Basal body positioning is controlled by flagellum formation in Trypanosoma brucei

DNA segregation in mitochondria and beyond: insights from the trypanosomal tripartite attachment complex

The tripartite attachment complex (TAC) of the single mitochondrion of trypanosomes allows precise segregation of its single nucleoid mitochondrial genome during cytokinesis. It couples the segregation of the duplicated mitochondrial genome to the segregation of the basal bodies of the flagella. Here, we provide a model of the molecular architecture of the TAC that explains how its eight essential subunits connect the basal body, across the mitochondrial membranes, with the mitochondrial genome. We also discuss how the TAC subunits are imported into the mitochondrion and how they assemble to form a new TAC. Finally, we present a comparative analysis of the trypanosomal TAC with open and closed mitotic spindles, which reveals conserved concepts between these diverse DNA segregation systems.

The microtubule quartet protein SNAP1 in Trypanosoma brucei facilitates flagellum and cell division plane positioning by promoting basal body segregation

The unicellular protozoan Trypanosoma brucei has a single flagellum that is involved in cell motility, cell morphogenesis, and cell division. Inheritance of the newly assembled flagellum during the cell cycle requires its correct positioning, which depends on the faithful duplication or segregation of multiple flagellum-associated cytoskeletal structures, including the basal body, the flagellum attachment zone, and the hook complex. Along the flagellum attachment zone sites a set of four microtubules termed the microtubule quartet (MtQ), whose molecular function remains enigmatic. We recently reported that the MtQ-localized protein NHL1 interacts with the microtubule-binding protein TbSpef1 and regulates flagellum inheritance by promoting basal body rotation and segregation. Here, we identified a TbSpef1- and NHL1-associated protein named SNAP1, which co-localizes with NHL1 and TbSpef1 at the proximal portion of the MtQ, depends on TbSpef1 for localization and is required for NHL1 localization to the MtQ. Knockdown of SNAP1 impairs the rotation and segregation of the basal body, the elongation of the flagellum attachment zone filament, and the positioning of the newly assembled flagellum, thereby causing mis-placement of the cell division plane, a halt in cleavage furrow ingression, and an inhibition of cytokinesis completion. Together, these findings uncover a coordinating role of SNAP1 with TbSpef1 and NHL1 in facilitating flagellum positioning and cell division plane placement for the completion of cytokinesis.

TFK1, a basal body transition fibre protein that is essential for cytokinesis in Trypanosoma brucei

In Trypanosoma brucei, transition fibres (TF) form a nine-bladed pattern-like structure connecting the base of the flagellum to the flagellar pocket membrane. Despite the characterization of two TF proteins, CEP164C and TbRP2, little is known about the organization of these fibres. Here, we report the identification and characterization of the first kinetoplastid-specific TF protein named TFK1 (Tb927.6.1180). Bioinformatics and functional domain analysis identified three TFK1 distinct domains: an N-terminal domain of an unpredicted function, a coiled-coil domain involved in TFK1-TFK1 interaction and a C-terminal intrinsically disordered region potentially involved in protein interaction. Cellular immuno-localization showed that TFK1 is a newly identified basal body maturation marker. Further, using ultrastructure expansion and immuno-electron microscopies we localized CEP164C and TbRP2 at the TF and TFK1 on the distal appendage matrix of the TF. Importantly, RNAi knockdown of TFK1 in bloodstream form cells induced misplacement of basal bodies, a defect in the furrow or fold generation and eventually cell death. We hypothesize that TFK1 is a basal body positioning specific actor and a key regulator of cytokinesis in the bloodstream form Trypanosoma brucei.

A specific basal body linker protein provides the connection function for basal body inheritance in trypanosomes

Centrioles and basal bodies (CBBs) are found in physically linked pairs, and in mammalian cells intercentriole connections (G1-G2 tether and S-M linker) regulate centriole duplication and function. In trypanosomes BBs are not associated with the spindle and function in flagellum/cilia nucleation with an additional role in mitochondrial genome (kinetoplast DNA [kDNA]) segregation. Here, we describe BBLP, a BB/pro-BB (pBB) linker protein in Trypanosoma brucei predicted to be a large coiled-coil protein conserved in the kinetoplastida. Colocalization with the centriole marker SAS6 showed that BBLP localizes between the BB/pBB pair, throughout the cell cycle, with a stronger signal in the old flagellum BB/pBB pair. Importantly, RNA interference (RNAi) depletion of BBLP leads to a conspicuous splitting of the BB/pBB pair associated only with the new flagellum. BBLP RNAi is lethal in the bloodstream form of the parasite and perturbs mitochondrial kDNA inheritance. Immunogold labeling confirmed that BBLP is localized to a cytoskeletal component of the BB/pBB linker, and tagged protein induction showed that BBLP is incorporated de novo in both new and old flagella BB pairs of dividing cells. We show that the two aspects of CBB disengagement-loss of orthogonal orientation and ability to separate and move apart-are consistent but separable events in evolutionarily diverse cells and we provide a unifying model explaining centriole/BB linkage differences between such cells.

Casein kinase TbCK1.2 regulates division of kinetoplast DNA, and movement of basal bodies in the African trypanosome

The single mitochondrial nucleoid (kinetoplast) of Trypanosoma brucei is found proximal to a basal body (mature (mBB)/probasal body (pBB) pair). Kinetoplast inheritance requires synthesis of, and scission of kinetoplast DNA (kDNA) generating two kinetoplasts that segregate with basal bodies into daughter cells. Molecular details of kinetoplast scission and the extent to which basal body separation influences the process are unavailable. To address this topic, we followed basal body movements in bloodstream trypanosomes following depletion of protein kinase TbCK1.2 which promotes kinetoplast division. In control cells we found that pBBs are positioned 0.4 um from mBBs in G1, and they mature after separating from mBBs by at least 0.8 um: mBB separation reaches ~2.2 um. These data indicate that current models of basal body biogenesis in which pBBs mature in close proximity to mBBs may need to be revisited. Knockdown of TbCK1.2 produced trypanosomes containing one kinetoplast and two nuclei (1K2N), increased the percentage of cells with uncleaved kDNA 400%, decreased mBB spacing by 15%, and inhibited cytokinesis 300%. We conclude that (a) separation of mBBs beyond a threshold of 1.8 um correlates with division of kDNA, and (b) TbCK1.2 regulates kDNA scission. We propose a Kinetoplast Division Factor hypothesis that integrates these data into a pathway for biogenesis of two daughter mitochondrial nucleoids.

