Flagellar and ciliary beating in trypanosome motility

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.

Wave-like oscillations of clamped microtubules driven by collective dynein transport

Microtubules (MTs) are observed to move and buckle driven by ATP-dependent molecular motors in both mitotic and interphasic eukaryotic cells as well as in specialized structures such as flagella and cilia with a stereotypical geometry. In previous work, clamped MTs driven by a few kinesin motors were seen to buckle and occasionally flap in what was referred to as flagella-like motion. Theoretical models of active-filament dynamics and a following force have predicted that, with sufficient force and binding-unbinding, such clamped filaments should spontaneously undergo periodic buckling oscillations. However, a systematic experimental test of the theory and reconciliation to a model was lacking. Here, we have engineered a minimal system of MTs clamped at their plus ends and transported by a sheet of dynein motors that demonstrate the emergence of spontaneous traveling-wave oscillations along single filaments. The frequencies of tip oscillations are in the millihertz range and are statistically indistinguishable in the onset and recovery phases. We develop a 2D computational model of clamped MTs binding and unbinding stochastically to motors in a "gliding-assay" geometry. The simulated MTs oscillate with a frequency comparable to experiment. The model predicts the effect of MT length and motor density on qualitative transitions between distinct phases of flapping, regular oscillations, and looping. We develop an effective "order parameter" based on the relative deflection along the filament and orthogonal to it. The transitions predicted in simulations are validated by experimental data. These results demonstrate a role for geometry, MT buckling, and collective molecular motor activity in the emergence of oscillatory dynamics.

Assembly of FAP93 at the proximal axoneme in Chlamydomonas cilia

To identify proteins specific to the proximal ciliary axoneme, we used iTRAQ to compare short (~2 μm) and full-length (~11 μm) axonemes of Chlamydomonas. Known compoents of the proximal axoneme such as minor dynein heavy chains and LF5 kinase as well as the ciliary tip proteins FAP256 (CEP104) and EB1 were enriched in short axonemes whereas proteins present along the length of the axoneme were of similar abundance in both samples. The iTRAQ analysis revealed that FAP93, a protein of unknown function, and protein phosphatase 2A (PP2A) are enriched in the short axonemes. Consistently, immunoblots show enrichment of FAP93 and PP2A in short axonemes and immunofluorescence confirms the localization of FAP93 and enrichment of PP2A at the proximal axoneme. Ciliary regeneration reveals that FAP93 assembles continuously but more slowly than other axonemal structures and terminates at 1.03 μm in steady-state axonemes. The length of FAP93 assembly correlates with ciliary length, demonstrating ciliary length-dependent assembly of FAP93. Dikaryon rescue experiments show that FAP93 can assemble independently of IFT transport. In addition, FRAP analysis of GFP-tagged FAP93 demonstrates that FAP93 is stably anchored in axoneme. FAP93 may function as a scaffold for assembly of other specific proteins at the proximal axoneme.

A sticky situation: When trypanosomatids attach to insect tissues

Transmission of trypanosomatids to their mammalian hosts requires a complex series of developmental transitions in their insect vectors, including stable attachment to an insect tissue. While there are many ultrastructural descriptions of attached cells, we know little about the signaling events and molecular mechanisms involved in this process. Each trypanosomatid species attaches to a specific tissue in the insect at a particular stage of its life cycle. Attachment is mediated by the flagellum, which is modified to accommodate a filament-rich plaque within an expanded region of the flagellar membrane. Attachment immediately precedes differentiation to the mammal-infectious stage and in some cases a direct mechanistic link has been demonstrated. In this review, we summarize the current state of knowledge of trypanosomatid attachment in insects, including structure, function, signaling, candidate molecules, and changes in gene expression. We also highlight remaining questions about this process and how the field is poised to address them through modern approaches.

Light chain 2 is a Tctex-type related axonemal dynein light chain that regulates directional ciliary motility in Trypanosoma brucei

Flagellar motility is essential for the cell morphology, viability, and virulence of pathogenic kinetoplastids. Trypanosoma brucei flagella beat with a bending wave that propagates from the flagellum's tip to its base, rather than base-to-tip as in other eukaryotes. Thousands of dynein motor proteins coordinate their activity to drive ciliary bending wave propagation. Dynein-associated light and intermediate chains regulate the biophysical mechanisms of axonemal dynein. Tctex-type outer arm dynein light chain 2 (LC2) regulates flagellar bending wave propagation direction, amplitude, and frequency in Chlamydomonas reinhardtii. However, the role of Tctex-type light chains in regulating T. brucei motility is unknown. Here, we used a combination of bioinformatics, in-situ molecular tagging, and immunofluorescence microscopy to identify a Tctex-type light chain in the procyclic form of T. brucei (TbLC2). We knocked down TbLC2 expression using RNAi in both wild-type and FLAM3, a flagellar attachment zone protein, knockdown cells and quantified TbLC2's effects on trypanosome cell biology and biophysics. We found that TbLC2 knockdown reduced the directional persistence of trypanosome cell swimming, induced an asymmetric ciliary bending waveform, modulated the bias between the base-to-tip and tip-to-base beating modes, and increased the beating frequency. Together, our findings are consistent with a model of TbLC2 as a down-regulator of axonemal dynein activity that stabilizes the forward tip-to-base beating ciliary waveform characteristic of trypanosome cells. Our work sheds light on axonemal dynein regulation mechanisms that contribute to pathogenic kinetoplastids' unique tip-to-base ciliary beating nature and how those mechanisms underlie dynein-driven ciliary motility more generally.

