A cell-body groove housing the new flagellum tip suggests an adaptation of cellular morphogenesis for parasitism in the bloodstream form of Trypanosoma brucei

Characterisation of TbSmee1 suggests endocytosis allows surface-bound cargo to enter the trypanosome flagellar pocket

All endocytosis and exocytosis in the African trypanosome Trypanosoma brucei occurs at a single subdomain of the plasma membrane. This subdomain, the flagellar pocket, is a small vase-shaped invagination containing the root of the single flagellum of the cell. Several cytoskeleton-associated multiprotein complexes are coiled around the neck of the flagellar pocket on its cytoplasmic face. One of these, the hook complex, was proposed to affect macromolecule entry into the flagellar pocket lumen. In previous work, knockdown of T. brucei (Tb)MORN1, a hook complex component, resulted in larger cargo being unable to enter the flagellar pocket. In this study, the hook complex component TbSmee1 was characterised in bloodstream form T. brucei and found to be essential for cell viability. TbSmee1 knockdown resulted in flagellar pocket enlargement and impaired access to the flagellar pocket membrane by surface-bound cargo, similar to depletion of TbMORN1. Unexpectedly, inhibition of endocytosis by knockdown of clathrin phenocopied TbSmee1 knockdown, suggesting that endocytic activity itself is a prerequisite for the entry of surface-bound cargo into the flagellar pocket.

FAZ assembly in bloodstream form Trypanosoma brucei requires kinesin KIN-E

Trypanosoma brucei, the causative agent of African sleeping sickness, uses its flagellum for movement, cell division, and signaling. The flagellum is anchored to the cell body membrane via the flagellum attachment zone (FAZ), a complex of proteins, filaments, and microtubules that spans two membranes with elements on both flagellum and cell body sides. How FAZ components are carried into place to form this complex is poorly understood. Here, we show that the trypanosome-specific kinesin KIN-E is required for building the FAZ in bloodstream-form parasites. KIN-E is localized along the flagellum with a concentration at its distal tip. Depletion of KIN-E by RNAi rapidly inhibits flagellum attachment and leads to cell death. A detailed analysis reveals that KIN-E depletion phenotypes include failure in cytokinesis completion, kinetoplast DNA missegregation, and transport vesicle accumulation. Together with previously published results in procyclic form parasites, these data suggest KIN-E plays a critical role in FAZ assembly in T. brucei.

Subcellular protein localisation of Trypanosoma brucei bloodstream form-upregulated proteins maps stage-specific adaptations

Background: Genome-wide subcellular protein localisation in Trypanosoma brucei, through our TrypTag project, has comprehensively dissected the molecular organisation of this important pathogen. Powerful as this resource is , T. brucei has multiple developmental forms and we previously only analysed the procyclic form. This is an insect life cycle stage, leaving the mammalian bloodstream form unanalysed. The expectation is that between life stages protein localisation would not change dramatically (completely unchanged or shifting to analogous stage-specific structures). However, this has not been specifically tested. Similarly, which organelles tend to contain proteins with stage-specific expression can be predicted from known stage specific adaptations but has not been comprehensively tested. Methods: We used endogenous tagging with mNG to determine the sub-cellular localisation of the majority of proteins encoded by transcripts significantly upregulated in the bloodstream form, and performed comparison to the existing localisation data in procyclic forms. Results: We have confirmed the localisation of known stage-specific proteins and identified the localisation of novel stage-specific proteins. This gave a map of which organelles tend to contain stage specific proteins: the mitochondrion for the procyclic form, and the endoplasmic reticulum, endocytic system and cell surface in the bloodstream form. Conclusions: This represents the first genome-wide map of life cycle stage-specific adaptation of organelle molecular machinery in T. brucei.

Subcellular protein localisation of Trypanosoma brucei bloodstream form-upregulated proteins maps stage-specific adaptations