The kinetoplastid-specific phosphatase KPP1 attenuates PLK activity to facilitate flagellum inheritance in Trypanosoma brucei

Trypanosoma brucei, an important human parasite, has a flagellum that controls cell motility, morphogenesis, proliferation, and cell-cell communication. Inheritance of the newly assembled flagellum during the cell cycle requires the Polo-like kinase homolog TbPLK and the kinetoplastid-specific protein phosphatase KPP1, although whether TbPLK acts on KPP1 or vice versa has been unclear. Here, we showed that dephosphorylation of TbPLK on Thr¹²⁵ by KPP1 maintained low TbPLK activity in the flagellum-associated hook complex structure, thereby ensuring proper flagellum positioning and attachment. This dephosphorylation event required the recognition of phosphorylated Thr¹⁹⁸ in the activation loop of TbPLK by the N-terminal Plus3 domain of KPP1 and the dephosphorylation of phosphorylated Thr¹²⁵ in TbPLK by the C-terminal catalytic domain of KPP1. Dephosphorylation of TbPLK by KPP1 prevented hyperphosphorylation of the hook complex protein TbCentrin2, thereby allowing timely dephosphorylation of phosphorylated TbCentrin2 for hook complex duplication and flagellum positioning and attachment. Thus, KPP1 attenuates TbPLK activity by dephosphorylating TbPLK to facilitate flagellum inheritance.

Timing and original features of flagellum assembly in trypanosomes during development in the tsetse fly

Background

Trypanosoma brucei exhibits a complex life-cycle alternating between tsetse flies and mammalian hosts. When parasites infect the fly, cells differentiate to adapt to life in various tissues, which is accompanied by drastic morphological and biochemical modifications especially in the proventriculus. This key step represents a bottleneck for salivary gland infection.

Methods

Here, we monitored flagellum assembly in trypanosomes during differentiation from the trypomastigote to the epimastigote stage, i.e. when the nucleus migrates to the posterior end of the cell, by using three-dimensional electron microscopy (focused ion beam scanning electron microscopy, FIB-SEM) and immunofluorescence assays.

Results

The combination of light and electron microscopy approaches provided structural and molecular evidence that the new flagellum is assembled while the nucleus migrates towards the posterior region of the body. Two major differences with well-known procyclic cells are reported. First, growth of the new flagellum begins when the associated basal body is found in a posterior position relative to the mature flagellum. Secondly, the new flagellum acquires its own flagellar pocket before rotating on the left side of the anterior-posterior axis. FIB-SEM revealed the presence of a structure connecting the new and mature flagellum and serial sectioning confirmed morphological similarities with the flagella connector of procyclic cells. We discuss the potential function of the flagella connector in trypanosomes from the proventriculus.

Conclusions

These findings show that T. brucei finely modulates its cytoskeletal components to generate highly variable morphologies.

BOH1 cooperates with Polo-like kinase to regulate flagellum inheritance and cytokinesis initiation in Trypanosoma brucei

Trypanosoma brucei possesses a motile flagellum that determines cell morphology and the cell division plane. Inheritance of the newly assembled flagellum during the cell cycle is controlled by the Polo-like kinase homolog TbPLK, which also regulates cytokinesis initiation. How TbPLK is targeted to its subcellular locations remains poorly understood. Here we report the trypanosome-specific protein BOH1 that cooperates with TbPLK to regulate flagellum inheritance and cytokinesis initiation in the procyclic form of T. brucei BOH1 localizes to an unusual sub-domain in the flagellum-associated hook complex, bridging the hook complex, the centrin arm and the flagellum attachment zone. Depletion of BOH1 disrupts hook-complex morphology, inhibits centrin-arm elongation and abolishes flagellum attachment zone assembly, leading to flagellum mis-positioning and detachment. Further, BOH1 deficiency impairs the localization of TbPLK and the cytokinesis regulator CIF1 to the cytokinesis initiation site, providing a molecular mechanism for its role in cytokinesis initiation. These findings reveal the roles of BOH1 in maintaining hook-complex morphology and regulating flagellum inheritance, and establish BOH1 as an upstream regulator of the TbPLK-mediated cytokinesis regulatory pathway.

Indirubin Analogues Inhibit Trypanosoma brucei Glycogen Synthase Kinase 3 Short and T. brucei Growth

The protozoan parasite Trypanosoma brucei is the causative agent of human African trypanosomiasis (HAT). The disease is fatal if it remains untreated, whereas most drug treatments are inadequate due to high toxicity, difficulties in administration, and low central nervous system penetration. T. brucei glycogen synthase kinase 3 short (TbGSK3s) is essential for parasite survival and thus represents a potential drug target that could be exploited for HAT treatment. Indirubins, effective leishmanicidals, provide a versatile scaffold for the development of potent GSK3 inhibitors. Herein, we report on the screening of 69 indirubin analogues against T. brucei bloodstream forms. Of these, 32 compounds had potent antitrypanosomal activity (half-maximal effective concentration = 0.050 to 3.2 μM) and good selectivity for the analogues over human HepG2 cells (range, 7.4- to over 641-fold). The majority of analogues were potent inhibitors of TbGSK3s, and correlation studies for an indirubin subset, namely, the 6-bromosubstituted 3'-oxime bearing an extra bulky substituent on the 3' oxime [(6-BIO-3'-bulky)-substituted indirubins], revealed a positive correlation between kinase inhibition and antitrypanosomal activity. Insights into this indirubin-TbGSK3s interaction were provided by structure-activity relationship studies. Comparison between 6-BIO-3'-bulky-substituted indirubin-treated parasites and parasites silenced for TbGSK3s by RNA interference suggested that the above-described compounds may target TbGSK3s in vivo To further understand the molecular basis of the growth arrest brought about by the inhibition or ablation of TbGSK3s, we investigated the intracellular localization of TbGSK3s. TbGSK3s was present in cytoskeletal structures, including the flagellum and basal body area. Overall, these results give insights into the mode of action of 6-BIO-3'-bulky-substituted indirubins that are promising hits for antitrypanosomal drug discovery.

Dependency relationships between IFT-dependent flagellum elongation and cell morphogenesis in Leishmania

Flagella have multiple functions that are associated with different axonemal structures. Motile flagella typically have a 9 + 2 arrangement of microtubules, whereas sensory flagella normally have a 9 + 0 arrangement. Leishmania exhibits both of these flagellum forms and differentiation between these two flagellum forms is associated with cytoskeletal and cell shape changes. We disrupted flagellum elongation in Leishmania by deleting the intraflagellar transport (IFT) protein IFT140 and examined the effects on cell morphogenesis. Δift140 cells have no external flagellum, having only a very short flagellum within the flagellar pocket. This short flagellum had a collapsed 9 + 0 (9v) axoneme configuration reminiscent of that in the amastigote and was not attached to the pocket membrane. Although amastigote-like changes occurred in the flagellar cytoskeleton, the cytoskeletal structures of Δift140 cells retained their promastigote configurations, as examined by fluorescence microscopy of tagged proteins and serial electron tomography. Thus, Leishmania promastigote cell morphogenesis does not depend on the formation of a long flagellum attached at the neck. Furthermore, our data show that disruption of the IFT system is sufficient to produce a switch from the 9 + 2 to the collapsed 9 + 0 (9v) axonemal structure, echoing the process that occurs during the promastigote to amastigote differentiation.

