Basal body and flagellum mutants reveal a rotational constraint of the central pair microtubules in the axonemes of trypanosomes

Intraflagellar transport speed is sensitive to genetic and mechanical perturbations to flagellar beating

Two sets of motor proteins underpin motile cilia/flagella function. The axoneme-associated inner and outer dynein arms drive sliding of adjacent axoneme microtubule doublets to periodically bend the flagellum for beating, while intraflagellar transport (IFT) kinesins and dyneins carry IFT trains bidirectionally along the axoneme. Despite assembling motile cilia and flagella, IFT train speeds have only previously been quantified in immobilized flagella-mechanical immobilization or genetic paralysis. This has limited investigation of the interaction between IFT and flagellar beating. Here, in uniflagellate Leishmania parasites, we use high-frequency, dual-color fluorescence microscopy to visualize IFT train movement in beating flagella. We discovered that adhesion of flagella to a microscope slide is detrimental, reducing IFT train speed and increasing train stalling. In flagella free to move, IFT train speed is not strongly dependent on flagella beat type; however, permanent disruption of flagella beating by deletion of genes necessary for formation or regulation of beating showed an inverse correlation of beat frequency and IFT train speed.

Proximity-dependent biotinylation and identification of flagellar proteins in Trypanosoma cruzi

The flagellated kinetoplastid protozoan and causative agent of human Chagas disease, Trypanosoma cruzi , inhabits both invertebrate and mammalian hosts over the course of its complex life cycle. In these disparate environments, T. cruzi uses its single flagellum to propel motile life stages and in some instances, to establish intimate contact with the host. Beyond its role in motility, the functional capabilities of the T. cruzi flagellum have not been defined. Moreover, the lack of proteomic information for this organelle, in any parasite life stage, has limited functional investigation. In this study, we employed a proximity-dependent biotinylation approach based on the differential targeting of the biotin ligase, TurboID, to the flagellum or cytosol in replicative stages of T. cruzi , to identify flagellar-enriched proteins by mass spectrometry. Proteomic analysis of the resulting biotinylated protein fractions yielded 218 candidate flagellar proteins in T. cruzi epimastigotes (insect stage) and 99 proteins in intracellular amastigotes (mammalian stage). Forty of these flagellar-enriched proteins were common to both parasite life stages and included orthologs of known flagellar proteins in other trypanosomatid species, proteins specific to the T. cruzi lineage and hypothetical proteins. With the validation of flagellar localization for several of the identified candidates, our results demonstrate that TurboID-based proximity proteomics is an effective tool for probing subcellular compartments in T. cruzi . The proteomic datasets generated in this work offer a valuable resource to facilitate functional investigation of the understudied T. cruzi flagellum.

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.

Structure of the trypanosome paraflagellar rod and insights into non-planar motility of eukaryotic cells

Eukaryotic flagella (synonymous with cilia) rely on a microtubule-based axoneme, together with accessory filaments to carryout motility and signaling functions. While axoneme structures are well characterized, 3D ultrastructure of accessory filaments and their axoneme interface are mostly unknown, presenting a critical gap in understanding structural foundations of eukaryotic flagella. In the flagellum of the protozoan parasite Trypanosoma brucei (T. brucei), the axoneme is accompanied by a paraflagellar rod (PFR) that supports non-planar motility and signaling necessary for disease transmission and pathogenesis. Here, we employed cryogenic electron tomography (cryoET) with sub-tomographic averaging, to obtain structures of the PFR, PFR-axoneme connectors (PACs), and the axonemal central pair complex (CPC). The structures resolve how the 8 nm repeat of the axonemal tubulin dimer interfaces with the 54 nm repeat of the PFR, which consist of proximal, intermediate, and distal zones. In the distal zone, stacked "density scissors" connect with one another to form a "scissors stack network (SSN)" plane oriented 45° to the axoneme axis; and ~370 parallel SSN planes are connected by helix-rich wires into a paracrystalline array with ~90% empty space. Connections from these wires to the intermediate zone, then to overlapping layers of the proximal zone and to the PACs, and ultimately to the CPC, point to a contiguous pathway for signal transmission. Together, our findings provide insights into flagellum-driven, non-planar helical motility of T. brucei and have broad implications ranging from cell motility and tensegrity in biology, to engineering principles in bionics.

Delta and epsilon tubulin in mammalian development

Delta (δ-) and epsilon (ε-) tubulin are lesser-known cousins of alpha (α-) and beta (β-) tubulin. They are likely to regulate centriole function in a broad range of species; however, their in vivo role and mechanism of action in mammals remain mysterious. In unicellular species and mammalian cell lines, mutations in δ- and ε-tubulin cause centriole destabilization and atypical mitosis and, in the most severe cases, cell death. Beyond the centriole, δ- and ε-tubulin localize to the manchette during murine spermatogenesis and interact with katanin-like 2 (KATNAL2), a protein with microtubule (MT)-severing properties, indicative of novel non-centriolar functions. Herein we summarize the current knowledge surrounding δ- and ε-tubulin, identify pathways for future research, and highlight how and why spermatogenesis and embryogenesis are ideal systems to define δ- and ε-tubulin function in vivo.

Central Apparatus, the Molecular Kickstarter of Ciliary and Flagellar Nanomachines

Motile cilia and homologous organelles, the flagella, are an early evolutionarily invention, enabling primitive eukaryotic cells to survive and reproduce. In animals, cilia have undergone functional and structural speciation giving raise to typical motile cilia, motile nodal cilia, and sensory immotile cilia. In contrast to other cilia types, typical motile cilia are able to beat in complex, two-phase movements. Moreover, they contain many additional structures, including central apparatus, composed of two single microtubules connected by a bridge-like structure and assembling numerous complexes called projections. A growing body of evidence supports the important role of the central apparatus in the generation and regulation of the motile cilia movement. Here we review data concerning the central apparatus structure, protein composition, and the significance of its components in ciliary beating regulation.

The single flagellum of Leishmania has a fixed polarisation of its asymmetric beat

Eukaryotic flagella undertake different beat types as necessary for different functions; for example, the Leishmania parasite flagellum undergoes a symmetric tip-to-base beat for forward swimming and an asymmetric base-to-tip beat to rotate the cell. In multi-ciliated tissues or organisms, the asymmetric beats are coordinated, leading to movement of the cell, organism or surrounding fluid. This coordination involves a polarisation of power stroke direction. Here, we asked whether the asymmetric beat of the single Leishmania flagellum also has a fixed polarisation. We developed high frame rate dual-colour fluorescence microscopy to visualise flagellar-associated structures in live swimming cells. This showed that the asymmetric Leishmania beat is polarised, with power strokes only occurring in one direction relative to the asymmetric flagellar machinery. Polarisation of bending was retained in deletion mutants whose flagella cannot beat but have a static bend. Furthermore, deletion mutants for proteins required for asymmetric extra-axonemal and rootlet-like flagellum-associated structures also retained normal polarisation. Leishmania beat polarisation therefore likely arises from either the nine-fold rotational symmetry of the axoneme structure or is due to differences between the outer doublet decorations.