Background: Genome-wide subcellular protein localisation in Trypanosoma brucei, through our TrypTag project, has comprehensively dissected the molecular organisation of this important pathogen. Powerful as this resource is, T. brucei has multiple developmental forms and we previously only analysed the procyclic form. This is an insect life cycle stage, leaving the mammalian bloodstream form unanalysed. The expectation is that between life stages protein localisation would not change dramatically (completely unchanged or shifting to analogous stage-specific structures). However, this has not been specifically tested. Similarly, which organelles tend to contain proteins with stage-specific expression can be predicted from known stage specific adaptations but has not been comprehensively tested. Methods: We used endogenous tagging with mNG to determine the sub-cellular localisation of the majority of proteins encoded by transcripts significantly upregulated in the bloodstream form, and performed comparison to the existing localisation data in procyclic forms. Results: We have confirmed the localisation of known and identified the localisation of novel stage-specific proteins. This gave a map of which organelles tend to contain stage specific proteins: the mitochondrion for the procyclic form, and the endoplasmic reticulum, endocytic system and cell surface in the bloodstream form. Conclusions: This represents the first genome-wide map of life cycle stage-specific adaptation of organelle molecular machinery in T. brucei.

Diversity in new flagellum tip attachment in bloodstream form African trypanosomes

The closely related parasites Trypanosoma brucei, T. congolense, and T. vivax cause neglected tropical diseases collectively known as African Trypanosomiasis. A characteristic feature of bloodstream form T. brucei is the flagellum that is laterally attached to the side of the cell body. During the cell cycle, the new flagellum is formed alongside the old flagellum, with the new flagellum tip embedded within a mobile transmembrane junction called the groove. The molecular composition of the groove is currently unknown, which limits the analysis of this junction and assessment of its conservation in related trypanosomatids. Here, we identified 13 proteins that localize to the flagellar groove through a small-scale tagging screen. Functional analysis of a subset of these proteins by RNAi and gene deletion revealed three proteins, FCP4/TbKin15, FCP7, and FAZ45, that are involved in new flagellum tip attachment to the groove. Despite possessing orthologues of all 13 groove proteins, T. congolense and T. vivax did not assemble a canonical groove around the new flagellum tip according to 3D electron microscopy. This diversity in new flagellum tip attachment points to the rapid evolution of membrane-cytoskeleton structures that can occur without large changes in gene complement and likely reflects the niche specialization of each species.

Structure, function and druggability of the African trypanosome flagellum

African trypanosomes are early branching protists that cause human and animal diseases, termed trypanosomiases. They have been under intensive study for more than 100 years and have contributed significantly to our understanding of eukaryotic biology. The combination of conserved and parasite‐specific features mean that their flagellum has gained particular attention. Here, we discuss the different structural features of the flagellum and their role in transmission and virulence. We highlight the possibilities of targeting flagellar function to cure trypanosome infections and help in the fight to eliminate trypanosomiases.

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.

Expansion microscopy facilitates quantitative super-resolution studies of cytoskeletal structures in kinetoplastid parasites

Expansion microscopy (ExM) has become a powerful super-resolution method in cell biology. It is a simple, yet robust approach, which does not require any instrumentation or reagents beyond those present in a standard microscopy facility. In this study, we used kinetoplastid parasites Trypanosoma brucei and Leishmania major, which possess a complex, yet well-defined microtubule-based cytoskeleton, to demonstrate that this method recapitulates faithfully morphology of structures as previously revealed by a combination of sophisticated electron microscopy (EM) approaches. Importantly, we also show that due to the rapidness of image acquisition and three-dimensional reconstruction of cellular volumes ExM is capable of complementing EM approaches by providing more quantitative data. This is demonstrated on examples of less well-appreciated microtubule structures, such as the neck microtubule of T. brucei or the pocket, cytosolic and multivesicular tubule-associated microtubules of L. major. We further demonstrate that ExM enables identifying cell types rare in a population, such as cells in mitosis and cytokinesis. Three-dimensional reconstruction of an entire volume of these cells provided details on the morphology of the mitotic spindle and the cleavage furrow. Finally, we show that established antibody markers of major cytoskeletal structures function well in ExM, which together with the ability to visualize proteins tagged with small epitope tags will facilitate studies of the kinetoplastid cytoskeleton.

Redistribution of FLAgellar Member 8 during the trypanosome life cycle: Consequences for cell fate prediction

The single flagellum of African trypanosomes is essential in multiple aspects of the parasites' development. The FLAgellar Member 8 protein (FLAM8), localised to the tip of the flagellum in cultured insect forms of Trypanosoma brucei, was identified as a marker of the locking event that controls flagellum length. Here, we investigated whether FLAM8 could also reflect the flagellum maturation state in other parasite cycle stages. We observed that FLAM8 distribution extended along the entire flagellar cytoskeleton in mammalian‐infective forms. Then, a rapid FLAM8 concentration to the distal tip occurs during differentiation into early insect forms, illustrating the remodelling of an existing flagellum. In the tsetse cardia, FLAM8 further localises to the entire length of the new flagellum during an asymmetric division. Strikingly, in parasites dividing in the tsetse midgut and in the salivary glands, the amount and distribution of FLAM8 in the new flagellum were seen to predict the daughter cell fate. We propose and discuss how FLAM8 could be considered a meta‐marker of the flagellum stage and maturation state in trypanosomes.