An orphan kinesin controls trypanosome morphology transitions by targeting FLAM3 to the flagellum

Trypanosoma brucei undergoes life cycle form transitions from trypomastigotes to epimastigotes in the insect vector by re-positioning the mitochondrial genome and re-locating the flagellum and flagellum-associated cytoskeletal structures. The mechanism underlying these dramatic morphology transitions remains poorly understood. Here we report the regulatory role of the orphan kinesin KIN-E in controlling trypanosome morphology transitions. KIN-E localizes to the flagellum and is enriched at the flagellar tip, and this localization depends on the C-terminal m-calpain domain III-like domains. Depletion of KIN-E in the trypomastigote form of T. brucei causes major morphology changes and a gradual increase in the level of EP procyclin, generating epimastigote-like cells. Mechanistically, through its C-terminal importin α-like domain, KIN-E targets FLAM3, a flagellar protein involved in morphology transitions, to the flagellum to promote elongation of the flagellum attachment zone and positioning of the flagellum and flagellum-associated cytoskeletal structure, thereby maintaining trypomastigote cell morphology. Our findings suggest that morphology transitions in trypanosomes require KIN-E-mediated transport of FLAM3 to the flagellum.

Trypanosoma brucei, the causative agent of sleeping sickness in humans and nagana in cattle in sub-Saharan Africa, has a complex life cycle by alternating between the tsetse fly vector and the mammalian hosts. In the gut of tsetse flies, trypanosomes undergo life cycle transitions from the trypomastigote form to the epimastigote form by re-positioning the mitochondrial genome and re-locating the flagellum and flagellum-associated cytoskeletal structures. Previous work demonstrated that elongation of the flagellum attachment zone plays an important role in controlling morphology transitions, but how it is regulated remains poorly understood. This work discovered that an orphan kinesin plays an essential role in regulating trypanosome morphology transitions. This novel kinesin localizes to the flagellum and targets FLAM3, one of the two flagellar proteins involved in morphology transitions, to the flagellum. This work suggests that trypanosome morphology transitions require kinesin-mediated transport of FLAM3 to the flagellum to promote the elongation of the flagellum attachment zone, thereby maintaining flagellum-cell body attachment and positioning the flagellum and flagellum-associated cytoskeletal structures to assume trypomastigote cell morphology.

Flagellum inheritance in Trypanosoma brucei requires a kinetoplastid-specific protein phosphatase

Trypanosoma brucei causes sleeping sickness in humans and nagana in cattle in sub-Saharan Africa and alternates between its mammalian hosts and its insect vector, the tsetse fly. T. brucei uses a flagellum for motility, cell division, and cell-cell communication. Proper positioning and attachment of the newly assembled flagellum rely on the faithful duplication and segregation of flagellum-associated cytoskeletal structures. These processes are regulated by the polo-like kinase homolog TbPLK, whose activity and abundance are under stringent control to ensure spatiotemporally regulated phosphorylation of its substrates. However, it remains unclear whether a protein phosphatase that counteracts TbPLK activity is also involved in this regulation. Here, we report that a putative kinetoplastid-specific protein phosphatase, named KPP1, has essential roles in regulating flagellum positioning and attachment in T. brucei KPP1 localized to multiple flagellum-associated cytoskeletal structures and co-localized with TbPLK in several cytoskeletal structures at different cell-cycle stages. KPP1 depletion abolished basal body segregation, inhibited the duplication of the centrin arm and the hook complex of the bilobe structure, and disrupted the elongation of the flagellum attachment zone, leading to flagellum misplacement and detachment and cytokinesis arrest. Importantly, KPP1-depleted cells lacked dephosphorylation of TbCentrin2, a TbPLK substrate, at late cell-cycle stages. Together, these results suggest that KPP1-mediated protein dephosphorylation regulates the duplication and segregation of flagellum-associated cytoskeletal structures, thereby promoting flagellum positioning and attachment. These findings highlight the requirement of reversible protein phosphorylation, mediated by TbPLK and KPP1, in regulating flagellum inheritance in T. brucei.

Giant FAZ10 is required for flagellum attachment zone stabilization and furrow positioning in Trypanosoma brucei

The flagellum and flagellum attachment zone (FAZ) are important cytoskeletal structures in trypanosomatids, being required for motility, cell division and cell morphogenesis. Trypanosomatid cytoskeletons contain abundant high molecular mass proteins (HMMPs), but many of their biological functions are still unclear. Here, we report the characterization of the giant FAZ protein, FAZ10, in Trypanosoma brucei, which, using immunoelectron microscopy, we show localizes to the intermembrane staples in the FAZ intracellular domain. Our data show that FAZ10 is a giant cytoskeletal protein essential for normal growth and morphology in both procyclic and bloodstream parasite life cycle stages, with its depletion leading to defects in cell morphogenesis, flagellum attachment, and kinetoplast and nucleus positioning. We show that the flagellum attachment defects are probably brought about by reduced tethering of the proximal domain of the paraflagellar rod to the FAZ filament. Further, FAZ10 depletion also reduces abundance of FAZ flagellum domain protein, ClpGM6. Moreover, ablation of FAZ10 impaired the timing and placement of the cleavage furrow during cytokinesis, resulting in premature or asymmetrical cell division.

CRL4WDR1 Controls Polo-like Kinase Protein Abundance to Promote Bilobe Duplication, Basal Body Segregation and Flagellum Attachment in Trypanosoma brucei

The Polo-like kinase homolog in Trypanosoma brucei, TbPLK, plays essential roles in basal body segregation, flagellum attachment and cytokinesis. The level of TbPLK protein is tightly controlled, but the underlying mechanism remains elusive. Here, we report a Cullin-RING ubiquitin ligase composed of Cullin4, the DNA damage-binding protein 1 homolog TbDDB1 and a WD40-repeat protein WDR1 that controls TbPLK abundance in the basal body and the bilobe. WDR1, through its C-terminal domain, interacts with the PEST motif in TbPLK and, through its N-terminal WD40 motif, binds to TbDDB1. Depletion of WDR1 inhibits bilobe duplication and basal body segregation, disrupts the assembly of the new flagellum attachment zone filament and detaches the new flagellum. Consistent with its role in TbPLK degradation, depletion of WDR1 causes excessive accumulation of TbPLK in the basal body and the bilobe, leading to continuous phosphorylation of TbCentrin2 in the bilobe at late cell cycle stages. Together, these results identify a novel WD40-repeat protein as a TbPLK receptor in the Cullin4-DDB1 ubiquitin ligase complex for degrading TbPLK in the basal body and the bilobe after the G1/S cell cycle transition, thereby promoting bilobe duplication, basal body separation and flagellum-cell body adhesion.

