Characterization of a coiled coil protein present in the basal body of Trypanosoma brucei

The conserved hypothetical protein Tb427.10.13790 is required for cytokinesis in Trypanosoma brucei

Trypanosoma brucei, a flagellated protozoan causing the deadly tropical disease Human African Trypanosomiasis (HAT), affects people in sub-Saharan Africa. HAT therapy relies upon drugs which use is limited by toxicity and rigorous treatment regimes, while development of vaccines remains elusive, due to the effectiveness of the parasite´s antigenic variation. Here, we evaluate a hypothetical protein Tb427.10.13790, as a potential drug target. This protein is conserved among all kinetoplastids, but lacks homologs in all other pro- and eukaryotes. Knockdown of Tb427.10.13790 resulted in appearance of monster cells containing multiple nuclei and multiple flagella, a considerable enlargement of the flagellar pocket and eventually a lethal phenotype. Furthermore, analysis of kinetoplast and nucleus division in the knockdown cell line revealed a partial cell cycle arrest and failure to initiate cytokinesis. Likewise, overexpression of the respective protein fused with enhanced green fluorescent protein was also lethal for T. brucei. In these cells, the labelled protein appeared as a single dot near kinetoplast and flagellar pocket. Our results reveal that Tb427.10.13790 is essential for the parasite´s viability and may be a suitable new anti-trypanosomatid drug target candidate. Furthermore, we suggest that it might be worthwhile to investigate also other of the many so far just annotated trypanosome genes as a considerable number of them to lack human homologs but may be of critical importance for the kinetoplastid parasites.

The Centriole Cartwheel Protein SAS-6 in Trypanosoma brucei Is Required for Probasal Body Biogenesis and Flagellum Assembly

The centriole in eukaryotes functions as the cell's microtubule-organizing center (MTOC) to nucleate spindle assembly, and its biogenesis requires an evolutionarily conserved protein, SAS-6, which assembles the centriole cartwheel. Trypanosoma brucei, an early branching protozoan, possesses the basal body as its MTOC to nucleate flagellum biogenesis. However, little is known about the components of the basal body and their roles in basal body biogenesis and flagellum assembly. Here, we report that the T. brucei SAS-6 homolog, TbSAS-6, is localized to the mature basal body and the probasal body throughout the cell cycle. RNA interference (RNAi) of TbSAS-6 inhibited probasal body biogenesis, compromised flagellum assembly, and caused cytokinesis arrest. Surprisingly, overexpression of TbSAS-6 in T. brucei also impaired probasal body duplication and flagellum assembly, contrary to SAS-6 overexpression in humans, which produces supernumerary centrioles. Furthermore, we showed that depletion of T. brucei Polo-like kinase, TbPLK, or inhibition of TbPLK activity did not abolish TbSAS-6 localization to the basal body, in contrast to the essential role of Polo-like kinase in recruiting SAS-6 to centrioles in animals. Altogether, these results identified the essential role of TbSAS-6 in probasal body biogenesis and flagellum assembly and suggest the presence of a TbPLK-independent pathway governing basal body duplication in T. brucei.

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.

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.

Polo-like kinase is necessary for flagellum inheritance in Trypanosoma brucei

Polo-like kinases play an important role in a variety of mitotic events in mammalian cells, ranging from centriole separation and chromosome congression to abscission. To fulfill these roles, Polo-like kinase homologs move to different cellular locations as the cell cycle progresses, starting at the centrosome, progressing to the spindle poles and then the midbody. In the protist parasite Trypanosoma brucei, the single polo-like kinase homolog T. brucei PLK (TbPLK) is essential for cytokinesis and is necessary for the correct duplication of a centrin-containing cytoskeletal structure known as the bilobe. We show that TbPLK has a dynamic localization pattern during the cell cycle. The kinase localizes to the basal body, which nucleates the flagellum, and then successively localizes to a series of cytoskeletal structures that regulate the position and attachment of the flagellum to the cell body. The kinase localizes to each of these structures as they are duplicating. TbPLK associates with a specialized set of microtubules, known as the microtubule quartet, which might transport the kinase during its migration. Depletion of TbPLK causes defects in basal body segregation and blocks the duplication of the regulators that position the flagellum, suggesting that its presence on these structures might be necessary for their proper biogenesis. TbPLK migrates throughout the cell in T. brucei, but the specific locations to which it targets and its functions are geared towards the inheritance of a properly positioned and attached flagellum.