Highlighted Article: By using high speed, high-resolution fluorescence microscopy of swimming Leishmania cells, we showed that the asymmetric flagellar beat always wafts in the same direction and investigate which structures are involved.

Control of assembly of extra-axonemal structures: the paraflagellar rod of trypanosomes

Eukaryotic flagella are complex microtubule-based organelles that, in many organisms, contain extra-axonemal structures, such as the outer dense fibres of mammalian sperm and the paraflagellar rod (PFR) of trypanosomes. Flagellum assembly is a complex process occurring across three main compartments, the cytoplasm, the transition zone and the flagellum itself. The process begins with the translation of protein components followed by their sorting and trafficking into the flagellum, transport to the assembly site and incorporation. Flagella are formed from over 500 proteins and the principles governing assembly of the axonemal components are relatively clear. However, the coordination and location of assembly of extra-axonemal structures are less clear. We have discovered two cytoplasmic proteins in Trypanosoma brucei that are required for PFR formation, PFR assembly factors 1 and 2 (PFR-AF1 and PFR-AF2, respectively). Deletion of either PFR-AF1 or PFR-AF2 dramatically disrupted PFR formation and caused a reduction in the amount of major PFR proteins. The existence of cytoplasmic factors required for PFR formation aligns with the concept that processes facilitating axoneme assembly occur across multiple compartments, and this is likely a common theme for extra-axonemal structure assembly.

The unity and diversity of the ciliary central apparatus

Nearly all motile cilia and flagella (terms here used interchangeably) have a '9+2' axoneme containing nine outer doublet microtubules and two central microtubules. The central pair of microtubules plus associated projections, termed the central apparatus (CA), is involved in the control of flagellar motility and is essential for the normal movement of '9+2' cilia. Research using the green alga Chlamydomonas reinhardtii, an important model system for studying cilia, has provided most of our knowledge of the protein composition of the CA, and recent work using this organism has expanded the number of known and candidate CA proteins nearly threefold. Here we take advantage of this enhanced proteome to examine the genomes of a wide range of eukaryotic organisms, representing all of the major phylogenetic groups, to identify predicted orthologues of the C. reinhardtii CA proteins and explore how widely the proteins are conserved and whether there are patterns to this conservation. We also discuss in detail two contrasting groups of CA proteins-the ASH-domain proteins, which are broadly conserved, and the PAS proteins, which are restricted primarily to the volvocalean algae. This article is part of the Theo Murphy meeting issue 'Unity and diversity of cilia in locomotion and transport'.

CEP44 ensures the formation of bona fide centriole wall, a requirement for the centriole-to-centrosome conversion

Centrosomes are essential organelles with functions in microtubule organization that duplicate once per cell cycle. The first step of centrosome duplication is the daughter centriole formation followed by the pericentriolar material recruitment to this centriole. This maturation step was termed centriole-to-centrosome conversion. It was proposed that CEP295-dependent recruitment of pericentriolar proteins drives centriole conversion. Here we show, based on the analysis of proteins that promote centriole biogenesis, that the developing centriole structure helps drive centriole conversion. Depletion of the luminal centriole protein CEP44 that binds to the A-microtubules and interacts with POC1B affecting centriole structure and centriole conversion, despite CEP295 binding to centrioles. Impairment of POC1B, TUBE1 or TUBD1, which disturbs integrity of centriole microtubules, also prevents centriole-to-centrosome conversion. We propose that the CEP295, CEP44, POC1B, TUBE1 and TUBD1 centriole biogenesis pathway that functions in the centriole lumen and on the cytoplasmic side is essential for the centriole-to-centrosome conversion.

During cell division, centrosomes duplicate and newly formed centrioles must undergo centriole-to-centrosome conversion, but the molecular details are unclear. Here, the authors report that the centriole microtubule-triplet 9-fold structure scaffolds pericentriolar proteins and permits the conversion of centrioles to fully functional centrosomes.

LAX28 is required for the stable assembly of the inner dynein arm f complex, and the tether and tether head complex in Leishmania flagella

Motile eukaryotic flagella beat through coordinated activity of dynein motor proteins; however, the mechanisms of dynein coordination and regulation are incompletely understood. The inner dynein arm (IDA) f complex (also known as the I1 complex), and the tether and tether head (T/TH) complex are thought to be key regulators of dynein action but, unlike the IDA f complex, T/TH proteins remain poorly characterised. Here, we characterised T/TH-associated proteins in the protist Leishmania mexicana Proteome analysis of axonemes from null mutants for the CFAP44 T/TH protein showed that they lacked the IDA f protein IC140 and a novel 28-kDa axonemal protein, LAX28. Sequence analysis identified similarities between LAX28 and the uncharacterised human sperm tail protein TEX47, both sharing features with sensory BLUF-domain-containing proteins. Leishmania lacking LAX28, CFAP44 or IC140 retained some motility, albeit with reduced swimming speed and directionality and a propensity for flagellar curling. Expression of tagged proteins in different null mutant backgrounds showed that the axonemal localisation of LAX28 requires CFAP44 and IC140, and the axonemal localisations of CFAP44 and IC140 both depend on LAX28. These data demonstrate a role for LAX28 in motility and show mutual dependencies of IDA f and T/TH-associated proteins for axonemal assembly in Leishmania.

It takes two (centrioles) to tango

Cells that divide during embryo development require precisely two centrioles during interphase and four centrioles during mitosis. This precise number is maintained by allowing each centriole to nucleate only one centriole per cell cycle (i.e. centriole duplication). Yet, how the first cell of the embryo, the zygote, obtains two centrioles has remained a mystery in most mammals and insects. The mystery arose because the female gamete (oocyte) is thought to have no functional centrioles and the male gamete (spermatozoon) is thought to have only one functional centriole, resulting in a zygote with a single centriole. However, recent studies in fruit flies, beetles and mammals, including humans, suggest an alternative explanation: spermatozoa have a typical centriole and an atypical centriole. The sperm typical centriole has a normal structure but distinct protein composition, whereas the sperm atypical centriole is distinct in both. During fertilization, the atypical centriole is released into the zygote, nucleates a new centriole and participates in spindle pole formation. Thus, the spermatozoa's atypical centriole acts as a second centriole in the zygote. Here, we review centriole biology in general and especially in reproduction, we describe the discovery of the spermatozoon atypical centriole, and we provide an updated model for centriole inherence during sexual reproduction. While we focus on humans and other non-rodent mammals, we also provide a broader evolutionary perspective.