Intraflagellar transport is required for the maintenance of the trypanosome flagellum composition but not its length

Intraflagellar transport (IFT) is required for construction of most cilia and flagella. Here, we used electron microscopy, immunofluorescence and live video microscopy to show that IFT is absent or arrested in the mature flagellum of Trypanosoma brucei upon RNA interference (RNAi)-mediated knockdown of IFT88 and IFT140, respectively. Flagella assembled prior to RNAi did not shorten, showing that IFT is not essential for the maintenance of flagella length. Although the ultrastructure of the axoneme was not visibly affected, flagellar beating was strongly reduced and the distribution of several flagellar components was drastically modified. The R subunit of the protein kinase A was no longer concentrated in the flagellum but was largely found in the cell body whereas the kinesin 9B motor was accumulating at the distal tip of the flagellum. In contrast, the distal tip protein FLAM8 was dispersed along the flagellum. This reveals that IFT also functions in maintaining the distribution of some flagellar proteins after construction of the organelle is completed.

Motility, Force Generation, and Energy Consumption of Unicellular Parasites

Motility is a key factor for pathogenicity of unicellular parasites, enabling them to infiltrate and evade host cells, and perform several of their life-cycle events. State-of-the-art methods of motility analysis rely on a combination of optical tweezers with high-resolution microscopy and microfluidics. With this technology, propulsion forces, energies, and power generation can be determined so as to shed light on the motion mechanisms, chemotactic behavior, and specific survival strategies of unicellular parasites. With these new tools in hand, we can elucidate the mechanisms of motility and force generation of unicellular parasites, and identify ways to manipulate and eventually inhibit them.

Basal body structure and cell cycle-dependent biogenesis in Trypanosoma brucei

Basal bodies are microtubule-based organelles that assemble cilia and flagella, which are critical for motility and sensory functions in all major eukaryotic lineages. The core structure of the basal body is highly conserved, but there is variability in biogenesis and additional functions that are organism and cell type specific. Work carried out in the protozoan parasite Trypanosoma brucei has arguably produced one of the most detailed dissections of basal body structure and biogenesis within the context of the flagellar pocket and associated organelles. In this review, we provide a detailed overview of the basic basal body structure in T. brucei along with the accessory structures and show how basal body movements during the basal body duplication cycle orchestrate cell and organelle morphogenesis. With this in-depth three-dimensional knowledge, identification of many basal body genes coupled with excellent genetic tools makes it an attractive model organism to study basal body biogenesis and maintenance.

Knockdown of Inner Arm Protein IC138 in Trypanosoma brucei Causes Defective Motility and Flagellar Detachment

Motility in the protozoan parasite Trypanosoma brucei is conferred by a single flagellum, attached alongside the cell, which moves the cell forward using a beat that is generated from tip-to-base. We are interested in characterizing components that regulate flagellar beating, in this study we extend the characterization of TbIC138, the ortholog of a dynein intermediate chain that regulates axonemal inner arm dynein f/I1. TbIC138 was tagged In situ-and shown to fractionate with the inner arm components of the flagellum. RNAi knockdown of TbIC138 resulted in significantly reduced protein levels, mild growth defect and significant motility defects. These cells tended to cluster, exhibited slow and abnormal motility and some cells had partially or fully detached flagella. Slight but significant increases were observed in the incidence of mis-localized or missing kinetoplasts. To document development of the TbIC138 knockdown phenotype over time, we performed a detailed analysis of flagellar detachment and motility changes over 108 hours following induction of RNAi. Abnormal motility, such as slow twitching or irregular beating, was observed early, and became progressively more severe such that by 72 hours-post-induction, approximately 80% of the cells were immotile. Progressively more cells exhibited flagellar detachment over time, but this phenotype was not as prevalent as immotility, affecting less than 60% of the population. Detached flagella had abnormal beating, but abnormal beating was also observed in cells with no flagellar detachment, suggesting that TbIC138 has a direct, or primary, effect on the flagellar beat, whereas detachment is a secondary phenotype of TbIC138 knockdown. Our results are consistent with the role of TbIC138 as a regulator of motility, and has a phenotype amenable to more extensive structure-function analyses to further elucidate its role in the control of flagellar beat in T. brucei.

SAS-4 Protein in Trypanosoma brucei Controls Life Cycle Transitions by Modulating the Length of the Flagellum Attachment Zone Filament

The evolutionarily conserved centriole/basal body protein SAS-4 regulates centriole duplication in metazoa and basal body duplication in flagellated and ciliated organisms. Here, we report that the SAS-4 homolog in the flagellated protozoan Trypanosoma brucei, TbSAS-4, plays an unusual role in controlling life cycle transitions by regulating the length of the flagellum attachment zone (FAZ) filament, a specialized cytoskeletal structure required for flagellum adhesion and cell morphogenesis. TbSAS-4 is concentrated at the distal tip of the FAZ filament, and depletion of TbSAS-4 in the trypomastigote form disrupts the elongation of the new FAZ filament, generating cells with a shorter FAZ associated with a longer unattached flagellum and repositioned kinetoplast and basal body, reminiscent of epimastigote-like morphology. Further, we show that TbSAS-4 associates with six additional FAZ tip proteins, and depletion of TbSAS-4 disrupts the enrichment of these FAZ tip proteins at the new FAZ tip, suggesting a role of TbSAS-4 in maintaining the integrity of this FAZ tip protein complex. Together, these results uncover a novel function of TbSAS-4 in regulating the length of the FAZ filament to control basal body positioning and life cycle transitions in T. brucei.

A Novel Basal Body Protein That Is a Polo-like Kinase Substrate Is Required for Basal Body Segregation and Flagellum Adhesion in Trypanosoma brucei

The Polo-like kinase (PLK) in Trypanosoma brucei plays multiple roles in basal body segregation, flagellum attachment, and cytokinesis. However, the mechanistic role of TbPLK remains elusive, mainly because most of its substrates are not known. Here, we report a new substrate of TbPLK, SPBB1, and its essential roles in T. brucei. SPBB1 was identified through yeast two-hybrid screening with the kinase-dead TbPLK as the bait. It interacts with TbPLK in vitro and in vivo, and is phosphorylated by TbPLK in vitro. SPBB1 localizes to both the mature basal body and the probasal body throughout the cell cycle, and co-localizes with TbPLK at the basal body during early cell cycle stages. RNAi against SPBB1 in procyclic trypanosomes inhibited basal body segregation, disrupted the new flagellum attachment zone filament, detached the new flagellum, and caused defective cytokinesis. Moreover, RNAi of SPBB1 confined TbPLK at the basal body and the bilobe structure, resulting in constitutive phosphorylation of TbCentrin2 at the bilobe. Altogether, these results identified a basal body protein as a TbPLK substrate and its essential role in promoting basal body segregation and flagellum attachment zone filament assembly for flagellum adhesion and cytokinesis initiation.