The bank of swimming organisms at the micron scale (BOSO-Micro)

Unicellular microscopic organisms living in aqueous environments outnumber all other creatures on Earth. A large proportion of them are able to self-propel in fluids with a vast diversity of swimming gaits and motility patterns. In this paper we present a biophysical survey of the available experimental data produced to date on the characteristics of motile behaviour in unicellular microswimmers. We assemble from the available literature empirical data on the motility of four broad categories of organisms: bacteria (and archaea), flagellated eukaryotes, spermatozoa and ciliates. Whenever possible, we gather the following biological, morphological, kinematic and dynamical parameters: species, geometry and size of the organisms, swimming speeds, actuation frequencies, actuation amplitudes, number of flagella and properties of the surrounding fluid. We then organise the data using the established fluid mechanics principles for propulsion at low Reynolds number. Specifically, we use theoretical biophysical models for the locomotion of cells within the same taxonomic groups of organisms as a means of rationalising the raw material we have assembled, while demonstrating the variability for organisms of different species within the same group. The material gathered in our work is an attempt to summarise the available experimental data in the field, providing a convenient and practical reference point for future studies.

The biomechanical role of extra-axonemal structures in shaping the flagellar beat of Euglena gracilis

We propose and discuss a model for flagellar mechanics in Euglena gracilis. We show that the peculiar non-planar shapes of its beating flagellum, dubbed 'spinning lasso', arise from the mechanical interactions between two of its inner components, namely, the axoneme and the paraflagellar rod. The spontaneous shape of the axoneme and the resting shape of the paraflagellar rod are incompatible. Thus, the complex non-planar configurations of the coupled system emerge as the energetically optimal compromise between the two antagonistic components. The model is able to reproduce the experimentally observed flagellar beats and the characteristic geometric signature of spinning lasso, namely, traveling waves of torsion with alternating sign along the length of the flagellum.

Long-term segmentation-free assessment of head-flagellum movement and intracellular calcium in swimming human sperm

Human spermatozoa are the archetype of long-term self-organizing transport in nature and are critical for reproductive success. They utilize coordinated head and flagellar movements to swim long distances within the female reproductive tract in order to find and fertilize the egg. However, to date, long-term analysis of the sperm head-flagellar movements, or indeed those of other flagellated microorganisms, remains elusive due to limitations in microscopy and flagellar-tracking techniques. Here, we present a novel methodology based on local orientation and isotropy of bio-images to obtain long-term kinematic and physiological parameters of individual free-swimming spermatozoa without requiring image segmentation (thresholding). This computer-assisted segmentation-free method evaluates, for the first time, characteristics of the head movement and flagellar beating for up to 9.2 min. We demonstrate its powerful use by showing how releasing Ca²⁺ from internal stores significantly alters long-term sperm behavior. The method allows for straightforward generalization to other bio-imaging applications, such as studies of bull sperm and Trypanosoma, or indeed of other flagellated microorganisms - appealing to communities other than those investigating sperm biology.

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.

Motility Analysis of Trypanosomatids

Motility analysis of microswimmers has long been limited to a few model cell types and broadly restricted by technical challenges of high-resolution in vivo microscopy. Recently, interdisciplinary interest in detailed analysis of the motile behavior of various species has gained momentum. Here we describe a basic protocol for motility analysis of an important, highly diverse group of eukaryotic flagellate microswimmers, using high spatiotemporal resolution videomicroscopy. Further, we provide a special, time-dependent tomographic approach for the proof of rotational locomotion of periodically oscillating microswimmers, using the same data. Taken together, the methods describe part of an integrative approach to generate decisive information on three-dimensional in vivo motility from standard two-dimensional videomicroscopy data.

Cilia oscillations

Cilia, or eukaryotic flagella, are microscopic active filaments expressed on the surface of many eukaryotic cells, from single-celled protozoa to mammalian epithelial surfaces. Cilia are characterized by a highly conserved and intricate internal structure in which molecular motors exert forces on microtubule doublets causing cilia oscillations. The spatial and temporal regulations of this molecular machinery are not well understood. Several theories suggest that geometric feedback control from cilium deformations to molecular activity is needed. Here, we implement a recent sliding control model, where the unbinding of molecular motors is dictated by the sliding motion between microtubule doublets. We investigate the waveforms exhibited by the model cilium, as well as the associated molecular motor dynamics, for hinged and clamped boundary conditions. Hinged filaments exhibit base-to-tip oscillations while clamped filaments exhibit both base-to-tip and tip-to-base oscillations. We report the change in oscillation frequencies and amplitudes as a function of motor activity and sperm number, and we discuss the validity of these results in the context of experimental observations of cilia behaviour. This article is part of the Theo Murphy meeting issue 'Unity and diversity of cilia in locomotion and transport'.

High-speed multifocal plane fluorescence microscopy for three-dimensional visualisation of beating flagella

Analysis of flagellum and cilium beating in three dimensions (3D) is important for understanding cell motility, and using fluorescence microscopy to do so would be extremely powerful. Here, high-speed multifocal plane fluorescence microscopy, where the light path is split to visualise multiple focal planes simultaneously, was used to reconstruct Trypanosoma brucei and Leishmania mexicana movement in 3D. These species are uniflagellate unicellular parasites for which motility is vital. It was possible to use either a fluorescent stain or a genetically-encoded fluorescent protein to visualise flagellum and cell movement at 200 Hz frame rates. This addressed two open questions regarding Trypanosoma and Leishmania flagellum beating, which contributes to their swimming behaviours: 1) how planar is the L. mexicana flagellum beat, and 2) what is the nature of flagellum beating during T. brucei 'tumbling'? We showed that L. mexicana has notable deviations from a planar flagellum beat, and that during tumbling the T. brucei flagellum bends the cell and beats only in the distal portion to achieve cell reorientation. This demonstrates high-speed multifocal plane fluorescence microscopy as a powerful tool for the analysis of beating flagella.

Swimming eukaryotic microorganisms exhibit a universal speed distribution

One approach to quantifying biological diversity consists of characterizing the statistical distribution of specific properties of a taxonomic group or habitat. Microorganisms living in fluid environments, and for whom motility is key, exploit propulsion resulting from a rich variety of shapes, forms, and swimming strategies. Here, we explore the variability of swimming speed for unicellular eukaryotes based on published data. The data naturally partitions into that from flagellates (with a small number of flagella) and from ciliates (with tens or more). Despite the morphological and size differences between these groups, each of the two probability distributions of swimming speed are accurately represented by log-normal distributions, with good agreement holding even to fourth moments. Scaling of the distributions by a characteristic speed for each data set leads to a collapse onto an apparently universal distribution. These results suggest a universal way for ecological niches to be populated by abundant microorganisms.