Three-Dimensional Imaging in Stem Cell-Based Researches

Stem cells have an important role in regenerative therapies, developmental biology studies and drug screening. Basic and translational research in stem cell technology needs more detailed imaging techniques. The possibility of cell-based therapeutic strategies has been validated in the stem cell field over recent years, a more detailed characterization of the properties of stem cells is needed for connectomics of large assemblies and structural analyses of these cells. The aim of stem cell imaging is the characterization of differentiation state, cellular function, purity and cell location. Recent progress in stem cell imaging field has included ultrasound-based technique to study living stem cells and florescence microscopy-based technique to investigate stem cell three-dimensional (3D) structures. Here, we summarized the fundamental characteristics of stem cells via 3D imaging methods and also discussed the emerging literatures on 3D imaging in stem cell research and the applications of both classical 2D imaging techniques and 3D methods on stem cells biology.

Trypanosomes have divergent kinesin-2 proteins that function differentially in flagellum biosynthesis and cell viability

Trypanosoma brucei, the causative agent of African sleeping sickness, has a flagellum that is crucial for motility, pathogenicity, and viability. In most eukaryotes, the intraflagellar transport (IFT) machinery drives flagellum biogenesis, and anterograde IFT requires kinesin-2 motor proteins. In this study, we investigated the function of the two T. brucei kinesin-2 proteins, TbKin2a and TbKin2b, in bloodstream form trypanosomes. We found that, compared to kinesin-2 proteins across other phyla, TbKin2a and TbKin2b show greater variation in neck, stalk and tail domain sequences. Both kinesins contributed additively to flagellar lengthening. Silencing TbKin2a inhibited cell proliferation, cytokinesis and motility, whereas silencing TbKin2b did not. TbKin2a was localized on the flagellum and colocalized with IFT components near the basal body, consistent with it performing a role in IFT. TbKin2a was also detected on the flagellar attachment zone, a specialized structure that connects the flagellum to the cell body. Our results indicate that kinesin-2 proteins in trypanosomes play conserved roles in flagellar biosynthesis and exhibit a specialized localization, emphasizing the evolutionary flexibility of motor protein function in an organism with a large complement of kinesins.

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.

Touching the Surface: Diverse Roles for the Flagellar Membrane in Kinetoplastid Parasites

While flagella have been studied extensively as motility organelles, with a focus on internal structures such as the axoneme, more recent research has illuminated the roles of the flagellar surface in a variety of biological processes. Parasitic protists of the order Kinetoplastida, which include trypanosomes and Leishmania species, provide a paradigm for probing the role of flagella in host-microbe interactions and illustrate that this interface between the flagellar surface and the host is of paramount importance. An increasing body of knowledge indicates that the flagellar membrane serves a multitude of functions at this interface: attachment of parasites to tissues within insect vectors, close interactions with intracellular organelles of vertebrate cells, transactions between flagella from different parasites, junctions between the flagella and the parasite cell body, emergence of nanotubes and exosomes from the parasite directed to either host or microbial targets, immune evasion, and sensing of the extracellular milieu. Recent whole-organelle or genome-wide studies have begun to identify protein components of the flagellar surface that must mediate these diverse host-parasite interactions. The increasing corpus of knowledge on kinetoplastid flagella will likely prove illuminating for other flagellated or ciliated pathogens as well.

Coordination of the Cell Cycle in Trypanosomes

Trypanosomes have complex life cycles within which there are both proliferative and differentiation cell divisions. The coordination of the cell cycle to achieve these different divisions is critical for the parasite to infect both host and vector. From studying the regulation of the proliferative cell cycle of the Trypanosoma brucei procyclic life cycle stage, three subcycles emerge that control the duplication and segregation of ( a) the nucleus, ( b) the kinetoplast, and ( c) a set of cytoskeletal structures. We discuss how the clear dependency relationships within these subcycles, and the potential for cross talk between them, are likely required for overall cell cycle coordination. Finally, we look at the implications this interdependence has for proliferative and differentiation divisions through the T. brucei life cycle and in related parasitic trypanosomatid species.