γ-Tubulin complex in Trypanosoma brucei: molecular composition, subunit interdependence and requirement for axonemal central pair protein assembly

γ-Tubulin complex constitutes a key component of the microtubule-organizing center and nucleates microtubule assembly. This complex differs in complexity in different organisms: the budding yeast contains the γ-tubulin small complex (γTuSC) composed of γ-tubulin, gamma-tubulin complex protein (GCP)2 and GCP3, whereas animals contain the γ-tubulin ring complex (γTuRC) composed of γTuSC and three additional proteins, GCP4, GCP5 and GCP6. In Trypanosoma brucei, the composition of the γ-tubulin complex remains elusive, and it is not known whether it also regulates assembly of the subpellicular microtubules and the spindle microtubules. Here we report that the γ-tubulin complex in T. brucei is composed of γ-tubulin and three GCP proteins, GCP2-GCP4, and is primarily localized in the basal body throughout the cell cycle. Depletion of GCP2 and GCP3, but not GCP4, disrupted the axonemal central pair microtubules, but not the subpellicular microtubules and the spindle microtubules. Furthermore, we showed that the γTuSC is required for assembly of two central pair proteins and that γTuSC subunits are mutually required for stability. Together, these results identified an unusual γ-tubulin complex in T. brucei, uncovered an essential role of γTuSC in central pair protein assembly, and demonstrated the interdependence of individual γTuSC components for maintaining a stable complex.

Assembly and maintenance of the flagellum attachment zone filament in Trypanosoma brucei

Adhesion of motile flagella to the cell body in Trypanosoma brucei requires a filamentous cytoskeletal structure termed the flagellum attachment zone (FAZ). Despite its essentiality, the complete molecular composition of the FAZ filament and its roles in FAZ filament assembly remain poorly understood. By localization-based screening, we here identified a new FAZ protein, which we called FAZ2. Knockdown of FAZ2 disrupted the FAZ filament, destabilized multiple FAZ filament proteins and caused a cytokinesis defect. We also showed that FAZ2 depletion destabilized another new FAZ filament protein and several flagellum and cytoskeleton proteins. Furthermore, we identified CC2D and KMP11 as FAZ2 partners through affinity purification, and showed that they are each required for maintaining a stable complex. Finally, we demonstrated that FAZ filament proteins are incorporated into the FAZ filament from the proximal region, in contrast to the flagellum components, which are incorporated from the distal tip. In summary, we identified three new FAZ filament proteins and a FAZ filament protein complex, and our results suggest that assembly of the FAZ filament occurs at the proximal region and is essential to maintain the stability of FAZ filament proteins.

The bi-lobe-associated LRRP1 regulates Ran activity in Trypanosoma brucei

Cilia and flagella are conserved eukaryotic organelles important for motility and sensory. The RanGTPase, best known for nucleocytoplasmic transport functions, may also play a role in protein trafficking into the specialized flagellar/ciliary compartments, although the regulatory mechanisms controlling Ran activity at the flagellum remain unclear. The unicellular parasite Trypanosoma brucei contains a single flagellum necessary for cell movement, division and morphogenesis. Correct flagellum functions require flagellar attachment to the cell body, which is mediated by a specialized flagellum attachment zone (FAZ) complex that is assembled together with the flagellum during the cell cycle. We have previously identified the leucine-rich-repeat protein 1 LRRP1 on a bi-lobe structure at the proximal base of flagellum and FAZ. LRRP1 is essential for bi-lobe and FAZ biogenesis, consequently affecting flagellum-driven cell motility and division. Here, we show that LRRP1 forms a complex with Ran and a Ran-binding protein, and regulates Ran-GTP hydrolysis in T. brucei. In addition to mitotic inhibition, depletion of Ran inhibits FAZ assembly in T. brucei, supporting the presence of a conserved mechanism that involves Ran in the regulation of flagellum functions in an early divergent eukaryote.

The intraflagellar transport dynein complex of trypanosomes is made of a heterodimer of dynein heavy chains and of light and intermediate chains of distinct functions

Cilia and flagella are assembled by intraflagellar transport (IFT) of protein complexes that bring tubulin and other precursors to the incorporation site at their distal tip. Anterograde transport is driven by kinesin, whereas retrograde transport is ensured by a specific dynein. In the protist Trypanosoma brucei, two distinct genes encode fairly different dynein heavy chains (DHCs; ∼40% identity) termed DHC2.1 and DHC2.2, which form a heterodimer and are both essential for retrograde IFT. The stability of each heavy chain relies on the presence of a dynein light intermediate chain (DLI1; also known as XBX-1/D1bLIC). The presence of both heavy chains and of DLI1 at the base of the flagellum depends on the intermediate dynein chain DIC5 (FAP133/WDR34). In the IFT140(RNAi) mutant, an IFT-A protein essential for retrograde transport, the IFT dynein components are found at high concentration at the flagellar base but fail to penetrate the flagellar compartment. We propose a model by which the IFT dynein particle is assembled in the cytoplasm, reaches the base of the flagellum, and associates with the IFT machinery in a manner dependent on the IFT-A complex.

Motility and more: the flagellum of Trypanosoma brucei

Trypanosoma brucei is a pathogenic unicellular eukaryote that infects humans and other mammals in sub-Saharan Africa. A central feature of trypanosome biology is the single flagellum of the parasite, which is an essential and multifunctional organelle that facilitates cell propulsion, controls cell morphogenesis and directs cytokinesis. Moreover, the flagellar membrane is a specialized subdomain of the cell surface that mediates attachment to host tissues and harbours multiple virulence factors. In this Review, we discuss the structure, assembly and function of the trypanosome flagellum, including canonical roles in cell motility as well as novel and emerging roles in cell morphogenesis and host-parasite interactions.

Proteomic analysis of intact flagella of procyclic Trypanosoma brucei cells identifies novel flagellar proteins with unique sub-localization and dynamics

Cilia and flagella are complex organelles made of hundreds of proteins of highly variable structures and functions. Here we report the purification of intact flagella from the procyclic stage of Trypanosoma brucei using mechanical shearing. Structural preservation was confirmed by transmission electron microscopy that showed that flagella still contained typical elements such as the membrane, the axoneme, the paraflagellar rod, and the intraflagellar transport particles. It also revealed that flagella severed below the basal body, and were not contaminated by other cytoskeletal structures such as the flagellar pocket collar or the adhesion zone filament. Mass spectrometry analysis identified a total of 751 proteins with high confidence, including 88% of known flagellar components. Comparison with the cell debris fraction revealed that more than half of the flagellum markers were enriched in flagella and this enrichment criterion was taken into account to identify 212 proteins not previously reported to be associated to flagella. Nine of these were experimentally validated including a 14-3-3 protein not yet reported to be associated to flagella and eight novel proteins termed FLAM (FLAgellar Member). Remarkably, they localized to five different subdomains of the flagellum. For example, FLAM6 is restricted to the proximal half of the axoneme, no matter its length. In contrast, FLAM8 is progressively accumulating at the distal tip of growing flagella and half of it still needs to be added after cell division. A combination of RNA interference and Fluorescence Recovery After Photobleaching approaches demonstrated very different dynamics from one protein to the other, but also according to the stage of construction and the age of the flagellum. Structural proteins are added to the distal tip of the elongating flagellum and exhibit slow turnover whereas membrane proteins such as the arginine kinase show rapid turnover without a detectible polarity.