Genetic dissection of a Leishmania flagellar proteome demonstrates requirement for directional motility in sand fly infections

The protozoan parasite Leishmania possesses a single flagellum, which is remodelled during the parasite’s life cycle from a long motile flagellum in promastigote forms in the sand fly to a short immotile flagellum in amastigotes residing in mammalian phagocytes. This study examined the protein composition and in vivo function of the promastigote flagellum. Protein mass spectrometry and label free protein enrichment testing of isolated flagella and deflagellated cell bodies defined a flagellar proteome for L. mexicana promastigote forms (available via ProteomeXchange with identifier PXD011057). This information was used to generate a CRISPR-Cas9 knockout library of 100 mutants to screen for flagellar defects. This first large-scale knockout screen in a Leishmania sp. identified 56 mutants with altered swimming speed (52 reduced and 4 increased) and defined distinct mutant categories (faster swimmers, slower swimmers, slow uncoordinated swimmers and paralysed cells, including aflagellate promastigotes and cells with curled flagella and disruptions of the paraflagellar rod). Each mutant was tagged with a unique 17-nt barcode, providing a simple barcode sequencing (bar-seq) method for measuring the relative fitness of L. mexicana mutants in vivo. In mixed infections of the permissive sand fly vector Lutzomyia longipalpis, paralysed promastigotes and uncoordinated swimmers were severely diminished in the fly after defecation of the bloodmeal. Subsequent examination of flies infected with a single paralysed mutant lacking the central pair protein PF16 or an uncoordinated swimmer lacking the axonemal protein MBO2 showed that these promastigotes did not reach anterior regions of the fly alimentary tract. These data show that L. mexicana need directional motility for successful colonisation of sand flies.

Leishmania are protozoan parasites, transmitted between mammals by the bite of phlebotomine sand flies. Promastigote forms in the sand fly have a long flagellum, which is motile and used for anchoring the parasites to prevent clearance with the digested blood meal remnants. To dissect flagellar functions and their importance in life cycle progression, we generated here a comprehensive list of >300 flagellar proteins and produced a CRISPR-Cas9 gene knockout library of 100 mutant Leishmania. We studied their behaviour in vitro before examining their fate in the sand fly Lutzomyia longipalpis. Measuring mutant swimming speeds showed that about half behaved differently compared to the wild type: a few swam faster, many slower and some were completely paralysed. We also found a group of uncoordinated swimmers. To test whether flagellar motility is required for parasite migration from the fly midgut to the foregut from where they reach the next host, we infected sand flies with a mixed mutant population. Each mutant carried a unique tag and tracking these tags up to nine days after infection showed that paralysed and uncoordinated Leishmania were rapidly lost from flies. These data indicate that directional swimming is important for successful colonisation of sand flies.

Axonemal doublet microtubules can split into two complete singlets in human sperm flagellum tips

Motile flagella are crucial for human fertility and embryonic development. The distal tip of the flagellum is where growth and intra-flagellar transport are coordinated. In most model organisms, but not all, the distal tip includes a 'singlet region', where axonemal doublet microtubules (dMT) terminate and only complete A-tubules extend as singlet microtubules (sMT) to the tip. How a human flagellar tip is structured is unknown. Here, the flagellar tip structure of human spermatozoa was investigated by cryo-electron tomography, revealing the formation of a complete sMT from both the A-tubule and B-tubule of dMTs. This different tip arrangement in human spermatozoa shows the need to investigate human flagella directly in order to understand their role in health and disease.

Boundary behaviours of Leishmania mexicana: A hydrodynamic simulation study

It is well established that the parasites of the genus Leishmania exhibit complex surface interactions with the sandfly vector midgut epithelium, but no prior study has considered the details of their hydrodynamics. Here, the boundary behaviours of motile Leishmania mexicana promastigotes are explored in a computational study using the boundary element method, with a model flagellar beating pattern that has been identified from digital videomicroscopy. In particular a simple flagellar kinematics is observed and quantified using image processing and mode identification techniques, suggesting a simple mechanical driver for the Leishmania beat. Phase plane analysis and long-time simulation of a range of Leishmania swimming scenarios demonstrate an absence of stable boundary motility for an idealised model promastigote, with behaviours ranging from boundary capture to deflection into the bulk both with and without surface forces between the swimmer and the boundary. Indeed, the inclusion of a short-range repulsive surface force results in the deflection of all surface-bound promastigotes, suggesting that the documented surface detachment of infective metacyclic promastigotes may be the result of their particular morphology and simple hydrodynamics. Further, simulation elucidates a remarkable morphology-dependent hydrodynamic mechanism of boundary approach, hypothesised to be the cause of the well-established phenomenon of tip-first epithelial attachment of Leishmania promastigotes to the sandfly vector midgut.

Effect of inhibition of axonemal dynein ATPases on the regulation of flagellar and ciliary waveforms in Leishmania parasites

Trypanosomes of the genus Leishmania swim by undulating motions of a single flagellum driven by axonemal dynein ATPases, essential for parasite survival and infectivity. The flagellum possesses two waveforms; flagellar (tip-to-base) responsible for forward movements and ciliary (base-to-tip) possibly responsible for reorientation in response to changes in surroundings. However, the role of dyneins in regulating the two waveforms remains unknown. Moreover, the unpredictable nature of the parasite ciliary waveform makes it difficult to study. We have previously reported a detergent-extracted, ATP-reactivated model ideal for investigating flagellar motility regulation in Leishmania that allows one to generate reactivated Leishmania flagella with constitutively beating ciliary waves in presence of cyclic-AMP. Here, using three dynein inhibitors [erythro-9-(2-hydroxy-3-nonyl) adenine, ciliobrevin A and vanadate] we investigated the role of dyneins in regulating the two waveforms of Leishmania. Using high speed videomicroscopy we observed differential inhibition of beat frequencies and waveforms of flagellar and ciliary beats in live (in vivo) and ATP-reactivated (in vitro) parasites. Beat frequency of flagellar waveform was more strongly reduced than ciliary waveform. Surprisingly, inhibition of the ciliary waveform led to an altered phenotype with the distal half of the flagellum paralysed. ATPase assays confirmed that dynein activity of flagellar cells was more strongly inhibited compared to ciliary cells irrespective of the mechanism of inhibition. Possibly the two different waveforms are an outcome of changes in the mechanical properties of axonemal dyneins present at the tip of the flagellum that contains a sliding resistive structure. Our study suggests that dyneins responsible for the two waveforms in Leishmania bear different structural and functional conformations. Moreover, during ciliary beating, there is heterogeneity along the flagellum.

