Assembly and function of complex flagellar structures illustrated by the paraflagellar rod of trypanosomes

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.

Intrabody-Induced Cell Death by Targeting the T. brucei Cytoskeletal Protein TbBILBO1

Trypanosoma brucei belongs to a genus of protists that cause life-threatening and economically important diseases of human and animal populations in Sub-Saharan Africa. T. brucei cells are covered in surface glycoproteins, some of which are used to escape the host immune system. Exo-/endocytotic trafficking of these and other molecules occurs via a single copy organelle called the flagellar pocket (FP). The FP is maintained and enclosed around the flagellum by the flagellar pocket collar (FPC). To date, the most important cytoskeletal component of the FPC is an essential calcium-binding, polymer-forming protein called TbBILBO1. In searching for novel tools to study this protein, we raised nanobodies (Nb) against purified, full-length TbBILBO1. Nanobodies were selected according to their binding properties to TbBILBO1, tested as immunofluorescence tools, and expressed as intrabodies (INb). One of them, Nb48, proved to be the most robust nanobody and intrabody. We further demonstrate that inducible, cytoplasmic expression of INb48 was lethal to these parasites, producing abnormal phenotypes resembling those of TbBILBO1 RNA interference (RNAi) knockdown. Our results validate the feasibility of generating functional single-domain antibody-derived intrabodies to target trypanosome cytoskeleton proteins.

IMPORTANCETrypanosoma brucei belongs to a group of important zoonotic parasites. We investigated how these organisms develop their cytoskeleton (the internal skeleton that controls cell shape) and focused on an essential protein (BILBO1) first described in T. brucei. To develop our analysis, we used purified BILBO1 protein to immunize an alpaca to make nanobodies (Nb). Nanobodies are derived from the antigen-binding portion of a novel antibody type found only in the camel and shark families of animals. Anti-BILBO1 nanobodies were obtained, and their encoding genes were inducibly expressed within the cytoplasm of T. brucei as intrabodies (INb). Importantly, INb48 expression rapidly killed parasites producing phenotypes normally observed after RNA knockdown, providing clear proof of principle. The importance of this study is derived from this novel approach, which can be used to study neglected and emerging pathogens as well as new model organisms, especially those that do not have the RNAi system.

An overview on kinetoplastid paraflagellar rod

Kinetoplastids, the evolutionary ancient organisms exhibit a rich and diverse biology which epitomizes many of the fascinating topics of recent interest and study. These organisms possess a multifunctional organelle, the flagellum containing a canonical 9 + 2 axoneme which is involved in vital roles, viz. parasite cell division, morphogenesis, motility and immune evasion. Since Antony Van Leeuwenhoek's innovative explanation of 'little legs' helping the movements of microbes in 1975, this biological nanomachine has captured the thoughts of scientists. The core structure of kinetoplastid flagellum is embroidered with a range of extra-axonemal structures such as paraflagellar rod (PFR), a large lattice like structure which extends alongside the axoneme from the flagellar pocket to the flagellar tip. The coding sequences for significant components of PFR are highly conserved throughout the Kinetoplastida and Euglenida. The high order organization and restricted evolutionary distribution of the PFR components and structure makes the PFR a particularly valuable therapeutic and prophylactic target. This review focuses on the recent developments in identification of ultra structural components of PFR in order to understand the function of this intriguing organelle and devising strategies for therapeutic interventions.

The eukaryotic flagellum makes the day: novel and unforeseen roles uncovered after post-genomics and proteomics data

This review will summarize and discuss the current biological understanding of the motile eukaryotic flagellum, as posed out by recent advances enabled by post-genomics and proteomics approaches. The organelle, which is crucial for motility, survival, differentiation, reproduction, division and feeding, among other activities, of many eukaryotes, is a great example of a natural nanomachine assembled mostly by proteins (around 350-650 of them) that have been conserved throughout eukaryotic evolution. Flagellar proteins are discussed in terms of their arrangement on to the axoneme, the canonical “9+2” microtubule pattern, and also motor and sensorial elements that have been detected by recent proteomic analyses in organisms such as Chlamydomonas reinhardtii, sea urchin, and trypanosomatids. Such findings can be remarkably matched up to important discoveries in vertebrate and mammalian types as diverse as sperm cells, ciliated kidney epithelia, respiratory and oviductal cilia, and neuro-epithelia, among others. Here we will focus on some exciting work regarding eukaryotic flagellar proteins, particularly using the flagellar proteome of C. reinhardtii as a reference map for exploring motility in function, dysfunction and pathogenic flagellates. The reference map for the eukaryotic flagellar proteome consists of 652 proteins that include known structural and intraflagellar transport (IFT) proteins, less well-characterized signal transduction proteins and flagellar associated proteins (FAPs), besides almost two hundred unannotated conserved proteins, which lately have been the subject of intense investigation and of our present examination.