Bidirectional intraflagellar transport is restricted to two sets of microtubule doublets in the trypanosome flagellum

Intraflagellar transport (IFT) is the movement of large protein complexes responsible for the construction of cilia and flagella. Using a combination of three-dimensional electron microscopy and high-resolution live imaging, Bertiaux et al. show that IFT takes place on only four microtubule doublets out of the nine available in the trypanosome flagellum.

Intraflagellar transport (IFT) is the rapid bidirectional movement of large protein complexes driven by kinesin and dynein motors along microtubule doublets of cilia and flagella. In this study, we used a combination of high-resolution electron and light microscopy to investigate how and where these IFT trains move within the flagellum of the protist Trypanosoma brucei. Focused ion beam scanning electron microscopy (FIB-SEM) analysis of trypanosomes showed that trains are found almost exclusively along two sets of doublets (3–4 and 7–8) and distribute in two categories according to their length. High-resolution live imaging of cells expressing mNeonGreen::IFT81 or GFP::IFT52 revealed for the first time IFT trafficking on two parallel lines within the flagellum. Anterograde and retrograde IFT occurs on each of these lines. At the distal end, a large individual anterograde IFT train is converted in several smaller retrograde trains in the space of 3–4 s while remaining on the same side of the axoneme.

Revisiting Centrioles in Nematodes-Historic Findings and Current Topics

Theodor Boveri is considered as the “father” of centrosome biology. Boveri’s fundamental findings have laid the groundwork for decades of research on centrosomes. Here, we briefly review his early work on centrosomes and his first description of the centriole. Mainly focusing on centriole structure, duplication, and centriole assembly factors in C. elegans, we will highlight the role of this model in studying germ line centrosomes in nematodes. Last but not least, we will point to future directions of the C. elegans centrosome field.

Direction of flagellum beat propagation is controlled by proximal/distal outer dynein arm asymmetry

The 9 + 2 axoneme structure of the motile flagellum/cilium is an iconic, apparently symmetrical cellular structure. Recently, asymmetries along the length of motile flagella have been identified in a number of organisms, typically in the inner and outer dynein arms. Flagellum-beat waveforms are adapted for different functions. They may start either near the flagellar tip or near its base and may be symmetrical or asymmetrical. We hypothesized that proximal/distal asymmetry in the molecular composition of the axoneme may control the site of waveform initiation and the direction of waveform propagation. The unicellular eukaryotic pathogens Trypanosoma brucei and Leishmania mexicana often switch between tip-to-base and base-to-tip waveforms, making them ideal for analysis of this phenomenon. We show here that the proximal and distal portions of the flagellum contain distinct outer dynein arm docking-complex heterodimers. This proximal/distal asymmetry is produced and maintained through growth by a concentration gradient of the proximal docking complex, generated by intraflagellar transport. Furthermore, this asymmetry is involved in regulating whether a tip-to-base or base-to-tip beat occurs, which is linked to a calcium-dependent switch. Our data show that the mechanism for generating proximal/distal flagellar asymmetry can control waveform initiation and propagation direction.

The ABCs of Centriole Architecture: The Form and Function of Triplet Microtubules

The centriole is a defining feature of many eukaryotic cells. It nucleates a cilium, organizes microtubules as part of the centrosome, and is duplicated in coordination with the cell cycle. Centrioles have a remarkable structure, consisting of microtubules arranged in a barrel with ninefold radial symmetry. At their base, or proximal end, centrioles have unique triplet microtubules, formed from three microtubules linked to each other. This microtubule organization is not found anywhere else in the cell, is conserved in all major branches of the eukaryotic tree, and likely was present in the last eukaryotic common ancestor. At their tip, or distal end, centrioles have doublet microtubules, which template the cilium. Here, we consider the structures of the compound microtubules in centrioles and discuss potential mechanisms for their formation and their function. We propose that triplet microtubules are required for the structural integrity of centrioles, allowing the centriole to serve as the essential nucleator of the cilium.

Centriole triplet microtubules are required for stable centriole formation and inheritance in human cells

Centrioles are composed of long-lived microtubules arranged in nine triplets. However, the contribution of triplet microtubules to mammalian centriole formation and stability is unknown. Little is known of the mechanism of triplet microtubule formation, but experiments in unicellular eukaryotes indicate that delta-tubulin and epsilon-tubulin, two less-studied tubulin family members, are required. Here, we report that centrioles in delta-tubulin and epsilon-tubulin null mutant human cells lack triplet microtubules and fail to undergo centriole maturation. These aberrant centrioles are formed de novo each cell cycle, but are unstable and do not persist to the next cell cycle, leading to a futile cycle of centriole formation and disintegration. Disintegration can be suppressed by paclitaxel treatment. Delta-tubulin and epsilon-tubulin physically interact, indicating that these tubulins act together to maintain triplet microtubules and that these are necessary for inheritance of centrioles from one cell cycle to the next.

A CRISPR Cas9 high-throughput genome editing toolkit for kinetoplastids

Clustered regularly interspaced short palindromic repeats (CRISPR), CRISPR-associated gene 9 (Cas9) genome editing is set to revolutionize genetic manipulation of pathogens, including kinetoplastids. CRISPR technology provides the opportunity to develop scalable methods for high-throughput production of mutant phenotypes. Here, we report development of a CRISPR-Cas9 toolkit that allows rapid tagging and gene knockout in diverse kinetoplastid species without requiring the user to perform any DNA cloning. We developed a new protocol for single-guide RNA (sgRNA) delivery using PCR-generated DNA templates which are transcribed in vivo by T7 RNA polymerase and an online resource (LeishGEdit.net) for automated primer design. We produced a set of plasmids that allows easy and scalable generation of DNA constructs for transfections in just a few hours. We show how these tools allow knock-in of fluorescent protein tags, modified biotin ligase BirA*, luciferase, HaloTag and small epitope tags, which can be fused to proteins at the N- or C-terminus, for functional studies of proteins and localization screening. These tools enabled generation of null mutants in a single round of transfection in promastigote form Leishmania major, Leishmania mexicana and bloodstream form Trypanosoma brucei; deleted genes were undetectable in non-clonal populations, enabling for the first time rapid and large-scale knockout screens.