Non-equivalence in old- and new-flagellum daughter cells of a proliferative division in Trypanosoma brucei

Differentiation of Trypanosoma brucei, a flagellated protozoan parasite, between life cycle stages typically occurs through an asymmetric cell division process, producing two morphologically distinct daughter cells. Conversely, proliferative cell divisions produce two daughter cells, which look similar but are not identical. To examine in detail differences between the daughter cells of a proliferative division of procyclic T. brucei we used the recently identified constituents of the flagella connector. These segregate asymmetrically during cytokinesis allowing the new‐flagellum and the old‐flagellum daughters to be distinguished. We discovered that there are distinct morphological differences between the two daughters, with the new‐flagellum daughter in particular re‐modelling rapidly and extensively in early G1. This re‐modelling process involves an increase in cell body, flagellum and flagellum attachment zone length and is accompanied by architectural changes to the anterior cell end. The old‐flagellum daughter undergoes a different G1 re‐modelling, however, despite this there was no difference in G1 duration of their respective cell cycles. This work demonstrates that the two daughters of a proliferative division of T. brucei are non‐equivalent and enables more refined morphological analysis of mutant phenotypes. We suggest all proliferative divisions in T. brucei and related organisms will involve non‐equivalence.

Functional Analyses of Cytokinesis Regulators in Bloodstream Stage Trypanosoma brucei Parasites Identify Functions and Regulations Specific to the Life Cycle Stage

The early divergent protozoan parasite Trypanosoma brucei is the causative agent of sleeping sickness in humans and nagana in cattle in sub-Saharan Africa. This parasite has a complex life cycle by alternating between the insect vector and the mammalian hosts and proliferates by binary cell fission. The control of cell division in trypanosomes appears to be distinct from that in its human host and differs substantially between two life cycle stages, the procyclic (insect) form and the bloodstream form. Cytokinesis, the final step of binary cell fission, is regulated by a novel signaling cascade consisting of two evolutionarily conserved protein kinases and a cohort of trypanosome-specific regulators in the procyclic form, but whether this signaling pathway operates in a similar manner in the bloodstream form is unclear. In this report, we performed a functional analysis of multiple cytokinesis regulators and discovered their distinct functions and regulations in the bloodstream form.

The early divergent protozoan parasite Trypanosoma brucei alternates between the insect vector and the mammalian hosts during its life cycle and proliferates through binary cell fission. The cell cycle control system in T. brucei differs substantially from that in its mammalian hosts and possesses distinct mitosis-cytokinesis checkpoint controls between two life cycle stages, the procyclic form and the bloodstream form. T. brucei undergoes an unusual mode of cytokinesis, which is controlled by a novel signaling cascade consisting of evolutionarily conserved protein kinases and trypanosome-specific regulatory proteins in the procyclic form. However, given the distinct mitosis-cytokinesis checkpoints between the two forms, it is unclear whether the cytokinesis regulatory pathway discovered in the procyclic form also operates in a similar manner in the bloodstream form. Here, we showed that the three regulators of cytokinesis initiation, cytokinesis initiation factor 1 (CIF1), CIF2, and CIF3, are interdependent for subcellular localization but not for protein stability as in the procyclic form. Further, we demonstrated that KLIF, a regulator of cytokinesis completion in the procyclic form, plays limited roles in cytokinesis in the bloodstream form. Finally, we showed that the cleavage furrow-localizing protein FRW1 is required for cytokinesis initiation in the bloodstream form but is nonessential for cytokinesis in the procyclic form. Together, these results identify conserved and life cycle-specific functions of cytokinesis regulators, highlighting the distinction in the regulation of cytokinesis between different life cycle stages of T. brucei.

IMPORTANCE The early divergent protozoan parasite Trypanosoma brucei is the causative agent of sleeping sickness in humans and nagana in cattle in sub-Saharan Africa. This parasite has a complex life cycle by alternating between the insect vector and the mammalian hosts and proliferates by binary cell fission. The control of cell division in trypanosomes appears to be distinct from that in its human host and differs substantially between two life cycle stages, the procyclic (insect) form and the bloodstream form. Cytokinesis, the final step of binary cell fission, is regulated by a novel signaling cascade consisting of two evolutionarily conserved protein kinases and a cohort of trypanosome-specific regulators in the procyclic form, but whether this signaling pathway operates in a similar manner in the bloodstream form is unclear. In this report, we performed a functional analysis of multiple cytokinesis regulators and discovered their distinct functions and regulations in the bloodstream form.