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.

Flagellum couples cell shape to motility in Trypanosoma brucei

Trypanosoma brucei is a highly invasive pathogen capable of penetrating deeply into host tissues. To understand how flagellar motility facilitates cell penetration, we used cryo-electron tomography (cryo-ET) to visualize two genetically anucleate mutants with different flagellar motility behaviors. We found that the T. brucei cell body is highly deformable as defined by changes in cytoskeletal twist and spacing, in response to flagellar beating and environmental conditions. Based on the cryo-ET models, we proposed a mechanism of how flagellum motility is coupled to cell shape changes, which may facilitate penetration through size-limiting barriers.

In the unicellular parasite Trypanosoma brucei, the causative agent of human African sleeping sickness, complex swimming behavior is driven by a flagellum laterally attached to the long and slender cell body. Using microfluidic assays, we demonstrated that T. brucei can penetrate through an orifice smaller than its maximum diameter. Efficient motility and penetration depend on active flagellar beating. To understand how active beating of the flagellum affects the cell body, we genetically engineered T. brucei to produce anucleate cytoplasts (zoids and minis) with different flagellar attachment configurations and different swimming behaviors. We used cryo-electron tomography (cryo-ET) to visualize zoids and minis vitrified in different motility states. We showed that flagellar wave patterns reflective of their motility states are coupled to cytoskeleton deformation. Based on these observations, we propose a mechanism for how flagellum beating can deform the cell body via a flexible connection between the flagellar axoneme and the cell body. This mechanism may be critical for T. brucei to disseminate in its host through size-limiting barriers.

Construction of artificial cilia from microtubules and kinesins through a well-designed bottom-up approach

Self-organized structures of biomolecular motor systems, such as cilia and flagella, play key roles in the dynamic processes of living organisms, like locomotion or the transportation of materials. Although fabrication of such self-organized structures from reconstructed biomolecular motor systems has attracted much attention in recent years, a systematic construction methodology is still lacking. In this work, through a bottom-up approach, we fabricated artificial cilia from a reconstructed biomolecular motor system, microtubule/kinesin. The artificial cilia exhibited a beating motion upon the consumption, by the kinesins, of the chemical energy obtained from the hydrolysis of adenosine triphosphate (ATP). Several design parameters, such as the length of the microtubules, the density of the kinesins along the microtubules, the depletion force among the microtubules, etc., have been identified, which permit tuning of the beating frequency of the artificial cilia. The beating frequency of the artificial cilia increases upon increasing the length of the microtubules, but declines for the much longer microtubules. A high density of the kinesins along the microtubules is favorable for the beating motion of the cilia. The depletion force induced bundling of the microtubules accelerated the beating motion of the artificial cilia and increased the beating frequency. This work helps understand the role of self-assembled structures of the biomolecular motor systems in the dynamics of living organisms and is expected to expedite the development of artificial nanomachines, in which the biomolecular motors may serve as actuators.

Simultaneous two-color imaging in digital holographic microscopy

We demonstrate the use of two-color digital holographic microscopy (DHM) for imaging microbiological subjects. The use of two wavelengths significantly reduces artifacts present in the reconstructed data, allowing us to image weakly-scattering objects in close proximity to strongly-scattering objects. We demonstrate this by reconstructing the shape of the flagellum of a unicellular eukaryotic parasite Leishmania mexicana in close proximity to a more strongly-scattering cell body. Our approach also yields a reduction of approximately one third in the axial position uncertainty when tracking the motion of swimming cells at low magnification, which we demonstrate with a sample of Escherichia coli bacteria mixed with polystyrene beads. The two-wavelength system that we describe introduces minimal additional complexity into the optical system, and provides significant benefits.

Use of chiral cell shape to ensure highly directional swimming in trypanosomes

Swimming cells typically move along a helical path or undergo longitudinal rotation as they swim, arising from chiral asymmetry in hydrodynamic drag or propulsion bending the swimming path into a helix. Helical paths are beneficial for some forms of chemotaxis, but why asymmetric shape is so prevalent when a symmetric shape would also allow highly directional swimming is unclear. Here, I analyse the swimming of the insect life cycle stages of two human parasites; Trypanosoma brucei and Leishmania mexicana. This showed quantitatively how chirality in T. brucei cell shape confers highly directional swimming. High speed videomicrographs showed that T. brucei, L. mexicana and a T. brucei RNAi morphology mutant have a range of shape asymmetries, from wild-type T. brucei (highly chiral) to L. mexicana (near-axial symmetry). The chiral cells underwent longitudinal rotation while swimming, with more rapid longitudinal rotation correlating with swimming path directionality. Simulation indicated hydrodynamic drag on the chiral cell shape caused rotation, and the predicted geometry of the resulting swimming path matched the directionality of the observed swimming paths. This simulation of swimming path geometry showed that highly chiral cell shape is a robust mechanism through which microscale swimmers can achieve highly directional swimming at low Reynolds number. It is insensitive to random variation in shape or propulsion (biological noise). Highly symmetric cell shape can give highly directional swimming but is at risk of giving futile circular swimming paths in the presence of biological noise. This suggests the chiral T. brucei cell shape (associated with the lateral attachment of the flagellum) may be an adaptation associated with the bloodstream-inhabiting lifestyle of this parasite for robust highly directional swimming. It also provides a plausible general explanation for why swimming cells tend to have strong asymmetries in cell shape or propulsion.