Evolution of Cilia

Anton van Leeuwenhoek's startling microscopic observations in the 1600s first stimulated fascination with the way that cells use cilia to generate currents and to swim in a fluid environment. Research in recent decades has yielded deep knowledge about the mechanical and biochemical nature of these organelles but only opened a greater fascination about how such beautifully intricate and multifunctional structures arose during evolution. Answers to this evolutionary puzzle are not only sought to satisfy basic curiosity, but also, as stated so eloquently by Dobzhansky (Am Zool 4: 443 [1964]), because "nothing in biology makes sense except in the light of evolution." Here I attempt to summarize current knowledge of what ciliary organelles of the last eukaryotic common ancestor (LECA) were like, explore the ways in which cilia have evolved since that time, and speculate on the selective processes that might have generated these organelles during early eukaryotic evolution.

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.

Basal body multipotency and axonemal remodelling are two pathways to a 9+0 flagellum

Eukaryotic cilia/flagella exhibit two characteristic ultrastructures reflecting two main functions; a 9+2 axoneme for motility and a 9+0 axoneme for sensation and signalling. Whether, and if so how, they interconvert is unclear. Here we analyse flagellum length, structure and molecular composition changes in the unicellular eukaryotic parasite Leishmania during the transformation of a life cycle stage with a 9+2 axoneme (the promastigote) to one with a 9+0 axoneme (the amastigote). We show 9+0 axonemes can be generated by two pathways: by de novo formation and by restructuring of existing 9+2 axonemes associated with decreased intraflagellar transport. Furthermore, pro-basal bodies formed under conditions conducive for 9+2 axoneme formation can form a 9+0 axoneme de novo. We conclude that pro-centrioles/pro-basal bodies are multipotent and not committed to form either a 9+2 or 9+0 axoneme. In an alternative pathway structures can also be removed from existing 9+2 axonemes to convert them to 9+0.

Whether basal bodies are pre-committed to form 9+2 motile or 9+0 sensory axonemes and whether interconversion occurs between the two types of axonemes is not clear. Here, the authors used the unicellular eukaryote Leishmania as a model system to demonstrate that 9+0 axonemes can be formed de novo or by restructuring of 9+2 axonemes.

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.

A MEIG1/PACRG complex in the manchette is essential for building the sperm flagella

A key event in the process of spermiogenesis is the formation of the flagella, which enables sperm to reach eggs for fertilization. Yeast two-hybrid studies revealed that meiosis-expressed gene 1 (MEIG1) and Parkin co-regulated gene (PACRG) interact, and that sperm-associated antigen 16, which encodes an axoneme central apparatus protein, is also a binding partner of MEIG1. In spermatocytes of wild-type mice, MEIG1 is expressed in the whole germ cell bodies, but the protein migrates to the manchette, a unique structure at the base of elongating spermatid that directs formation of the flagella. In the elongating spermatids of wild-type mice, PACRG colocalizes with α-tubulin, a marker for the manchette, whereas this localization was not changed in the few remaining elongating spermatids of Meig1-deficient mice. In addition, MEIG1 no longer localizes to the manchette in the remaining elongating spermatids of Pacrg-deficient mice, indicating that PACRG recruits MEIG1 to the manchette. PACRG is not stable in mammalian cells, but can be stabilized by MEIG1 or by inhibition of proteasome function. SPAG16L is present in the spermatocyte cytoplasm of wild-type mice, and in the manchette of elongating spermatids, but in the Meig1 or Pacrg-deficient mice, SPAG16L no longer localizes to the manchette. By contrast, MEIG1 and PACRG are still present in the manchette of Spag16L-deficient mice, indicating that SPAG16L is a downstream partner of these two proteins. Together, our studies demonstrate that MEIG1/PACRG forms a complex in the manchette and that this complex is necessary to transport cargos, such as SPAG16L, to build the sperm flagella.

Summary: In the manchette, a structure at the base of the elongating spermatid, the proteins MEIG1 and PACRG act in a complex to control cargo transport and direct formation of the flagellum.

Space-dependent formation of central pair microtubules and their interactions with radial spokes

Cilia and flagella contain nine outer doublet microtubules and a pair of central microtubules. The central pair of microtubules (CP) is important for cilia/flagella beating, as clearly shown by primary ciliary dyskinesia resulting from the loss of the CP. The CP is thought to regulate axonemal dyneins through interaction with radial spokes (RSs). However, the nature of the CP-RS interaction is poorly understood. Here we examine the appearance of CPs in the axonemes of a Chlamydomonas mutant, bld12, which produces axonemes with 8 to 11 outer-doublets. Most of its 8-doublet axonemes lack CPs. However, in the double mutant of bld12 and pf14, a mutant lacking the RS, most 8-doublet axonemes contain the CP. Thus formation of the CP apparently depends on the internal space limited by the outer doublets and RSs. In 10- or 11-doublet axonemes, only 3–5 RSs are attached to the CP and the doublet arrangement is distorted most likely because the RSs attached to the CP pull the outer doublets toward the axonemal center. The CP orientation in the axonemes varies in double mutants formed between bld12 and mutants lacking particular CP projections. The mutant bld12 thus provides the first direct and visual information about the CP-RS interaction, as well as about the mechanism of CP formation.

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.

The GTPase IFT27 is involved in both anterograde and retrograde intraflagellar transport

The construction of cilia and flagella depends on intraflagellar transport (IFT), the bidirectional movement of two protein complexes (IFT-A and IFT-B) driven by specific kinesin and dynein motors. IFT-B and kinesin are associated to anterograde transport whereas IFT-A and dynein participate to retrograde transport. Surprisingly, the small GTPase IFT27, a member of the IFT-B complex, turns out to be essential for retrograde cargo transport in Trypanosoma brucei. We reveal that this is due to failure to import both the IFT-A complex and the IFT dynein into the flagellar compartment. To get further molecular insight about the role of IFT27, GDP- or GTP-locked versions were expressed in presence or absence of endogenous IFT27. The GDP-locked version is unable to enter the flagellum and to interact with other IFT-B proteins and its sole expression prevents flagellum formation. These findings demonstrate that a GTPase-competent IFT27 is required for association to the IFT complex and that IFT27 plays a role in the cargo loading of the retrograde transport machinery.

DOI:http://dx.doi.org/10.7554/eLife.02419.001

Long, thin structures called cilia and flagella are found on the surface of many cells, and perform a range of roles, including propelling the cells around or sensing changes in the surrounding environment. A process called intraflagellar transport (IFT for short) is responsible for flagellum construction in eukaryotic cells. Protein complexes called IFT trains carry the building blocks that make up flagella along microtubule ‘tracks’ between the base and the tip of a flagellum.