A fixable probe for visualizing flagella and plasma membranes of the African trypanosome

The protozoan Trypanosoma brucei sp. cause diseases in humans and animals. Studies of T. brucei cell biology have revealed unique features, such as major endocytic events being limited to a single region, and mitochondrial genome segregation mediated via basal bodies. Further understanding of trypanosome cell biology can be facilitated with super-resolution fluorescence microscopy. Lack of a plasma membrane probe for fixed trypanosomes remains a persistent problem in need of a working solution. Herein, we report protocols developed using mCLING in super-resolution structured illumination fluorescence microscopy (SR-SIM). mCLING comprehensively labels flagellar membranes, including nascent intracellular stages. To extend its usefulness for trypanosome biology we optimized mCLING in combination with organelle-specific antibodies for immunofluorescence of basal bodies or mitochondria. Then in work with live trypanosomes, we demonstrated internalization of mCLING into endocytic stations that overlap with LysoTracker in acidic organelles. Greater detail of the intracellular location of mCLING was obtained with SR-SIM after pulsing trypanosomes with the probe, and allowing continuous uptake of fluorescent concanavalin A (ConA) destined for lysosomes. In most cases, ConA and mCLING vesicles were juxtaposed but not coincident. A video of the complete image stack at the 15 min time point shows zones of mCLING staining surrounding patches of ConA, consistent with persistence of mCLING in membranes of compartments that contain luminal ConA. In summary, these studies establish mCLING as a versatile trypanosome membrane probe compatible with super-resolution microscopy that can be used for detailed analysis of flagellar membrane biogenesis. In addition, mCLING can be used for immunofluorescence in fixed, permeabilized trypanosomes. Its robust staining of the plasma membrane eliminates a need to overlay transmitted light images on fluorescence pictures obtained from widefield, confocal, or super-resolution microscopy.

New advances in scanning microscopy and its application to study parasitic protozoa

Scanning electron microscopy has been used to observe and study parasitic protozoa for at least 40 years. However, field emission electron sources, as well as improvements in lenses and detectors, brought the resolution power of scanning electron microscopes (SEM) to a new level. Parallel to the refinement of instruments, protocols for preservation of the ultrastructure, immunolabeling, exposure of cytoskeleton and inner structures of parasites and host cells were developed. This review is focused on protozoan parasites of medical and veterinary relevance, e.g., Toxoplasma gondii, Tritrichomonas foetus, Giardia intestinalis, and Trypanosoma cruzi, compilating the main achievements in describing the fine ultrastructure of their surface, cytoskeleton and interaction with host cells. Two new resources, namely, Helium Ion Microscopy (HIM) and Slice and View, using either Focused Ion Beam (FIB) abrasion or Microtome Serial Sectioning (MSS) within the microscope chamber, combined to backscattered electron imaging of fixed (chemically or by quick freezing followed by freeze substitution and resin embedded samples is bringing an exponential amount of valuable information. In HIM there is no need of conductive coating and the depth of field is much higher than in any field emission SEM. As for FIB- and MSS-SEM, high resolution 3-D models of areas and volumes larger than any other technique allows can be obtained. The main results achieved with all these technological tools and some protocols for sample preparation are included in this review. In addition, we included some results obtained with environmental/low vacuum scanning microscopy and cryo-scanning electron microscopy, both promising, but not yet largely employed SEM modalities.

Let's get fISSical: fast in silico synchronization as a new tool for cell division cycle analysis

Cell cycle progression is a question of fundamental biological interest. The coordinated duplication and segregation of all cellular structures and organelles is however an extremely complex process, and one which remains only partially understood even in the most intensively researched model organisms. Trypanosomes are in an unusual position in this respect - they are both outstanding model systems for fundamental questions in eukaryotic cell biology, and pathogens that are the causative agents of three of the neglected tropical diseases. As a failure to successfully complete cell division will be deleterious or lethal, analysis of the cell division cycle is of relevance both to basic biology and drug design efforts. Cell division cycle analysis is however experimentally challenging, as the analysis of phenotypes associated with it remains hypothesis-driven and therefore biased. Current methods of analysis are extremely labour-intensive, and cell synchronization remains difficult and unreliable. Consequently, there exists a need - both in basic and applied trypanosome biology - for a global, unbiased, standardized and high-throughput analysis of cell division cycle progression. In this review, the requirements - both practical and computational - for such a system are considered and compared with existing techniques for cell cycle analysis.