Swimming cells often follow a helical swimming path, however the advantage of helical paths over a simple straight line path is not clear. To analyse this phenomenon, I analysed the swimming of the human parasites Trypanosoma brucei (which causes sleeping sickness/trypanosomiasis) and Leishmania mexicana (which causes leishmaniasis). Using new computational methods to determine the three dimensional shape of swimming cells I showed that T. brucei have a helical shape which causes rotation as the cell swims, and the geometry of the resulting swimming path makes the cell movement highly directional. In contrast, L. mexicana are symmetrical, do not rotate, and their swimming paths are curved and have low directionality. Using a T. brucei mutant I showed that the cell structure responsible for the helical shape while swimming is the flagellum attachment zone. This explains a key function of this structure. Finally, simulations showed the phenomenon of rotation while swimming is a way cells can ensure highly directional swimming along a controlled helical path, overcoming random variation in cell shape or propulsion. This provides a general explanation for why swimming cells are often asymmetric and tend to follow helical paths.

Reactivation of flagellar motility in demembranated Leishmania reveals role of cAMP in flagellar wave reversal to ciliary waveform

The flagellum of parasitic trypanosomes is a multifunctional appendage essential for its viability and infectivity. However, the biological mechanisms that make the flagellum so dynamic remains unexplored. No method is available to access and induce axonemal motility at will to decipher motility regulation in trypanosomes. For the first time we report the development of a detergent-extracted/demembranated ATP-reactivated model for studying flagellar motility in Leishmania. Flagellar beat parameters of reactivated parasites were similar to live ones. Using this model we discovered that cAMP (both exogenous and endogenous) induced flagellar wave reversal to a ciliary waveform in reactivated parasites via cAMP-dependent protein kinase A. The effect was reversible and highly specific. Such an effect of cAMP on the flagellar waveform has never been observed before in any organism. Flagellar wave reversal allows parasites to change direction of swimming. Our findings suggest a possible cAMP-dependent mechanism by which Leishmania responds to its surrounding microenvironment, necessary for its survival. Our demembranated-reactivated model not only serves as an important tool for functional studies of flagellated eukaryotic parasites but has the potential to understand ciliary motility regulation with possible implication on human ciliopathies.

Independent Control of the Static and Dynamic Components of the Chlamydomonas Flagellar Beat

When the green alga Chlamydomonas reinhardtii swims, it uses the breaststroke beat of its two flagella to pull itself forward [1]. The flagellar waveform can be decomposed into a static component, corresponding to an asymmetric time-averaged shape, and a dynamic component, corresponding to the time-varying wave [2]. Extreme lightening conditions photoshock the cell, converting the breaststroke beat into a symmetric sperm-like beat, which causes a reversal of the direction of swimming [3]. Waveform conversion is achieved by a reduction in magnitude of the static component, whereas the dynamic component remains unchanged [2]. The coupling between static and dynamic components, however, is poorly understood, and it is not known whether the static component requires the dynamic component or whether it can exist independently. We used isolated and reactivated axonemes [4] to investigate the relation between the two beat components. We discovered that, when reactivated in the presence of low ATP concentrations, axonemes displayed the static beat component in absence of the dynamic component. Furthermore, we found that the amplitudes of the two components depend on ATP in qualitatively different ways. These results show that the decomposition into static and dynamic components is not just a mathematical concept but that the two components can independently control different aspects of cell motility: the static component controls swimming direction, whereas the dynamic component provides propulsion.

Mechanical properties of a primary cilium as measured by resonant oscillation

Primary cilia are ubiquitous mammalian cellular substructures implicated in an ever-increasing number of regulatory pathways. The well-established ciliary hypothesis states that physical bending of the cilium (for example, due to fluid flow) initiates signaling cascades, yet the mechanical properties of the cilium remain incompletely measured, resulting in confusion regarding the biological significance of flow-induced ciliary mechanotransduction. In this work we measure the mechanical properties of a primary cilium by using an optical trap to induce resonant oscillation of the structure. Our data indicate 1) the primary cilium is not a simple cantilevered beam; 2) the base of the cilium may be modeled as a nonlinear rotatory spring, with the linear spring constant k of the cilium base calculated to be (4.6 ± 0.62) × 10(-12) N/rad and nonlinear spring constant α to be (-1 ± 0.34) × 10(-10) N/rad(2); and 3) the ciliary base may be an essential regulator of mechanotransduction signaling. Our method is also particularly suited to measure mechanical properties of nodal cilia, stereocilia, and motile cilia-anatomically similar structures with very different physiological functions.

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

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

Mechanical oscillation of dynamic microtubule rings

Microtubules driven by kinesins self-assemble into ring-shaped structures which exhibit dynamic reorganization at an air-buffer interface.

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

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

Two-headed outer- and inner-arm dyneins of Leishmania sp bear conserved IQ-like motifs

Dyneins are high molecular weight microtubule based motor proteins responsible for beating of the flagellum. The flagellum is important for the viability of trypanosomes like Leishmania. However, very little is known about dynein and its role in flagellar motility in such trypanosomatid species. Here, we have identified genes in five species of Leishmania that code for outer-arm dynein (OAD) heavy chains α and β, and inner-arm dynein (IAD) heavy chains 1α and 1β using BLAST and MSA. Our sequence analysis indicates that unlike the three-headed outer-arm dyneins of Chlamydomonas and Tetrahymena, the outer-arm dyneins of the genus Leishmania are two-headed, lacking the γ chain like that of metazoans. N-terminal sequence analysis revealed a conserved IQ-like calmodulin binding motif in the outer-arm α and inner-arm 1α dynein heavy chain in the five species of Leishmania similar to Chlamydomonas reinhardtii outer-arm γ. It was predicted that both motifs were incapable of binding calmodulin. Phosphorylation site prediction revealed conserved serine and threonine residues in outer-arm dynein α and inner-arm 1α as putative phosphorylation sites exclusive to Leishmania but not in Trypanosoma brucei suggesting that regulation of dynein activity might be via phosphorylation of these IQ-like motifs in Leishmania sp.