Tools for analyzing intraflagellar transport in trypanosomes

African trypanosomes are evolutionary-divergent eukaryotes responsible for sleeping sickness. They duplicate their single flagellum while maintaining the old one, providing a unique model to examine mature and elongating flagella in the same cell. Like in most eukaryotes, the trypanosome flagellum is constructed by addition of novel subunits at its distal end via the action of intraflagellar transport (IFT). Almost all genes encoding IFT proteins and motors are conserved in trypanosomes and related species, with only a few exceptions. A dozen of IFT genes have been functionally investigated in this organism, thanks to the potent reverse genetic tools available. Several alternative techniques to trigger RNAi are accessible, either transient RNAi by transfection of long double-stranded RNA or by generation of clonal cell lines able to express long double-stranded RNA under the control of tetracycline-inducible promoters. In addition, we provide a series of techniques to investigate cellular phenotypes in trypanosomes where expression of IFT genes has been silenced. In this chapter, we describe different methods for tagging and expression of IFT proteins in trypanosomes and for visualizing IFT in live cells.

The Trypanosoma brucei flagellum: moving parasites in new directions

African trypanosomes are devastating human and animal pathogens. Trypanosoma brucei rhodesiense and T. b. gambiense subspecies cause the fatal human disease known as African sleeping sickness. It is estimated that several hundred thousand new infections occur annually and the disease is fatal if untreated. T. brucei is transmitted by the tsetse fly and alternates between bloodstream-form and insect-form life cycle stages that are adapted to survive in the mammalian host and the insect vector, respectively. The importance of the flagellum for parasite motility and attachment to the tsetse fly salivary gland epithelium has been appreciated for many years. Recent studies have revealed both conserved and novel features of T. brucei flagellum structure and composition, as well as surprising new functions that are outlined here. These discoveries are important from the standpoint of understanding trypanosome biology and identifying novel drug targets, as well as for advancing our understanding of fundamental aspects of eukaryotic flagellum structure and function.

Flagellum elongation is required for correct structure, orientation and function of the flagellar pocket in Trypanosoma brucei

In trypanosomes, the flagellum is rooted in the flagellar pocket, a surface micro-domain that is the sole site for endocytosis and exocytosis. By analysis of anterograde or retrograde intraflagellar transport in IFT88RNAi or IFT140RNAi mutant cells, we show that elongation of the new flagellum is not required for flagellar pocket formation but is essential for its organisation, orientation and function. Transmission electron microscopy revealed that the flagellar pocket exhibited a modified shape (smaller, distorted and/or deeper) in cells with abnormally short or no flagella. Scanning electron microscopy analysis of intact and detergent-extracted cells demonstrated that the orientation of the flagellar pocket collar was more variable in trypanosomes with short flagella. The structural protein BILBO1 was present but its localisation and abundance was altered. The membrane flagellar pocket protein CRAM leaked out of the pocket and reached the short flagella. CRAM also accumulated in intracellular compartments, indicating defects in routing of resident flagellar pocket proteins. Perturbations of vesicular trafficking were obvious; vesicles were observed in the lumen of the flagellar pocket or in the short flagella, and fluid-phase endocytosis was drastically diminished in non-flagellated cells. We propose a model to explain the role of flagellum elongation in correct flagellar pocket organisation and function.

Intraflagellar transport and functional analysis of genes required for flagellum formation in trypanosomes

Intraflagellar transport (IFT) is the bidirectional movement of protein complexes required for cilia and flagella formation. We investigated IFT by analyzing nine conventional IFT genes and five novel putative IFT genes (PIFT) in Trypanosoma brucei that maintain its existing flagellum while assembling a new flagellum. Immunostaining against IFT172 or expression of tagged IFT20 or green fluorescent protein GFP::IFT52 revealed the presence of IFT proteins along the axoneme and at the basal body and probasal body regions of both old and new flagella. IFT particles were detected by electron microscopy and exhibited a strict localization to axonemal microtubules 3-4 and 7-8, suggesting the existence of specific IFT tracks. Rapid (>3 microm/s) bidirectional intraflagellar movement of GFP::IFT52 was observed in old and new flagella. RNA interference silencing demonstrated that all individual IFT and PIFT genes are essential for new flagellum construction but the old flagellum remained present. Inhibition of IFTB proteins completely blocked axoneme construction. Absence of IFTA proteins (IFT122 and IFT140) led to formation of short flagella filled with IFT172, indicative of defects in retrograde transport. Two PIFT proteins turned out to be required for retrograde transport and three for anterograde transport. Finally, flagellum membrane elongation continues despite the absence of axonemal microtubules in all IFT/PIFT mutant.