IFT trains are made from two different protein complexes called IFT-A and IFT-B, which are dragged by various molecular motors. The IFT-B complex is necessary for the train to move towards the tip of the flagellum, and so enables the flagellum to grow. The IFT-A protein complex is required to recycle the train back towards the base of the flagellum.

Huet et al. examined the role that a protein called IFT27 plays in intraflagellar transport. IFT27 is part of the IFT-B complex, and so it was thought to only affect how flagella grow. However, short flagella still grow when IFT27 is absent, but they are filled with IFT trains that are not able to reverse back from the tip. Huet et al. reveal that the IFT-A complex and the molecular motor that is essential for reversing the train are not transported into the flagellum if IFT27 is not present. This is therefore an unusual case of an IFT-B protein affecting the IFT-A complex and the transport back to the base.

IFT27 also affects how the IFT-B complex forms. ITF27 can bind to some small molecules, which can switch the protein ‘on’ or ‘off’. Huet et al. found that when IFT27 is switched off it is not transported into flagella, and also cannot bind to some of the other proteins in the IFT-B complex. This means that if IFT27 is locked in an inactive state, the IFT-B complex does not form, and a flagellum cannot grow. Therefore, activated IFT27 is needed for putting together the IFT train and to ensure its movement in either direction along the microtubule tracks.

DOI:http://dx.doi.org/10.7554/eLife.02419.002

Modes of flagellar assembly in Chlamydomonas reinhardtii and Trypanosoma brucei

Defects in flagella growth are related to a number of human diseases. Central to flagellar growth is the organization of microtubules that polymerize from basal bodies to form the axoneme, which consists of hundreds of proteins. Flagella exist in all eukaryotic phyla, but neither the mechanism by which flagella grow nor the conservation of this process in evolution are known. Here, we study how protein complexes assemble onto the growing axoneme tip using (cryo) electron tomography. In Chlamydomonas reinhardtii microtubules and associated proteins are added simultaneously. However, in Trypanosoma brucei, disorganized arrays of microtubules are arranged into the axoneme structure by the later addition of preformed protein complexes. Post assembly, the T. brucei transition zone alters structure and its association with the central pair loosens. We conclude that there are multiple ways to form a flagellum and that species-specific structural knowledge is critical before evaluating flagellar defects.

DOI:http://dx.doi.org/10.7554/eLife.01479.001

Some cells have a whip-like appendage called a flagellum. This is most often used to propel the cell, notably in sperm cells, but it can also be involved in sensing cues in the surrounding environment. Flagella are found in all three domains of life—the eukaryotes (which include the animals), bacteria and ancient, single-celled organisms called Archaea—and they perform similar functions in each domain. However, they also differ significantly in their protein composition, overall structure, and mechanism of propulsion.

The core of the flagellum in eukaryotes is made up of 20 hollow filaments called ‘microtubules’ arranged so that nine pairs of microtubules form a ring around two central microtubules. The core also contains many other proteins, but it is not clear how all these components come together to make a working flagellum. Moreover, it is not known if the flagella of different groups of eukaryotes are all assembled in the same way.

Now, Höög et al. have discovered that although the core structure of the eukaryote flagellum is highly conserved, it can be assembled in markedly different ways. Some species of eukaryote—such as Chlamydomonas reinhardtii, a single-celled green alga, and Trypanosoma brucei, the protist parasite that causes African sleeping sickness—must grow new flagella when their cells divide, so that each new cell can swim. Using a form of electron microscopy called electron tomography, Höög et al. could see the detailed structure of the growing flagella in three dimensions. At first the cores of the flagella in these two distantly related species grow in the same way. However as the flagella get longer their cores grow in completely different ways. The microtubule filaments in longer flagella grow in a synchronized manner in the alga, but in a disorganized way in the protist.

The results of Höög et al. illustrate that it is not advisable to draw generalised conclusions based on studies of a few model species. However, since defects in flagella are known to cause several diseases in humans, this knowledge might inform future studies aimed at developing treatments for infertility, respiratory problems, and certain kinds of cancer.

DOI:http://dx.doi.org/10.7554/eLife.01479.002

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

Flagella are highly conserved organelles present in a wide variety of species. In Trypanosoma brucei the single flagellum is necessary for morphogenesis, cell motility and pathogenesis, and is attached along the cell body. A new flagellum is formed alongside the old during the cell division cycle. In the (insect) procyclic form, the flagella connector (FC) attaches the tip of the new flagellum to the side of the old flagellum, ensuring faithful replication of cell architecture. The FC is not present in the bloodstream form of the parasite. We show here, using new imaging techniques including serial block-face scanning electron microscopy (SBF-SEM), that the distal tip of the new flagellum in the bloodstream form is embedded within an invagination in the cell body plasma membrane, named the groove. We suggest that the groove has a similar function to the flagella connector. The groove is a mobile junction located alongside the microtubule quartet (MtQ) and occurred within a gap in the subpellicular microtubule corset, causing significant modification of microtubules during elongation of the new flagellum. It appears likely that this novel form of morphogenetic structure has evolved to withstand the hostile immune response in the mammalian blood.

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.

Bending, twisting and beating trunk robot bioinspired from the '3 + 0' axoneme

The axoneme is the skeleton and motor axis of flagella and cilia in eukaryotic organisms. Basically it consists of a series of longitudinal fibers (outer doublets of microtubules) that design a cylinder and whose sliding, due to the coordinated activities of dedicated molecular motors (the dynein arms), is converted into a bending because outer doublets pairs are stabilized by elastic links (the nexine molecules). In spite of these interesting mechanical properties, mechanical and robotics engineers have never considered this amazing molecular machinery as a model. The aim of this paper is to propose the robotic design and the kinematic modeling of the '3 + 0' axoneme that makes motile the flagellum of Diplauxis hatti, the simplest that exists. The model that we propose bends and twists and combines the two movements. It is able to propagate wave trains that could be involved in the development of biomimetic actuators of various mechanisms such as (sub)aquatic robotic propellers as well as robotic trunks.

