology. The double and triple knockdown bloodstream-form cells were differentiated in vitro into the procyclic form, and the latter thus generated bore the typical morphology of a procyclic form without an extended/branched posterior end, albeit arrested in the G1 phase as the bloodstream-form precursor. There is thus a major distinction in the mechanisms regulating G1-S transition and posterior morphogenesis between the two life stages of T. brucei.