Tracking the biogenesis and inheritance of subpellicular microtubule in Trypanosoma brucei with inducible YFP-α-tubulin

The microtubule cytoskeleton forms the most prominent structural system in Trypanosoma brucei, undergoing extensive modifications during the cell cycle. Visualization of tyrosinated microtubules leads to a semiconservative mode of inheritance, whereas recent studies employing microtubule plus end tracking proteins have hinted at an asymmetric pattern of cytoskeletal inheritance. To further the knowledge of microtubule synthesis and inheritance during T. brucei cell cycle, the dynamics of the microtubule cytoskeleton was visualized by inducible YFP-α-tubulin expression. During new flagellum/flagellum attachment zone (FAZ) biogenesis and cell growth, YFP-α-tubulin was incorporated mainly between the old and new flagellum/FAZ complexes. Cytoskeletal modifications at the posterior end of the cells were observed with EB1, a microtubule plus end binding protein, particularly during mitosis. Additionally, the newly formed microtubules segregated asymmetrically, with the daughter cell inheriting the new flagellum/FAZ complex retaining most of the new microtubules. Together, our results suggest an intimate connection between new microtubule formation and new FAZ assembly, consequently leading to asymmetric microtubule inheritance and cell division.

New insights into the molecular mechanisms of mitosis and cytokinesis in trypanosomes

Trypanosoma brucei, a unicellular eukaryote and the causative agent of human sleeping sickness, possesses multiple single-copy organelles that all need to be duplicated and segregated during cell division. Trypanosomes undergo a closed mitosis in which the mitotic spindle is anchored on the nuclear envelope and connects the kinetochores made of novel protein components. Cytokinesis in trypanosomes is initiated from the anterior tip of the new flagellum attachment zone, and proceeds along the longitudinal axis without the involvement of the actomyosin contractile ring, the well-recognized cytokinesis machinery conserved from yeast to humans. Trypanosome appears to employ both evolutionarily conserved and trypanosome-specific proteins to regulate its cell cycle, and has evolved certain cell cycle regulatory pathways that are either distinct between its life cycle stages or different from its human host. Understanding the mechanisms of mitosis and cytokinesis in trypanosomes not only would shed novel light on the evolution of cell cycle control, but also could provide new drug targets for chemotherapy.

Cytokinesis in Trypanosoma brucei differs between bloodstream and tsetse trypomastigote forms: implications for microtubule-based morphogenesis and mutant analysis

Trypanosomes use a microtubule‐focused mechanism for cell morphogenesis and cytokinesis. We used scanning electron and video microscopy of living cells to provide the first detailed description of cell morphogenesis and cytokinesis in the early‐branching eukaryote Trypanosoma brucei. We outline four distinct stages of cytokinesis and show that an asymmetric division fold bisects the two daughter cells, with a cytoplasmic bridge‐like structure connecting the two daughters immediately prior to abscission. Using detection of tyrosinated α‐tubulin as a marker for new or growing microtubules and expression of XMAP215, a plus end binding protein, as a marker for microtubule plus ends we demonstrate spatial asymmetry in the underlying microtubule cytoskeleton throughout the cell division cycle. This leads to inheritance of different microtubule cytoskeletal patterns and demonstrates the major role of microtubules in achieving cytokinesis. RNA interference techniques have led to a large set of mutants, often with variations in phenotype between procyclic and bloodstream life cycle forms. Here, we show morphogenetic differences between these two life cycle forms of this parasite during new flagellum growth and cytokinesis. These discoveries are important tools to explain differences between bloodstream and procyclic form RNAi phenotypes involving organelle mis‐positioning during cell division and cytokinesis defects.

More than meets the eye: understanding Trypanosoma brucei morphology in the tsetse

T. brucei, the causative parasite for African trypanosomiasis, faces an interesting dilemma in its life cycle. It has to successfully complete its infection cycle in the tsetse vector to be able to infect other vertebrate hosts. T. brucei has to undergo multiple morphological changes as it invades the alimentary canal of the tsetse to finally achieve infectivity in the salivary glands. In this review, we attempt to elucidate how these morphological changes are possible for a parasite that has evolved a highly robust cell structure to survive the chemically and physically diverse environments it finds itself in. To achieve this, we juxtaposed the experimental evidence that has been collected from T. brucei forms that are cultured in vitro with the observations that have been carried out on tsetse-infective forms in vivo. Although the accumulated knowledge on T. brucei biology is by no means trivial, several outstanding questions remain for how the parasite mechanistically changes its morphology as it traverses the tsetse and how those changes are triggered. However, we conclude that with recent breakthroughs allowing for the replication of the tsetse-infection process of T. brucei in vitro, these outstanding questions can finally be addressed.

The cooperative roles of two kinetoplastid-specific kinesins in cytokinesis and in maintaining cell morphology in bloodstream trypanosomes

The cytoskeleton of Trypanosoma brucei, a unicellular eukaryote and a parasitic protozoan, is defined by the subpellicular microtubule corset that is arranged underneath the plasma membrane. We recently identified two orphan kinesins, TbKIN-C and TbKIN-D, that cooperate to regulate the organization of the subpellicular microtubule corset and thereby maintain cell morphology in the procyclic form of T. brucei. In this report, we characterize the function of TbKIN-C and TbKIN-D in the bloodstream form of T. brucei and investigate their functional cooperation in both the bloodstream and procyclic forms. TbKIN-C and TbKIN-D form a tight complex in vivo in the bloodstream form. TbKIN-C is strongly enriched at the posterior tip of the cell, whereas TbKIN-D is distributed throughout the cell body at all cell cycle stages. RNAi of TbKIN-C or TbKIN-D in the bloodstream form inhibits cell proliferation and leads to cell death, due to cytokinesis defects. RNAi of TbKIN-C and TbKIN-D also results in defects in basal body segregation, but does not affect the synthesis and segregation of the flagellum and the flagellum attachment zone (FAZ) filament. Knockdown of TbKIN-C and TbKIN-D does not disrupt the organization of the subpellicular microtubule corset, but produces multinucleated cells with an enlarged flagellar pocket and misplaced flagella. Interestingly, depletion of TbKIN-C results in rapid degradation of TbKIN-D and, similarly, knockdown of TbKIN-C destabilizes TbKIN-D, suggesting that formation of TbKIN-C/TbKIN-D complex stabilizes both kinesins and is required for the two kinesins to execute their essential cellular functions. Altogether, our results demonstrate the essential role of the two kinesins in cell morphogenesis and cytokinesis in the bloodstream form and the requirement of heteromeric complex formation for maintaining the stability of the two kinesins.