Conserved structural motifs in the central pair complex of eukaryotic flagella

Cilia and flagella are conserved hair-like appendages of eukaryotic cells that function as sensing and motility generating organelles. Motility is driven by thousands of axonemal dyneins that require precise regulation. One essential motility regulator is the central pair complex (CPC) and many CPC defects cause paralysis of cilia/flagella. Several human diseases, such as immotile cilia syndrome, show CPC abnormalities, but little is known about the detailed three-dimensional (3D) structure and function of the CPC. The CPC is located in the center of typical [9+2] cilia/flagella and is composed of two singlet microtubules (MTs), each with a set of associated projections that extend toward the surrounding nine doublet MTs. Using cryo-electron tomography coupled with subtomogram averaging, we visualized and compared the 3D structures of the CPC in both the green alga Chlamydomonas and the sea urchin Strongylocentrotus at the highest resolution published to date. Despite the evolutionary distance between these species, their CPCs exhibit remarkable structural conservation. We identified several new projections, including those that form the elusive sheath, and show that the bridge has a more complex architecture than previously thought. Organism-specific differences include the presence of MT inner proteins in Chlamydomonas, but not Strongylocentrotus, and different overall outlines of the highly connected projection network, which forms a round-shaped cylinder in algae, but is more oval in sea urchin. These differences could be adaptations to the mechanical requirements of the rotating CPC in Chlamydomonas, compared to the Strongylocentrotus CPC which has a fixed orientation.

Cryo-electron tomography and 3-D analysis of the intact flagellum in Trypanosoma brucei

Trypanosoma brucei is a uni-cellular protist that causes African sleeping sickness. These parasites have a flagellum that is attached to the cell body and is indispensible for its motility. The flagellum consists of a canonical 9+2 axoneme and a paraflagellar rod (PFR), an intricate tripartite, fibrous structure that is connected to the axoneme. In this paper we describe results from cryo-electron tomography of unperturbed flagella. This method revealed novel structures that are likely involved in attaching the flagellum to the cell. We also show the first cryo-electron tomographic images of a basal body in situ, revealing electron dense structures inside its triplet microtubules. Sub-tomogram averaging of the PFR revealed that its distal region is organized as an orthorhombic crystal.

Three-dimensional structure of the Trypanosome flagellum suggests that the paraflagellar rod functions as a biomechanical spring

Flagellum motility is critical for normal human development and for transmission of pathogenic protozoa that cause tremendous human suffering worldwide. Biophysical principles underlying motility of eukaryotic flagella are conserved from protists to vertebrates. However, individual cells exhibit diverse waveforms that depend on cell-specific elaborations on basic flagellum architecture. Trypanosoma brucei is a uniflagellated protozoan parasite that causes African sleeping sickness. The T. brucei flagellum is comprised of a 9+2 axoneme and an extra-axonemal paraflagellar rod (PFR), but the three-dimensional (3D) arrangement of the underlying structural units is poorly defined. Here, we use dual-axis electron tomography to determine the 3D architecture of the T. brucei flagellum. We define the T. brucei axonemal repeating unit. We observe direct connections between the PFR and axonemal dyneins, suggesting a mechanism by which mechanochemical signals may be transmitted from the PFR to axonemal dyneins. We find that the PFR itself is comprised of overlapping laths organized into distinct zones that are connected through twisting elements at the zonal interfaces. The overall structure has an underlying 57nm repeating unit. Biomechanical properties inferred from PFR structure lead us to propose that the PFR functions as a biomechanical spring that may store and transmit energy derived from axonemal beating. These findings provide insight into the structural foundations that underlie the distinctive flagellar waveform that is a hallmark of T. brucei cell motility.

Structure of Trypanosoma brucei flagellum accounts for its bihelical motion

Trypanosoma brucei is a parasitic protozoan that causes African sleeping sickness. It contains a flagellum required for locomotion and viability. In addition to a microtubular axoneme, the flagellum contains a crystalline paraflagellar rod (PFR) and connecting proteins. We show here, by cryoelectron tomography, the structure of the flagellum in three bending states. The PFR lattice in straight flagella repeats every 56 nm along the length of the axoneme, matching the spacing of the connecting proteins. During flagellar bending, the PFR crystallographic unit cell lengths remain constant while the interaxial angles vary, similar to a jackscrew. The axoneme drives the expansion and compression of the PFR lattice. We propose that the PFR modifies the in-plane axoneme motion to produce the characteristic trypanosome bihelical motility as captured by high-speed light microscope videography.

Reconstructing the evolutionary history of the centriole from protein components

Centrioles are highly conserved structures that fulfil important cellular functions, such as nucleation of cilia and flagella (basal-body function) and organisation of pericentriolar material to form the centrosome. The evolution of these functions can be inferred from the distribution of the molecular components of extant centrioles and centrosomes. Here, we undertake an evolutionary analysis of 53 proteins known either for centriolar association or for involvement in cilia-associated pathologies. By linking protein distribution in 45 diverse eukaryotes with organism biology, we provide molecular evidence to show that basal-body function is ancestral, whereas the presence of the centrosome is specific to the Holozoa. We define an ancestral centriolar inventory of 14 core proteins, Polo-like-kinase, and proteins associated with Bardet-Biedl syndrome (BBS) and Meckel-Gruber syndrome. We show that the BBSome is absent from organisms that produce cilia only for motility, predicting a dominant and ancient role for this complex in sensory function. We also show that the unusual centriole of Caenorhabditis elegans is highly divergent in both protein composition and sequence. Finally, we demonstrate a correlation between the presence of specific centriolar proteins and eye evolution. This correlation is used to predict proteins with functions in the development of ciliary, but not rhabdomeric, eyes.

Beyond 9+0: noncanonical axoneme structures characterize sensory cilia from protists to humans

The intracellular amastigote stages of parasites such as Leishmania are often referred to as aflagellate. They do, however, possess a short axoneme of cryptic function. Here, our examination of the structure of this axoneme leads to a testable hypothesis of its role in the cell biology of pathogenicity. We show a striking similarity between the microtubule axoneme structure of the Leishmania mexicana parasite infecting a macrophage and vertebrate primary cilia. In both, the 9-fold microtubule doublet symmetry is broken by the incursion of one or more microtubule doublets into the axoneme core, giving rise to an architecture that we term here the 9v (variable) axoneme. Three-dimensional reconstructions revealed that no particular doublet initiated the symmetry break, and moreover it often involved 2 doublets. The tip of the L. mexicana flagellum was frequently intimately associated with the macrophage vacuole membrane. We propose that the main function of the amastigote flagellum is to act as a sensory organelle with important functions in host-parasite interactions and signaling in the intracellular stage of the L. mexicana life cycle.