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.

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.

RNA interference mutant induction in vivo demonstrates the essential nature of trypanosome flagellar function during mammalian infection

We demonstrate that trypanosomes compromised in flagellar function are rapidly cleared from infected mice. Analysis of the PFR2 bloodstream RNA interference mutant revealed that defective cell motility occurred prior to cytokinesis failure. This validation provides a paradigm for the flagellum as a target for future assays and interventions against this human pathogen.

Cloning and expression analysis of two novel paraflagellar rod domain genes found in Trypanosoma cruzi

The eukaryotic flagellum is one of the most complex macromolecular structures found in cells, containing more than 250 proteins. One unique structure in the flagella of trypanomastids is the paraflagellar rod (PFR). The PFR constitutes a lattice of cytoskeletal filaments that lies alongside the axoneme in the flagella. This unique and complex structure is critical for cell motility, though little is known about its molecular assembly or its role in the lifecycle of trypanosomatids. These proteins are of particular importance in Trypanosoma cruzi, as purified or recombinant PFR proteins have been demonstrated to be immunogenic, protecting mice from a lethal challenge with the parasite. We have searched the T. cruzi databases and discovered two novel genes containing PFR domains. Both these genes are transcribed in vivo and are significantly larger than the previously described PFR genes identified in T. cruzi (>2 Kb). Real-time PCR was used to examine the relative expression levels of six PFR genes, including the two we describe here, in all three stages of T. cruzi's lifecycle. Database searches have further provided EST and genomic sequence support for the presence of these genes in two other pathogenic trypanosomatids, Trypanosoma brucei and Leishmania spp. One of these genes, designated PFR5 contains a carboxy terminal SH3 domain not previously seen in PFR family genes. We propose that this proline-binding SH3 domain may play an important role in the assembly of the PFR.

The flagellum of trypanosomes

Eukaryotic cilia and flagella are cytoskeletal organelles that are remarkably conserved from protists to mammals. Their basic unit is the axoneme, a well-defined cylindrical structure composed of microtubules and up to 250 associated proteins. These complex organelles are assembled by a dynamic process called intraflagellar transport. Flagella and cilia perform diverse motility and sensitivity functions in many different organisms. Trypanosomes are flagellated protozoa, responsible for various tropical diseases such as sleeping sickness and Chagas disease. In this review, we first describe general knowledge on the flagellum: its occurrence in the living world, its molecular composition, and its mode of assembly, with special emphasis on the exciting developments that followed the discovery of intraflagellar transport. We then present recent progress regarding the characteristics of the trypanosome flagellum, highlighting the original contributions brought by this organism. The most striking phenomenon is the involvement of the flagellum in several aspects of the trypanosome cell cycle, including cell morphogenesis, basal body migration, and cytokinesis.

Efficiency and specificity of RNA interference generated by intra- and intermolecular double stranded RNA in Trypanosoma brucei

In many eukaryotes, double-stranded (ds) RNA leads to specific degradation of RNA of cognate sequence, a process termed RNA interference (RNAi). Here we used the protozoan Trypanosoma brucei as a model to investigate efficiency and specificity of RNAi generated by expression of long dsRNA of PFRA and PFRC genes, which code for flagellar proteins required for cell motility. Consequences of RNAi were monitored at all three levels: target RNA expression, protein expression and phenotype observation, using population or individual cell analysis. Expression of PFRA dsRNA from an inverted repeat was extremely efficient, knocking down PFRA RNA and PFRA protein, and producing a severe paralysis phenotype. Silencing by expression of PFRA dsRNA using a dual facing promoter system was also very efficient, producing a clear phenotype, although low amounts of PFRA RNA and PFRA protein were detected. Expression via the dual facing promoters of PAR2 dsRNA (83% overall identity with PFRA, including nine blocks of >20 nt total identity) did not produce significant reduction of total amounts of PFRA RNA or PFRA protein. However, individual cell analysis by immunofluorescence revealed that 10-60% cells (depending on subclones) exhibited lower PFRA amounts in their flagellum, producing a reduced-motility phenotype.