Flagellar adhesion in Trypanosoma brucei relies on interactions between different skeletal structures present in the flagellum and in the cell body

The Trypanosoma brucei flagellum is an essential organelle anchored along the surface of the cell body via a specialized structure called the flagellum attachment zone (FAZ). Adhesion relies on the interaction of the extracellular portion of two transmembrane proteins termed FLA1 and FLA1BP. Analysis of the flagellum proteome identified FLAM3, a novel large protein associated to the flagellum skeleton whose ablation inhibits flagellum attachment. FLAM3 does not contain transmembrane domains and its flagellar localization matches closely but not exactly with that of the paraflagellar rod, an extra-axonemal structure present in the flagellum. Knockdown of FLA1 or FLAM3 triggers similar motility and morphogenesis defects, characterized by the assembly of a drastically reduced FAZ filament. FLAM3 remains associated to the flagellum skeleton even in the absence of adhesion or of a normal paraflagellar rod. However, the protein is dispersed in the cytoplasm when flagellum formation is inhibited. By contrast, FLA1 remains tightly associated to the FAZ filament even in the absence of a flagellum. In these conditions, the extracellular domain of FLA1 points to the cell surface. FLAM3 turns out to be essential for proper distribution of FLA1BP that is restricted to the very proximal portion of the flagellum upon FLAM3 knockdown. We propose that FLAM3 is a key component of the FAZ connectors that appear to link the axoneme to the adhesion zone, hence acting in an equivalent manner to the FAZ filament complex, but on the flagellum side.

Biochemical characterization of the bi-lobe reveals a continuous structural network linking the bi-lobe to other single-copied organelles in Trypanosoma brucei

Trypanosoma brucei, a unicellular parasite, contains several single-copied organelles that duplicate and segregate in a highly coordinated fashion during the cell cycle. In the procyclic stage, a bi-lobed structure is found adjacent to the single ER exit site and Golgi apparatus, forming both stable and dynamic association with other cytoskeletal components including the basal bodies that seed the flagellum and the flagellar pocket collar that is critical for flagellar pocket biogenesis. To further understand the bi-lobe and its association with adjacent organelles, we performed proteomic analyses on the immunoisolated bi-lobe complex. Candidate proteins were localized to the flagellar pocket, the basal bodies, a tripartite attachment complex linking the basal bodies to the kinetoplast, and a segment of microtubule quartet linking the flagellar pocket collar and bi-lobe to the basal bodies. These results supported an extensive connection among the single-copied organelles in T. brucei, a strategy employed by the parasite for orderly organelle assembly and inheritance during the cell cycle.

An intracellular membrane junction consisting of flagellum adhesion glycoproteins links flagellum biogenesis to cell morphogenesis in Trypanosoma brucei

African trypanosomes have a single, membrane-bounded flagellum that is attached to the cell cortex by membrane adhesion proteins and an intracellular flagellum attachment zone (FAZ) complex. The coordinated assembly of flagellum and FAZ, during the cell cycle and the life cycle development, plays a pivotal role in organelle positioning, cell division and cell morphogenesis. To understand how the flagellum and FAZ assembly are coordinated, we examined the domain organization of the flagellum adhesion protein 1 (FLA1), a glycosylated, transmembrane protein essential for flagellum attachment and cell division. By immunoprecipitation of a FLA1-truncation mutant that mislocalized to the flagellum, a novel FLA1-binding protein (FLA1BP) was identified in procyclic Trypanosoma brucei. The interaction between FLA1 on the cell membrane and FLA1BP on the flagellum membrane acts like a molecular zipper, joining flagellum membrane to cell membrane and linking flagellum biogenesis to FAZ elongation. By coordinating flagellum and FAZ assembly during the cell cycle, morphology information is transmitted from the flagellum to the cell body.

Trypanosoma brucei FKBP12 differentially controls motility and cytokinesis in procyclic and bloodstream forms

FKBP12 proteins are able to inhibit TOR kinases or calcineurin phosphatases upon binding of rapamycin or FK506 drugs, respectively. The Trypanosoma brucei FKBP12 homologue (TbFKBP12) was found to be a cytoskeleton-associated protein with specific localization in the flagellar pocket area of the bloodstream form. In the insect procyclic form, RNA interference-mediated knockdown of TbFKBP12 affected motility. In bloodstream cells, depletion of TbFKBP12 affected cytokinesis and cytoskeleton architecture. These last effects were associated with the presence of internal translucent cavities limited by an inside-out configuration of the normal cell surface, with a luminal variant surface glycoprotein coat lined up by microtubules. These cavities, which recreated the streamlined shape of the normal trypanosome cytoskeleton, might represent unsuccessful attempts for cell abscission. We propose that TbFKBP12 differentially affects stage-specific processes through association with the cytoskeleton.

Extra-glycosomal localisation of Trypanosoma brucei hexokinase 2

The majority of the glycolytic enzymes in the African trypanosome are compartmentalised within peroxisome-like organelles, the glycosomes. Polypeptides harbouring peroxisomal targeting sequences (PTS type 1 or 2) are targeted to these organelles. This targeting is essential to parasite viability, as compartmentalisation of glycolytic enzymes prevents unregulated ATP-dependent phosphorylation of intermediate metabolites. Here, we report the surprising extra-glycosomal localisation of a PTS-2 bearing trypanosomal hexokinase, TbHK2. In bloodstream form parasites, the protein localises to both glycosomes and to the flagellum. Evidence for this includes fractionation and immunofluorescence studies using antisera generated against the authentic protein as well as detection of epitope-tagged recombinant versions of the protein. In the insect stage parasite, distribution is different, with the polypeptide localised to glycosomes and proximal to the basal bodies. The function of the extra-glycosomal protein remains unclear. While its association with the basal body suggests that it may have a role in locomotion in the insect stage parasite, no detectable defect in directional motility or velocity of cell movement were observed for TbHK2-deficient cells, suggesting that the protein may have a different function in the cell.

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.

Proteomics and the Trypanosoma brucei cytoskeleton: advances and opportunities

Trypanosoma brucei is the etiological agent of devastating parasitic disease in humans and livestock in sub-saharan Africa. The pathogenicity and growth of the parasite are intimately linked to its shape and form. This is in turn derived from a highly ordered microtubule cytoskeleton that forms a tightly arrayed cage directly beneath the pellicular membrane and numerous other cytoskeletal structures such as the flagellum. The parasite undergoes extreme changes in cellular morphology during its life cycle and cell cycles which require a high level of integration and coordination of cytoskeletal processes. In this review we will discuss the role that proteomics techniques have had in advancing our understanding of the molecular composition of the cytoskeleton and its functions. We then consider future opportunities for the application of these techniques in terms of addressing some of the unanswered questions of trypanosome cytoskeletal cell biology with particular focus on the differences in the composition and organisation of the cytoskeleton through the trypanosome life-cycle.