•

Identified outer and inner-arm dynein heavy chain genes in five Leishmania species.

•

Outer-arm dyneins of the genus Leishmania are two-headed like metazoans.

•

Conserved IQ-like motif present in outer-arm α and inner-arm 1α in Leishmania sp.

•

Conserved serine and threonine residues in dynein arms exclusive to Leishmania sp.

•

Possible regulation of dynein activity via phosphorylation of these IQ-like motifs.

Effect of aliphatic, monocarboxylic, dicarboxylic, heterocyclic and sulphur-containing amino acids on Leishmania spp. chemotaxis

In the sand-fly mid gut, Leishmania promastigotes are exposed to acute changes in nutrients, e.g. amino acids (AAs). These metabolites are the main energy sources for the parasite, crucial for its differentiation and motility. We analysed the migratory behaviour and morphological changes produced by aliphatic, monocarboxylic, dicarboxylic, heterocyclic and sulphur-containing AAs in Leishmania amazonensis and Leishmania braziliensis and demonstrated that L-methionine (10-12 m), L-tryptophan (10-11 m), L-glutamine and L-glutamic acid (10-6 m), induced positive chemotactic responses, while L-alanine (10-7 m), L-methionine (10-11 and 10-7 m), L-tryptophan (10-11 m), L-glutamine (10-12 m) and L-glutamic acid (10-9 m) induced negative chemotactic responses. L-proline and L-cysteine did not change the migratory potential of Leishmania. The flagellum length of L. braziliensis, but not of L. amazonensis, decreased when incubated in hyperosmotic conditions. However, chemo-repellent concentrations of L-alanine (Hypo-/hyper-osmotic conditions) and L-glutamic acid (hypo-osmotic conditions) decreased L. braziliensis flagellum length and L-methionine (10-11 m, hypo-/hyper-osmotic conditions) decreased L. amazonensis flagellum length. This chemotactic responsiveness suggests that Leishmania discriminate between slight concentration differences of small and structurally closely related molecules and indicates that besides their metabolic effects, AAs play key roles linked to sensory mechanisms that might determine the parasite's behaviour.

Simulating the complex cell design of Trypanosoma brucei and its motility

The flagellate Trypanosoma brucei, which causes the sleeping sickness when infecting a mammalian host, goes through an intricate life cycle. It has a rather complex propulsion mechanism and swims in diverse microenvironments. These continuously exert selective pressure, to which the trypanosome adjusts with its architecture and behavior. As a result, the trypanosome assumes a diversity of complex morphotypes during its life cycle. However, although cell biology has detailed form and function of most of them, experimental data on the dynamic behavior and development of most morphotypes is lacking. Here we show that simulation science can predict intermediate cell designs by conducting specific and controlled modifications of an accurate, nature-inspired cell model, which we developed using information from live cell analyses. The cell models account for several important characteristics of the real trypanosomal morphotypes, such as the geometry and elastic properties of the cell body, and their swimming mechanism using an eukaryotic flagellum. We introduce an elastic network model for the cell body, including bending rigidity and simulate swimming in a fluid environment, using the mesoscale simulation technique called multi-particle collision dynamics. The in silico trypanosome of the bloodstream form displays the characteristic in vivo rotational and translational motility pattern that is crucial for survival and virulence in the vertebrate host. Moreover, our model accurately simulates the trypanosome's tumbling and backward motion. We show that the distinctive course of the attached flagellum around the cell body is one important aspect to produce the observed swimming behavior in a viscous fluid, and also required to reach the maximal swimming velocity. Changing details of the flagellar attachment generates less efficient swimmers. We also simulate different morphotypes that occur during the parasite's development in the tsetse fly, and predict a flagellar course we have not been able to measure in experiments so far.

Typanosoma brucei is a uni-cellular parasite that causes the sleeping sickness, a deadly disease for humans that also occurs in livestock. Injected into the mammalian host by the tsetse fly, the trypanosome travels through the blood stream, where it proliferates, and ultimately can be taken up again by a fly during a bloodmeal. In the tsetse fly, it continues its development with several morphological changes to the cell body plan. During its life cycle, the trypanosome meets different microenvironments, such as the mammalian's bloodstream and the tsetse fly's midgut, proventriculus, foregut, and salivary gland. The cell body of the trypanosome has the shape of a spindle along which an eukaryotic flagellum is attached. We have developed an accurate, in silico model trypanosome using information from live cell analyses. Performing computer simulations, we are able to reproduce all motility patterns of the blood-stream form in typical cell culture medium. Modifying the cell design, we show that the helical course of the flagellar attachment optimizes the trypanosome's swimming speed. We also design trypanosomal morphotypes that occur in the tsetse fly. Simulation science thereby provides an investigative tool to systematically explore the morphologcial diversity during the trypanosome's life cycle even beyond experimental capabilities.

αTAT1 is the major α-tubulin acetyltransferase in mice

Post-translational modification of tubulin serves as a powerful means for rapidly adjusting the functional diversity of microtubules. Acetylation of the ε-amino group of K40 in α-tubulin is one such modification that is highly conserved in ciliated organisms. Recently, αTAT1, a Gcn5-related N-acetyltransferase, was identified as an α-tubulin acetyltransferase in Tetrahymena and C. elegans. Here we generate mice with a targeted deletion of Atat1 to determine its function in mammals. Remarkably, we observe a loss of detectable K40 α-tubulin acetylation in these mice across multiple tissues and in cellular structures such as cilia and axons where acetylation is normally enriched. Mice are viable and develop normally, however, the absence of Atat1 impacts upon sperm motility and male mouse fertility, and increases microtubule stability. Thus, αTAT1 has a conserved function as the major α-tubulin acetyltransferase in ciliated organisms and has an important role in regulating subcellular specialization of subsets of microtubules.