Protein diversity in discrete structures at the distal tip of the trypanosome flagellum

The distal end of the eukaryotic flagellum/cilium is important for axonemal growth and signaling and has distinct biomechanical properties. Specific flagellum tip structures exist, yet their composition, dynamics, and functions are largely unknown. We used biochemical approaches to identify seven constituents of the flagella connector at the tip of an assembling trypanosome flagellum and three constituents of the axonemal capping structure at the tips of both assembling and mature flagella. Both tip structures contain evolutionarily conserved as well as kinetoplastid-specific proteins, and component assembly into the structures occurs very early during flagellum extension. Localization and functional studies reveal that the flagella connector membrane junction is attached to the tips of extending microtubules of the assembling flagellum by a kinesin-15 family member. On the opposite side, a kinetoplastid-specific kinesin facilitates attachment of the junction to the microtubules in the mature flagellum. Functional studies also suggest roles of several other components and the definition of subdomains in the tip structures.

Patterns of organelle ontogeny through a cell cycle revealed by whole-cell reconstructions using 3D electron microscopy

The major mammalian bloodstream form of the African sleeping sickness parasite Trypanosoma brucei multiplies rapidly, and it is important to understand how these cells divide. Organelle inheritance involves complex spatiotemporal re-arrangements to ensure correct distribution to daughter cells. Here, serial block face scanning electron microscopy (SBF-SEM) was used to reconstruct whole individual cells at different stages of the cell cycle to give an unprecedented temporal, spatial and quantitative view of organelle division, inheritance and abscission in a eukaryotic cell. Extensive mitochondrial branching occurred only along the ventral surface of the parasite, but the mitochondria returned to a tubular form during cytokinesis. Fission of the mitochondrion occurred within the cytoplasmic bridge during the final stage of cell division, correlating with cell abscission. The nuclei were located underneath each flagellum at mitosis and the mitotic spindle was located along the ventral surface, further demonstrating the asymmetric arrangement of cell cleavage in trypanosomes. Finally, measurements demonstrated that multiple Golgi bodies were accurately positioned along the flagellum attachment zone, suggesting a mechanism for determining the location of Golgi bodies along each flagellum during the cell cycle.

KHARON Is an Essential Cytoskeletal Protein Involved in the Trafficking of Flagellar Membrane Proteins and Cell Division in African Trypanosomes

African trypanosomes and related kinetoplastid parasites selectively traffic specific membrane proteins to the flagellar membrane, but the mechanisms for this trafficking are poorly understood. We show here that KHARON, a protein originally identified in Leishmania parasites, interacts with a putative trypanosome calcium channel and is required for its targeting to the flagellar membrane. KHARON is located at the base of the flagellar axoneme, where it likely mediates targeting of flagellar membrane proteins, but is also on the subpellicular microtubules and the mitotic spindle. Hence, KHARON is probably a multifunctional protein that associates with several components of the trypanosome cytoskeleton. RNA interference-mediated knockdown of KHARON mRNA results in failure of the calcium channel to enter the flagellar membrane, detachment of the flagellum from the cell body, and disruption of mitotic spindles. Furthermore, knockdown of KHARON mRNA induces a lethal failure of cytokinesis in both bloodstream (mammalian host) and procyclic (insect vector) life cycle stages, and KHARON is thus critical for parasite viability.

The Trypanosome Flagellar Pocket Collar and Its Ring Forming Protein-TbBILBO1

Sub-species of Trypanosoma brucei are the causal agents of human African sleeping sickness and Nagana in domesticated livestock. These pathogens have developed an organelle-like compartment called the flagellar pocket (FP). The FP carries out endo- and exocytosis and is the only structure this parasite has evolved to do so. The FP is essential for parasite viability, making it an interesting structure to evaluate as a drug target, especially since it has an indispensible cytoskeleton component called the flagellar pocket collar (FPC). The FPC is located at the neck of the FP where the flagellum exits the cell. The FPC has a complex architecture and division cycle, but little is known concerning its organization. Recent work has focused on understanding how the FP and the FPC are formed and as a result of these studies an important calcium-binding, polymer-forming protein named TbBILBO1 was identified. Cellular biology analysis of TbBILBO1 has demonstrated its uniqueness as a FPC component and until recently, it was unknown what structural role it played in forming the FPC. This review summarizes the recent data on the polymer forming properties of TbBILBO1 and how these are correlated to the FP cytoskeleton.