Flagellar oscillation: a commentary on proposed mechanisms

Eukaryotic flagella and cilia have a remarkably uniform internal 'engine' known as the '9+2' axoneme. With few exceptions, the function of cilia and flagella is to beat rhythmically and set up relative motion between themselves and the liquid that surrounds them. The molecular basis of axonemal movement is understood in considerable detail, with the exception of the mechanism that provides its rhythmical or oscillatory quality. Some kind of repetitive 'switching' event is assumed to occur; there are several proposals regarding the nature of the 'switch' and how it might operate. Herein I first summarise all the factors known to influence the rate of the oscillation (the beating frequency). Many of these factors exert their effect through modulating the mean sliding velocity between the nine doublet microtubules of the axoneme, this velocity being the determinant of bend growth rate and bend propagation rate. Then I explain six proposed mechanisms for flagellar oscillation and review the evidence on which they are based. Finally, I attempt to derive an economical synthesis, drawing for preference on experimental research that has been minimally disruptive of the intricate structure of the axoneme. The 'provisional synthesis' is that flagellar oscillation emerges from an effect of passive sliding direction on the dynein arms. Sliding in one direction facilitates force-generating cycles and dynein-to-dynein synchronisation along a doublet; sliding in the other direction is inhibitory. The direction of the initial passive sliding normally oscillates because it is controlled hydrodynamically through the alternating direction of the propulsive thrust. However, in the absence of such regulation, there can be a perpetual, mechanical self-triggering through a reversal of sliding direction due to the recoil of elastic structures that deform as a response to the prior active sliding. This provisional synthesis may be a useful basis for further examination of the problem.

Kinesin 9 family members perform separate functions in the trypanosome flagellum

KIF9B localizes to the axoneme and basal body and is needed for flagella assembly, whereas KIF9A localizes only to the axoneme and controls flagella motility without affecting their structure.

Numerous eukaryote genome projects have uncovered a variety of kinesins of unknown function. The kinesin 9 family is limited to flagellated species. Our phylogenetic experiments revealed two subfamilies: KIF9A (including Chlamydomonasreinhardtii KLP1) and KIF9B (including human KIF6). The function of KIF9A and KIF9B was investigated in the protist Trypanosoma brucei that possesses a single motile flagellum. KIF9A and KIF9B are strongly associated with the cytoskeleton and are required for motility. KIF9A is localized exclusively in the axoneme, and its depletion leads to altered motility without visible structural modifications. KIF9B is found in both the axoneme and the basal body, and is essential for the assembly of the paraflagellar rod (PFR), a large extra-axonemal structure. In the absence of KIF9B, cells grow abnormal flagella with excessively large blocks of PFR-like material that alternate with regions where only the axoneme is present. The functional diversity of the kinesin 9 family illustrates the capacity for adaptation of organisms to suit specific cytoskeletal requirements.

The paraflagellar rod of kinetoplastid parasites: from structure to components and function

The role of the eukaryotic flagellum in cell motility is well established but its importance in many other aspects of cell biology, from cell signalling to developmental regulation, is becoming increasingly apparent. In addition to this diversity of function the core structure of the flagellum, which has been inherited from the earliest ancestor of all eukaryotes, is embellished with a range of extra-axonemal structures in many organisms. One of the best studied of these structures is the paraflagellar rod of kinetoplastid protozoa in which the morphological characteristics have been well defined and some of the major protein constituents have been identified. Here we discuss recent advances in the identification of further molecular components of the paraflagellar rod, how these impact on our understanding of its function and regulation and the implications for therapeutic intervention in a number of devastating human pathologies.

Functional studies of an evolutionarily conserved, cytochrome b5 domain protein reveal a specific role in axonemal organisation and the general phenomenon of post-division axonemal growth in trypanosomes

Eukaryotic cilia and flagella are highly conserved structures composed of a canonical 9+2 microtubule axoneme. Several recent proteomic studies of cilia and flagella have been published, including a proteome of the flagellum of the protozoan parasite Trypanosoma brucei. Comparing proteomes reveals many novel proteins that appear to be widely conserved in evolution. Amongst these, we found a previously uncharacterised protein which localised to the axoneme in T. brucei, and therefore named it Trypanosome Axonemal protein (TAX)-2. Ablation of the protein using RNA interference in the procyclic form of the parasite has no effect on growth but causes a reduction in motility. Using transmission electron microscopy, various structural defects were seen in some axonemes, most frequently with microtubule doublets missing from the 9+2 arrangement. RNAi knockdown of TAX-2 expression in the bloodstream form of the parasite caused defects in growth and cytokinesis, a further example of the effects caused by loss of flagellar function in bloodstream form T. brucei. In procyclic cells we used a new set of vectors to ablate protein expression in cells expressing a GFP:TAX-2 fusion protein, which enabled us to easily quantify protein reduction and visualise axonemes made before and after RNAi induction. This establishes a useful generic technique but also revealed a specific observation that the new flagellum on the daughter trypanosome continues growth after cytokinesis. Our results provide evidence for TAX-2 function within the axoneme, where we suggest that it is involved in processes linking the outer doublet microtubules and the central pair.

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.

The flagellum of Trypanosoma brucei: new tricks from an old dog

African trypanosomes, i.e. Trypanosoma brucei and related sub-species, are devastating human and animal pathogens that cause significant human mortality and limit sustained economic development in sub-Saharan Africa. T. brucei is a highly motile protozoan parasite and coordinated motility is central to both disease pathogenesis in the mammalian host and parasite development in the tsetse fly vector. Therefore, understanding unique aspects of the T. brucei flagellum may uncover novel targets for therapeutic intervention in African sleeping sickness. Moreover, studies of conserved features of the T. brucei flagellum are directly relevant to understanding fundamental aspects of flagellum and cilium function in other eukaryotes, making T. brucei an important model system. The T. brucei flagellum contains a canonical 9+2 axoneme, together with additional features that are unique to kinetoplastids and a few closely-related organisms. Until recently, much of our knowledge of the structure and function of the trypanosome flagellum was based on analogy and inference from other organisms. There has been an explosion in functional studies in T. brucei in recent years, revealing conserved as well as novel and unexpected structural and functional features of the flagellum. Most notably, the flagellum has been found to be an essential organelle, with critical roles in parasite motility, morphogenesis, cell division and immune evasion. This review highlights recent discoveries on the T. brucei flagellum.