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.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

A coiled-coil- and C2-domain-containing protein is required for FAZ assembly and cell morphology in Trypanosoma brucei

Trypanosoma brucei, a flagellated protozoan parasite causing human sleeping sickness, relies on a subpellicular microtubule array for maintenance of cell morphology. The flagellum is attached to the cell body through a poorly understood flagellum attachment zone (FAZ), and regulates cell morphogenesis using an unknown mechanism. Here we identified a new FAZ component, CC2D, which contains coiled-coil motifs followed by a C-terminal C2 domain. T. brucei CC2D is present on the FAZ filament, FAZ-juxtaposed ER membrane and the basal bodies. Depletion of CC2D inhibits the assembly of a new FAZ filament, forming a FAZ stub with a relatively fixed size at the base of a detached, but otherwise normal, flagellum. Inhibition of new FAZ formation perturbs subpellicular microtubule organization and generates short daughter cells. The cell length shows a strong linear correlation with FAZ length, in both control cells and in cells with inhibited FAZ assembly. Together, our data support a direct function of FAZ assembly in determining new daughter cell length by regulating subpellicular microtubule synthesis.

Axonemal positioning and orientation in three-dimensional space for primary cilia: what is known, what is assumed, and what needs clarification

Two positional characteristics of the ciliary axoneme--its location on the plasma membrane as it emerges from the cell, and its orientation in three-dimensional (3D) space--are known to be critical for optimal function of actively motile cilia (including nodal cilia), as well as for modified cilia associated with special senses. However, these positional characteristics have not been analyzed to any significant extent for primary cilia. This review briefly summarizes the history of knowledge of these two positional characteristics across a wide spectrum of cilia, emphasizing their importance for proper function. Then the review focuses what is known about these same positional characteristics for primary cilia in all major tissue types where they have been reported. The review emphasizes major areas that would be productive for future research for understanding how positioning and 3D orientation of primary cilia may be related to their hypothesized signaling roles within different cellular populations.

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.

Molecular bases of cytoskeleton plasticity during the Trypanosoma brucei parasite cycle

African trypanosomes are flagellated protozoan parasites responsible for sleeping sickness and transmitted by tsetse flies. The accomplishment of their parasite cycle requires adaptation to highly diverse environments. These transitions take place in a strictly defined order and are accompanied by spectacular morphological modifications in cell size, shape and positioning of organelles. To understand the molecular bases of these processes, parasites isolated from different tissues of the tsetse fly were analysed by immunofluorescence with markers for specific cytoskeleton components and by a new immunofluorescence-based assay for evaluation of the cell volume. The data revealed striking differences between proliferative stages found in the midgut or in the salivary glands and the differentiating stage occurring in the proventriculus. Cell proliferation was characterized by a significant increase in cell volume, by a pronounced cell elongation marked by microtubule extension at the posterior end, and by the production of a new flagellum similar to the existing one. In contrast, the differentiating stage found in the proventriculus does not display any increase in cell volume neither in cell length, but is marked by a profound remodelling of the posterior part of the cytoskeleton and by changes in molecular composition and/or organization of the flagellum attachment zone.

The kinetoplast duplication cycle in Trypanosoma brucei is orchestrated by cytoskeleton-mediated cell morphogenesis

The mitochondrial DNA of Trypanosoma brucei is organized in a complex structure called the kinetoplast. In this study, we define the complete kinetoplast duplication cycle in T. brucei based on three-dimensional reconstructions from serial-section electron micrographs. This structural model was enhanced by analyses of the replication process of DNA maxi- and minicircles. Novel insights were obtained about the earliest and latest stages of kinetoplast duplication. We show that kinetoplast S phase occurs concurrently with the repositioning of the new basal body from the anterior to the posterior side of the old flagellum. This emphasizes the role of basal body segregation in kinetoplast division and suggests a possible mechanism for driving the rotational movement of the kinetoplast during minicircle replication. Fluorescence in situ hybridization with minicircle- and maxicircle-specific probes showed that maxicircle DNA is stretched out between segregated minicircle networks, indicating that maxicircle segregation is a late event in the kinetoplast duplication cycle. This new view of the complexities of kinetoplast duplication emphasizes the dependencies between the dynamic remodelling of the cytoskeleton and the inheritance of the mitochondrial genome.

Cyclosporin A treatment of Leishmania donovani reveals stage-specific functions of cyclophilins in parasite proliferation and viability

Background

Cyclosporin A (CsA) has important anti-microbial activity against parasites of the genus Leishmania, suggesting CsA-binding cyclophilins (CyPs) as potential drug targets. However, no information is available on the genetic diversity of this important protein family, and the mechanisms underlying the cytotoxic effects of CsA on intracellular amastigotes are only poorly understood. Here, we performed a first genome-wide analysis of Leishmania CyPs and investigated the effects of CsA on host-free L. donovani amastigotes in order to elucidate the relevance of these parasite proteins for drug development.

Methodology/Principal Findings

Multiple sequence alignment and cluster analysis identified 17 Leishmania CyPs with significant sequence differences to human CyPs, but with highly conserved functional residues implicated in PPIase function and CsA binding. CsA treatment of promastigotes resulted in a dose-dependent inhibition of cell growth with an IC50 between 15 and 20 µM as demonstrated by proliferation assay and cell cycle analysis. Scanning electron microscopy revealed striking morphological changes in CsA treated promastigotes reminiscent to developing amastigotes, suggesting a role for parasite CyPs in Leishmania differentiation. In contrast to promastigotes, CsA was highly toxic to amastigotes with an IC50 between 5 and 10 µM, revealing for the first time a direct lethal effect of CsA on the pathogenic mammalian stage linked to parasite thermotolerance, independent from host CyPs. Structural modeling, enrichment of CsA-binding proteins from parasite extracts by FPLC, and PPIase activity assays revealed direct interaction of the inhibitor with LmaCyP40, a bifunctional cyclophilin with potential co-chaperone function.

Conclusions/Significance

The evolutionary expansion of the Leishmania CyP protein family and the toxicity of CsA on host-free amastigotes suggest important roles of PPIases in parasite biology and implicate Leishmania CyPs in key processes relevant for parasite proliferation and viability. The requirement of Leishmania CyP functions for intracellular parasite survival and their substantial divergence form host CyPs defines these proteins as prime drug targets.

Visceral leishmanisasis, also known as Kala Azar, is caused by the protozoan parasite Leishmania donovani. The L. donovani infectious cycle comprises two developmental stages, a motile promastigote stage that proliferates inside the digestive tract of the phlebotomine insect host, and a non-motile amastigote stage that differentiates inside the macrophages of mammalian hosts. Intracellular parasite survival in mouse and macrophage infection assays has been shown to be strongly compromised in the presence of the inhibitor cyclosporin A (CsA), which binds to members of the cyclophilin (CyP) protein family. It has been suggested that the toxic effects of CsA on amastigotes occurs indirectly via host cyclophilins, which may be required for intracellular parasite development and growth. Using a host-free L. donovani culture system we revealed for the first time a direct and stage-specific effect of CsA on promastigote growth and amastigote viability. We provided evidence that parasite killing occurs through a heat sensitivity mechanism likely due to direct inhibition of the co-chaperone cyclophilin 40. Our data allow important new insights into the function of the Leishmania CyP protein family in differentiation, growth, and intracellular survival, and define this class of molecules as important drug targets.

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

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