Basal body positioning is controlled by flagellum formation in Trypanosoma brucei

To perform their multiple functions, cilia and flagella are precisely positioned at the cell surface by mechanisms that remain poorly understood. The protist Trypanosoma brucei possesses a single flagellum that adheres to the cell body where a specific cytoskeletal structure is localised, the flagellum attachment zone (FAZ). Trypanosomes build a new flagellum whose distal tip is connected to the side of the old flagellum by a discrete structure, the flagella connector. During this process, the basal body of the new flagellum migrates towards the posterior end of the cell. We show that separate inhibition of flagellum assembly, base-to-tip motility or flagella connection leads to reduced basal body migration, demonstrating that the flagellum contributes to its own positioning. We propose a model where pressure applied by movements of the growing new flagellum on the flagella connector leads to a reacting force that in turn contributes to migration of the basal body at the proximal end of the flagellum.

γ-Tubulin in Leishmania: cell cycle-dependent changes in subcellular localization and heterogeneity of its isoforms

A panel of six anti-peptide antibodies recognizing epitopes in different regions of the gamma-tubulin molecule was used for the characterization and localization of gamma-tubulin during cell cycle in Leishmania promastigotes. Immunofluorescence microscopy revealed the presence of gamma-tubulin in the basal bodies, posterior pole of the cell, and in the flagellum. Furthermore, the antibodies showed punctuate staining in the subpellicular microtubule. This complex localization pattern was observed in both interphase and dividing cells, where staining of posterior poles and the subpellicular corset was more prominent. In posterior poles, gamma-tubulin co-distributed with the 210-kDa microtubule-interacting protein and the 57-kDa protein immunodetected with anti-vimentin antibody. Immunogold electron microscopy on thin sections of isolated flagella showed that gamma-tubulin was associated with the paraflagellar rod (PFR) that runs adjacent to the axonemal microtubules. Under different extraction conditions, gamma-tubulin in Leishmania was found only in insoluble cytoskeletal fractions, in contrast to tubulin dimers that were both in soluble and cytoskeletal pool. Two-dimensional electrophoresis revealed multiple charge variants of gamma-tubulin. Posttranslational modifications of Leishmania gamma-tubulin might therefore have an important role in the regulation of microtubule nucleation and interaction with other proteins. The complex pattern of gamma-tubulin localization and its properties indicate that gamma-tubulin in Leishmania might have other function(s) besides microtubule nucleation.

T lymphocyte-triggering factor of african trypanosomes is associated with the flagellar fraction of the cytoskeleton and represents a new family of proteins that are present in several divergent eukaryotes

The trypanosome cytoskeleton consists almost entirely of microtubule-based structures. Although alpha- and beta-tubulin from Trypanosoma brucei have been well characterized, much less is known about other cytoskeleton-associated proteins in trypanosomes. Using biochemical fractionation, we demonstrate here that T lymphocyte-triggering factor (TLTF) from T. brucei is a component of the detergent-resistant and Ca(2+)-resistant fraction of the parasite cytoskeleton. This fraction contains the flagellar apparatus and a subset of cytoskeletal protein complexes that together function in cell motility, cytokinesis, and organelle inheritance. We also show that TLTF-related genes are present in several highly divergent eukaryotic organisms. Although the function of the corresponding proteins is not known, the mammalian TLTF-like gene (GAS11; growth arrest-specific gene 11) is up-regulated in growth-arrested cells and is a candidate tumor suppressor (Whitmore, S. A., Settasatian, C., Crawford, J., Lower, K. M., McCallum, B., Seshadri, R., Cornelisse, C. J., Moerland, E. W., Cleton-Jansen, A. M., Tipping, A. J., Mathew, C. G., Savnio, M., Savoia, A., Verlander, P., Auerbach, A. D., Van Berkel, C., Pronk, J. C., Doggett, N. A., and Callen, D. F. (1998) Genomics 52, 325-331), suggestive of a role in coordinating cytoskeleton activities. Consistent with this possibility, we show that the human GAS11 protein contains a 144-amino acid domain that co-localizes with microtubules when fused to the green fluorescent protein and expressed in mammalian cells. These findings suggest that TLTF represents a newly defined protein family, whose members contribute to cytoskeleton function in species as diverse as protozoa and mammals.