Leishmania braziliensis: cytotoxic, cytostatic and chemotactic effects of poly-lysine-methotrexate-conjugates

Chemotactic responses play a significant role during Leishmania differentiation, as well as in the course of parasite-host-cell interaction, a process that precedes a successful infection. The present study uses the modified "two-chamber capillary assay" to quantitatively evaluate the chemotactic properties and the toxic activities of methotrexate containing branched chain polymeric polypeptide conjugates in Leishmania. Our results demonstrate that this methodology quantitatively determines the taxis of Leishmania towards/against gradients of compounds. They also demonstrate that chemotaxis produced by the polypeptide-methotrexate conjugates depends on specific chemical characteristics. For example, the N-terminal amino acid (Ser or Glu) location at the branch significantly influences the elicited chemotaxis. Furthermore, the use of different attachment sites in the methotrexate conjugates (α- or γ-carboxylic groups) affect their chemotactic activity. Specific cytotoxic activities and cytostatic effects of the conjugates on parasites and on murine and human cells of the macrophage/monocyte system respectively, suggest that these ligands may be used as a group of anti-Leishmania substances acting selectively on Leishmania and different hosts.

Calmodulin is required for paraflagellar rod assembly and flagellum-cell body attachment in trypanosomes

In the flagellum of the African sleeping sickness parasite Trypanosoma brucei calmodulin (CaM) is found within the paraflagellar rod (PFR), an elaborate extra-axonemal structure, and the axoneme. In dissecting mechanisms of motility regulation we analysed CaM function using RNAi. Unexpectedly CaM depletion resulted in total and catastrophic failure in PFR assembly; even connections linking axoneme to PFR failed to form following CaM depletion. This provides an intriguing parallel with the role in the green alga Chlamydomonas of a CaM-related protein in docking outer-dynein arms to axoneme outer-doublet microtubules. Absence of CaM had no discernible effect on axoneme assembly, but the failure in PFR assembly was further compounded by loss of the normal linkage between PFR and axoneme to the flagellum attachment zone of the cell body. Thus, flagellum detachment was a secondary, time-dependent consequence of CaM RNAi, and coincided with the loss of normal trypomastigote morphology, thereby linking the presence of PFR architecture with maintenance of cell form, as well as cell motility. Finally, wider comparison between the flagellum detachment phenotypes of RNAi mutants for CaM and the FLA1 glycoprotein potentially provides new perspective into the function of the latter into establishing and maintaining flagellum-cell body attachment.

Predicting the proteins of Angomonas deanei, Strigomonas culicis and their respective endosymbionts reveals new aspects of the trypanosomatidae family

Endosymbiont-bearing trypanosomatids have been considered excellent models for the study of cell evolution because the host protozoan co-evolves with an intracellular bacterium in a mutualistic relationship. Such protozoa inhabit a single invertebrate host during their entire life cycle and exhibit special characteristics that group them in a particular phylogenetic cluster of the Trypanosomatidae family, thus classified as monoxenics. In an effort to better understand such symbiotic association, we used DNA pyrosequencing and a reference-guided assembly to generate reads that predicted 16,960 and 12,162 open reading frames (ORFs) in two symbiont-bearing trypanosomatids, Angomonas deanei (previously named as Crithidia deanei) and Strigomonas culicis (first known as Blastocrithidia culicis), respectively. Identification of each ORF was based primarily on TriTrypDB using tblastn, and each ORF was confirmed by employing getorf from EMBOSS and Newbler 2.6 when necessary. The monoxenic organisms revealed conserved housekeeping functions when compared to other trypanosomatids, especially compared with Leishmania major. However, major differences were found in ORFs corresponding to the cytoskeleton, the kinetoplast, and the paraflagellar structure. The monoxenic organisms also contain a large number of genes for cytosolic calpain-like and surface gp63 metalloproteases and a reduced number of compartmentalized cysteine proteases in comparison to other TriTryp organisms, reflecting adaptations to the presence of the symbiont. The assembled bacterial endosymbiont sequences exhibit a high A+T content with a total of 787 and 769 ORFs for the Angomonas deanei and Strigomonas culicis endosymbionts, respectively, and indicate that these organisms hold a common ancestor related to the Alcaligenaceae family. Importantly, both symbionts contain enzymes that complement essential host cell biosynthetic pathways, such as those for amino acid, lipid and purine/pyrimidine metabolism. These findings increase our understanding of the intricate symbiotic relationship between the bacterium and the trypanosomatid host and provide clues to better understand eukaryotic cell evolution.

Trypanosome motion represents an adaptation to the crowded environment of the vertebrate bloodstream

Blood is a remarkable habitat: it is highly viscous, contains a dense packaging of cells and perpetually flows at velocities varying over three orders of magnitude. Only few pathogens endure the harsh physical conditions within the vertebrate bloodstream and prosper despite being constantly attacked by host antibodies. African trypanosomes are strictly extracellular blood parasites, which evade the immune response through a system of antigenic variation and incessant motility. How the flagellates actually swim in blood remains to be elucidated. Here, we show that the mode and dynamics of trypanosome locomotion are a trait of life within a crowded environment. Using high-speed fluorescence microscopy and ordered micro-pillar arrays we show that the parasites mode of motility is adapted to the density of cells in blood. Trypanosomes are pulled forward by the planar beat of the single flagellum. Hydrodynamic flow across the asymmetrically shaped cell body translates into its rotational movement. Importantly, the presence of particles with the shape, size and spacing of blood cells is required and sufficient for trypanosomes to reach maximum forward velocity. If the density of obstacles, however, is further increased to resemble collagen networks or tissue spaces, the parasites reverse their flagellar beat and consequently swim backwards, in this way avoiding getting trapped. In the absence of obstacles, this flagellar beat reversal occurs randomly resulting in irregular waveforms and apparent cell tumbling. Thus, the swimming behavior of trypanosomes is a surprising example of micro-adaptation to life at low Reynolds numbers. For a precise physical interpretation, we compare our high-resolution microscopic data to results from a simulation technique that combines the method of multi-particle collision dynamics with a triangulated surface model. The simulation produces a rotating cell body and a helical swimming path, providing a functioning simulation method for a microorganism with a complex swimming strategy.