The biology of kinetoplastid parasites: insights and challenges from genomics and post-genomics

Kinetoplastid parasites exhibit a rich and diverse biology which mirrors many of the most interesting topics of current interest and study in the broader biological sciences. These evolutionarily ancient organisms possess intriguing mechanisms for control of gene expression, and exhibit complex patterns of cell morphogenesis orchestrated by an internal cytoskeleton. Their cell shapes change during a set of complex cell type differentiations in their life cycles. These differentiations are intimately linked to interactions with mammalian hosts or insect vectors, and often, these differentiations appear central to the successful transfer of the parasite between vector and host, and host and vector. The basics of this rich and complex cell and life cycle biology were described (with often rather forgotten clarity and prescience) in the early period of the last century. The last 30 years have seen major developments in our understanding of this biology. Ultrastructural differences in the various cells of the life cycle stages of Trypanosoma brucei, Trypanosoma cruzi and the various Leishmania species have been documented, and such studies have proven highly informative in defining important aspects of parasite adaptation. They have also proven to be a rich source of information for defining unusual aspects of parasite cell biology, novel organelles and cell architecture. This ultrastructural cell biology has been mirrored in a set of biochemical explanations defining unusual aspects of metabolism, surface molecules, and organelles. Finally, the application of molecular biology to these parasites revealed fascinating layers of complexity in the control of gene expression. These molecular studies have given us particular insights into polycistronic transcription, trans-splicing, RNA editing and gene rearrangements during antigenic variation. In contrast to other microbial systems, these cell biological, biochemical and molecular studies have not been greatly aided by insights gained from genetics--the diploid nature of the genome has discouraged the application of selectional genetics, mutant isolation and analysis. This is an important fact, since in general, it means that we have only recently started to analyse the phenotypes of mutants produced in the context of reverse genetics. In the following, I will argue that this lack of investment in the analysis of mutant phenotype is just one of the challenges that will need to be met if we are to gain the expected added value from the parasite genome projects. In this presentation, I will use some of the current areas of interest in the biology of T. brucei, T. cruzi and the Leishmania species to rehearse some of the insights and challenges that are likely to stem from the application of genomics and post-genomic studies to the kinetoplastid parasites. In some cases, I will exemplify points by illustrations from my laboratory's work, interests and hypotheses. The presentation slants therefore towards T. brucei biology, however, in each case the reader will, no doubt, see the generalities of application to other kinetoplastid parasites.

Inside and outside of the trypanosome flagellum:a multifunctional organelle

Amongst the earliest eukaryotes, trypanosomes have developed conventional organelles but sometimes with extreme features rarely seen in other organisms. This is the case of the flagellum, containing conventional and unique structures whose role in infectivity is still enigmatic.

Flagellar morphogenesis: protein targeting and assembly in the paraflagellar rod of trypanosomes

The paraflagellar rod (PFR) of the African trypanosome Trypanosoma brucei represents an excellent model to study flagellum assembly. The PFR is an intraflagellar structure present alongside the axoneme and is composed of two major proteins, PFRA and PFRC. By inducible expression of a functional epitope-tagged PFRA protein, we have been able to monitor PFR assembly in vivo. As T. brucei cells progress through their cell cycle, they possess both an old and a new flagellum. The induction of expression of tagged PFRA in trypanosomes growing a new flagellum provided an excellent marker of newly synthesized subunits. This procedure showed two different sites of addition: a major, polar site at the distal tip of the flagellum and a minor, nonpolar site along the length of the partially assembled PFR. Moreover, we have observed turnover of epitope-tagged PFRA in old flagella that takes place throughout the length of the PFR structure. Expression of truncated PFRA mutant proteins identified a sequence necessary for flagellum localization by import or binding. This sequence was not sufficient to confer full flagellum localization to a green fluorescent protein reporter. A second sequence, necessary for the addition of PFRA protein to the distal tip, was also identified. In the absence of this sequence, the mutant PFRA proteins were localized both in the cytosol and in the flagellum where they could still be added along the length of the PFR. This seven-amino-acid sequence is conserved in all PFRA and PFRC proteins and shows homology to a sequence in the flagellar dynein heavy chain of Chlamydomonas reinhardtii.