3D Architecture of the Trypanosoma brucei Flagella Connector, a Mobile Transmembrane Junction

BACKGROUND

Cellular junctions are crucial for the formation of multicellular organisms, where they anchor cells to each other and/or supportive tissue and enable cell-to-cell communication. Some unicellular organisms, such as the parasitic protist Trypanosoma brucei, also have complex cellular junctions. The flagella connector (FC) is a three-layered transmembrane junction that moves with the growing tip of a new flagellum and attaches it to the side of the old flagellum. The FC moves via an unknown molecular mechanism, independent of new flagellum growth. Here we describe the detailed 3D architecture of the FC suggesting explanations for how it functions and its mechanism of motility.

METHODOLOGY/PRINCIPAL FINDINGS

We have used a combination of electron tomography and cryo-electron tomography to reveal the 3D architecture of the FC. Cryo-electron tomography revealed layers of repetitive filamentous electron densities between the two flagella in the interstitial zone. Though the FC does not change in length and width during the growth of the new flagellum, the interstitial zone thickness decreases as the FC matures. This investigation also shows interactions between the FC layers and the axonemes of the new and old flagellum, sufficiently strong to displace the axoneme in the old flagellum. We describe a novel filament, the flagella connector fibre, found between the FC and the axoneme in the old flagellum.

CONCLUSIONS/SIGNIFICANCE

The FC is similar to other cellular junctions in that filamentous proteins bridge the extracellular space and are anchored to underlying cytoskeletal structures; however, it is built between different portions of the same cell and is unique because of its intrinsic motility. The detailed description of its structure will be an important tool to use in attributing structure / function relationships as its molecular components are discovered in the future. The FC is involved in the inheritance of cell shape, which is important for the life cycle of this human parasite.

Flagellar pocket restructuring through the Leishmania life cycle involves a discrete flagellum attachment zone

Leishmania promastigote parasites have a flagellum, which protrudes from the flagellar pocket at the cell anterior, yet, surprisingly, have homologs of many flagellum attachment zone (FAZ) proteins – proteins used in the related Trypanosoma species to laterally attach the flagellum to the cell body from the flagellar pocket to the cell posterior. Here, we use seven Leishmania mexicana cell lines that expressed eYFP fusions of FAZ protein homologs to show that the Leishmania flagellar pocket includes a FAZ structure. Electron tomography revealed a precisely defined 3D organisation for both the flagellar pocket and FAZ, with striking similarities to those of Trypanosoma brucei. Expression of two T. brucei FAZ proteins in L. mexicana showed that T. brucei FAZ proteins can assemble into the Leishmania FAZ structure. Leishmania therefore have a previously unrecognised FAZ structure, which we show undergoes major structural reorganisation in the transition from the promastigote (sandfly vector) to amastigote (in mammalian macrophages). Morphogenesis of the Leishmania flagellar pocket, a structure important for pathogenicity, is therefore intimately associated with a FAZ; a finding with implications for understanding shape changes involving component modules during evolution.

Summary:Leishmania parasites have a highly structured flagellar pocket, including a structure homologous to the Trypanosoma brucei flagellum attachment zone, which undergoes structural adaptations in different life cycle stages.

Proteomic identification of novel cytoskeletal proteins associated with TbPLK, an essential regulator of cell morphogenesis in Trypanosoma brucei

Trypanosoma brucei is the causative agent of African sleeping sickness, a devastating disease endemic to sub-Saharan Africa with few effective treatment options. The parasite is highly polarized, including a single flagellum that is nucleated at the posterior of the cell and adhered along the cell surface. These features are essential and must be transmitted to the daughter cells during division. Recently we identified the T. brucei homologue of polo-like kinase (TbPLK) as an essential morphogenic regulator. In the present work, we conduct proteomic screens to identify potential TbPLK binding partners and substrates to better understand the molecular mechanisms of kinase function. These screens identify a cohort of proteins, most of which are completely uncharacterized, which localize to key cytoskeletal organelles involved in establishing cell morphology, including the flagella connector, flagellum attachment zone, and bilobe structure. Depletion of these proteins causes substantial changes in cell division, including mispositioning of the kinetoplast, loss of flagellar connection, and prevention of cytokinesis. The proteins identified in these screens provide the foundation for establishing the molecular networks through which TbPLK directs cell morphogenesis in T. brucei.