Trypanosoma brucei Polo-like kinase is essential for basal body duplication, kDNA segregation and cytokinesis

Polo-like kinases (PLKs) are conserved eukaryotic cell cycle regulators, which play multiple roles, particularly during mitosis. The function of Trypanosoma brucei PLK was investigated in procyclic and bloodstream-form parasites. In procyclic trypanosomes, RNA interference (RNAi) of PLK, or overexpression of TY1-epitope-tagged PLK (PLKty), but not overexpression of a kinase-dead variant, resulted in the accumulation of cells that had divided their nucleus but not their kinetoplast (2N1K cells). Analysis of basal bodies and flagella in these cells suggested the defect in kinetoplast division arose because of an inhibition of basal body duplication, which occurred when PLK expression levels were altered. Additionally, a defect in kDNA replication was observed in the 2N1K cells. However, the 2N1K cells obtained by each approach were not equivalent. Following PLK depletion, the single kinetoplast was predominantly located between the two divided nuclei, while in cells overexpressing PLKty, the kinetoplast was mainly found at the posterior end of the cell, suggesting a role for PLK kinase activity in basal body and kinetoplast migration. PLK RNAi in bloodstream trypanosomes also delayed kinetoplast division, and was further observed to inhibit furrow ingression during cytokinesis. Notably, no additional roles were detected for trypanosome PLK in mitosis, setting this protein kinase apart from its counterparts in other eukaryotes.

Flagellar and ciliary beating in trypanosome motility

The single flagellum of Leishmania and Trypanosoma parasites is becoming an increasingly attractive model for the analysis of flagellar function-driven largely by the abundance of genomic and proteomic information available for the organelle, the genetic manipulability of the organisms and the importance of motility for the parasite lifecycle. However, as yet, there is a paucity of published data on the beating of any genetically malleable trypanosomatid species. Here we undertook an in-depth analysis using high-speed videomicroscopy of the beating of free-swimming Leishmania major cells in comparison to Crithidia species (for which there is some existing literature). In so doing, we describe a simple and generally-applicable technique to facilitate the quantitative analysis of free-swimming cells. Our analysis thoroughly defines the parameters of the expected tip-to-base symmetrical flagellar beat in these species. It also describes beat initiation from points other than the flagellum tip and a completely different, base-to-tip highly-asymmetric beat that represents a ciliary beat of trypanosomatid flagella. Moreover, detailed analysis of parameter interrelationships revealed an unexpected dependency of wavelength on oscillator length that may be the result of reversible constraint of doublet sliding at the tip or resonance of the flagellar beat.

The hydrocephalus inducing gene product, Hydin, positions axonemal central pair microtubules

Background

Impairment of cilia and flagella function underlies a growing number of human genetic diseases. Mutations in hydin in hy3 mice cause lethal communicating hydrocephalus with early onset. Hydin was recently identified as an axonemal protein; however, its function is as yet unknown.

Results

Here we use RNAi in Trypanosoma brucei to address this issue and demonstrate that loss of Hydin causes slow growth and a loss of cell motility. We show that two separate defects in newly-formed flagellar central pair microtubules underlie the loss of cell motility. At early time-points after RNAi induction, the central pair becomes mispositioned, while at later time points the central pair is lost. While the basal body is unaffected, both defects originate at the basal plate, reflecting a role for TbHydin throughout the length of the central pair.

Conclusion

Our data provide the first evidence of Hydin's role within the trypanosome axoneme, and reveal central pair anomalies and thus impairment of ependymal ciliary motility as the likely cause of the hydrocephalus observed in the hy3 mouse.

An essential quality control mechanism at the eukaryotic basal body prior to intraflagellar transport

Constructing a eukaryotic cilium/flagellum is a demanding task requiring the transport of proteins from their cytoplasmic synthesis site into a spatially and environmentally distinct cellular compartment. The clear potential hazard is that import of aberrant proteins could seriously disable cilia/flagella assembly or turnover processes. Here, we reveal that tubulin protein destined for incorporation into axonemal microtubules interacts with a tubulin cofactor C (TBCC) domain-containing protein that is specifically located at the mature basal body transitional fibres. RNA interference-mediated ablation of this protein results in axonemal microtubule defects but no effect on other microtubule populations within the cell. Bioinformatics analysis indicates that this protein belongs to a clade of flagellum-specific TBCC-like proteins that includes the human protein, XRP2, mutations which lead to certain forms of the hereditary eye disease retinitis pigmentosa. Taken with other observations regarding the role of transitional fibres in cilium/flagellum assembly, we suggest that a localized protein processing capacity embedded at transitional fibres ensures the 'quality' of tubulin imported into the cilium/flagellum, and further, that loss of a ciliary/flagellar quality control capability may underpin a number of human genetic disorders.

Centriole/basal body morphogenesis and migration during ciliogenesis in animal cells

Cilia, either motile or immotile, exist on most cells in the human body. There are several different mechanisms of ciliogenesis, which enable the production of many kinds of cilia and flagella: motile and immotile, transient and long-lived. These can be linked to the cell cycle or associated with differentiation. A primary cilium is extended from a basal body analogous to the mitotic centrioles, whereas the several hundred centrioles needed to form the cilia of a multi-ciliated cell can be generated by centriolar or acentriolar pathways. Little is known about the molecular control of these pathways and most of our knowledge comes from ultrastructural studies. The increasing number of genetic diseases linked to dysfunctional cilia and basal bodies has renewed interest in this area, and recent proteomic and cell biological studies in model organisms have helped to shed light on the molecular components of these enigmatic organelles.

Structural insights into microtubule doublet interactions in axonemes

Coordinated sliding of microtubule doublets, driven by dynein motors, produces periodic beating of eukaryotic cilia and flagella. Recent structural studies of the axoneme, which forms the core of cilia and flagella, have used cryo-electron tomography to reveal new details of the interactions between some of the multitude of proteins that form the axoneme and regulate its movement. Connections between the several types of dyneins, in particular, suggest ways in which their action might be coordinated. Study of the molecular architecture of isolated doublets has provided a structural basis for understanding mechanical properties related to the bending of the axoneme, and has also offered insight into the potential role of doublets in the mechanism of dynein activity regulation.

Cell cycle regulation in Trypanosoma brucei

Cell division is regulated by intricate and interconnected signal transduction pathways that precisely coordinate, in time and space, the complex series of events involved in replicating and segregating the component parts of the cell. In Trypanosoma brucei, considerable progress has been made over recent years in identifying molecular regulators of the cell cycle and elucidating their functions, although many regulators undoubtedly remain to be identified, and there is still a long way to go with respect to determining signal transduction pathways. However, it is clear that cell cycle regulation in T. brucei is unusual in many respects. Analyses of trypanosome orthologues of conserved eukaryotic cell cycle regulators have demonstrated divergence of their function in the parasite, and a number of other key regulators are missing from T. brucei. Cell cycle regulation differs in different parasite life cycle stages, and T. brucei appears to use different checkpoint control strategies compared to model eukaryotes. It is therefore probable that T. brucei has evolved novel pathways to control its cell cycle.