Combining quantitative 2D and 3D image analysis in the serial block face SEM: application to secretory organelles of pancreatic islet cells

A combination of two-dimensional (2D) and three-dimensional (3D) analyses of tissue volume ultrastructure acquired by serial block face scanning electron microscopy can greatly shorten the time required to obtain quantitative information from big data sets that contain many billions of voxels. Thus, to analyse the number of organelles of a specific type, or the total volume enclosed by a population of organelles within a cell, it is possible to estimate the number density or volume fraction of that organelle using a stereological approach to analyse randomly selected 2D block face views through the cells, and to combine such estimates with precise measurement of 3D cell volumes by delineating the plasma membrane in successive block face images. The validity of such an approach can be easily tested since the entire 3D tissue volume is available in the serial block face scanning electron microscopy data set. We have applied this hybrid 3D/2D technique to determine the number of secretory granules in the endocrine α and β cells of mouse pancreatic islets of Langerhans, and have been able to estimate the total insulin content of a β cell.

Scanning and three-dimensional electron microscopy methods for the study of Trypanosoma brucei and Leishmania mexicana flagella

Three-dimensional electron microscopy tools have revolutionized our understanding of cell structure and molecular complexes in biology. Here, we describe methods for studying flagellar ultrastructure and biogenesis in two unicellular parasites-Trypanosoma brucei and Leishmania mexicana. We describe methods for the preparation of these parasites for scanning electron microscopy cellular electron tomography, and serial block face scanning electron microscopy (SBFSEM). These parasites have a highly ordered cell shape and form, with a defined positioning of internal cytoskeletal structures and organelles. We show how knowledge of these can be used to dissect cell cycles in both parasites and identify the old flagellum from the new in T. brucei. Finally, we demonstrate the use of SBFSEM three-dimensional models for analysis of individual whole cells, demonstrating the excellent potential this technique has for future studies of mutant cell lines.

25 years of African trypanosome research: From description to molecular dissection and new drug discovery

The Molecular Parasitology conference was first held at the Marine Biological laboratory, Woods Hole, USA 25 years ago. Since that first meeting, the conference has evolved and expanded but has remained the showcase for the latest research developments in molecular parasitology. In this perspective, I reflect on the scientific discoveries focussed on African trypanosomes (Trypanosoma brucei spp.) that have occurred since the inaugural MPM meeting and discuss the current and future status of research on these parasites.

Quantitative analysis of mouse pancreatic islet architecture by serial block-face SEM

We have applied serial block-face scanning electron microscopy (SBF-SEM) to measure parameters that describe the architecture of pancreatic islets of Langerhans, microscopic endocrine organs that secrete insulin and glucagon for control of blood glucose. By analyzing entire mouse islets, we show that it is possible to determine (1) the distributions of alpha and beta cells, (2) the organization of blood vessels and pericapillary spaces, and (3) the ultrastructure of the individual secretory cells. Our results show that the average volume of a beta cell is nearly twice that of an alpha cell, and the total mitochondrial volume is about four times larger. In contrast, nuclear volumes in the two cell types are found to be approximately equal. Although the cores of alpha and beta secretory granules have similar diameters, the beta granules have prominent halos resulting in overall diameters that are twice those of alpha granules. Visualization of the blood vessels revealed that every secretory cell in the islet is in contact with the pericapillary space, with an average contact area of 9±5% of the cell surface area. Our data show that consistent results can be obtained by analyzing small numbers of islets. Due to the complicated architecture of pancreatic islets, such precision cannot easily be achieved by using TEM of thin sections.

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

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

Serial block face scanning electron microscopy--the future of cell ultrastructure imaging

One of the major drawbacks in transmission electron microscopy has been the production of three-dimensional views of cells and tissues. Currently, there is no one suitable 3D microscopy technique that answers all questions and serial block face scanning electron microscopy (SEM) fills the gap between 3D imaging using high-end fluorescence microscopy and the high resolution offered by electron tomography. In this review, we discuss the potential of the serial block face SEM technique for studying the three-dimensional organisation of animal, plant and microbial cells.

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.