Flagellar motility of Trypanosoma cruzi epimastigotes

The hemoflagellate Trypanosoma cruzi is the causative agent of American trypanosomiasis. Despite the importance of motility in the parasite life cycle, little is known about T. cruzi motility, and there is no quantitative description of its flagellar beating. Using video microscopy and quantitative vectorial analysis of epimastigote trajectories, we find a forward parasite motility defined by tip-to-base symmetrical flagellar beats. This motion is occasionally interrupted by base-to-tip highly asymmetric beats, which represent the ciliary beat of trypanosomatid flagella. The switch between flagellar and ciliary beating facilitates the parasite's reorientation, which produces a large variability of movement and trajectories that results in different distance ranges traveled by the cells. An analysis of the distance, speed, and rotational angle indicates that epimastigote movement is not completely random, and the phenomenon is highly dependent on the parasite behavior and is characterized by directed and tumbling parasite motion as well as their combination, resulting in the alternation of rectilinear and intricate motility paths.

Ultrastructure of cilia and flagella - back to the future!

Eukaryotic cilia and flagella perform motility and sensory functions which are essential for cell survival in protozoans, and to organism development and homoeostasis in metazoans. Their ultrastructure has been studied from the early beginnings of electron microscopy, and these studies continue to contribute to much of our understanding about ciliary biology. In the light of the progress made in the visualization of cellular structures over the last decade, we revisit the ultrastructure of cilia and flagella. We briefly describe the typical features of a 9+2 axoneme before focusing extensively on the transition zone, the ciliary necklace, the singlet zone, the ciliary cap and the ciliary crown. We discuss how the singlet zone is linked to sensory and/or motile function, the contribution of the ciliary crown to ovocyte and mucosal propulsion, and the relationship between the ciliary cap and microtubule growth and shortening, and its relation to ciliary beat. We further examine the involvement of the transition zone/the ciliary necklace in axonemal stabilization, autotomy and as a diffusion barrier.

Impact of microscopic motility on the swimming behavior of parasites: straighter trypanosomes are more directional

Microorganisms, particularly parasites, have developed sophisticated swimming mechanisms to cope with a varied range of environments. African Trypanosomes, causative agents of fatal illness in humans and animals, use an insect vector (the Tsetse fly) to infect mammals, involving many developmental changes in which cell motility is of prime importance. Our studies reveal that differences in cell body shape are correlated with a diverse range of cell behaviors contributing to the directional motion of the cell. Straighter cells swim more directionally while cells that exhibit little net displacement appear to be more bent. Initiation of cell division, beginning with the emergence of a second flagellum at the base, correlates to directional persistence. Cell trajectory and rapid body fluctuation correlation analysis uncovers two characteristic relaxation times: a short relaxation time due to strong body distortions in the range of 20 to 80 ms and a longer time associated with the persistence in average swimming direction in the order of 15 seconds. Different motility modes, possibly resulting from varying body stiffness, could be of consequence for host invasion during distinct infective stages.

Langevin dynamics deciphers the motility pattern of swimming parasites

The parasite African trypanosome swims in the bloodstream of mammals and causes the highly dangerous human sleeping sickness. Cell motility is essential for the parasite's survival within the mammalian host. We present an analysis of the random-walk pattern of a swimming trypanosome. From experimental time-autocorrelation functions for the direction of motion we identify two relaxation times that differ by an order of magnitude. They originate from the rapid deformations of the cell body and a slower rotational diffusion of the average swimming direction. Velocity fluctuations are athermal and increase for faster cells whose trajectories are also straighter. We demonstrate that such a complex dynamics is captured by two decoupled Langevin equations that decipher the complex trajectory pattern by referring it to the microscopic details of cell behavior.

Transitions between three swimming gaits in Paramecium escape

Paramecium and other protists are able to swim at velocities reaching several times their body size per second by beating their cilia in an organized fashion. The cilia beat in an asymmetric stroke, which breaks the time reversal symmetry of small scale flows. Here we show that Paramecium uses three different swimming gaits to escape from an aggression, applied in the form of a focused laser heating. For a weak aggression, normal swimming is sufficient and produces a steady swimming velocity. As the heating amplitude is increased, a higher acceleration and faster swimming are achieved through synchronized beating of the cilia, which begin by producing oscillating swimming velocities and later give way to the usual gait. Finally, escape from a life-threatening aggression is achieved by a "jumping" gait, which does not rely on the cilia but is achieved through the explosive release of a group of trichocysts in the direction of the hot spot. Measurements through high-speed video explain the role of trichocysts in defending against aggressions while showing unexpected transitions in the swimming of microorganisms. These measurements also demonstrate that Paramecium optimizes its escape pattern by taking advantage of its inertia.

Cell biology of the trypanosome genome

Trypanosomes are a group of protozoan eukaryotes, many of which are major parasites of humans and livestock. The genomes of trypanosomes and their modes of gene expression differ in several important aspects from those of other eukaryotic model organisms. Protein-coding genes are organized in large directional gene clusters on a genome-wide scale, and their polycistronic transcription is not generally regulated at initiation. Transcripts from these polycistrons are processed by global trans-splicing of pre-mRNA. Furthermore, in African trypanosomes, some protein-coding genes are transcribed by a multifunctional RNA polymerase I from a specialized extranucleolar compartment. The primary DNA sequence of the trypanosome genomes and their cellular organization have usually been treated as separate entities. However, it is becoming increasingly clear that in order to understand how a genome functions in a living cell, we will need to unravel how the one-dimensional genomic sequence and its trans-acting factors are arranged in the three-dimensional space of the eukaryotic nucleus. Understanding this cell biology of the genome will be crucial if we are to elucidate the genetic control mechanisms of parasitism. Here, we integrate the concepts of nuclear architecture, deduced largely from studies of yeast and mammalian nuclei, with recent developments in our knowledge of the trypanosome genome, gene expression, and nuclear organization. We also compare this nuclear organization to those in other systems in order to shed light on the evolution of nuclear architecture in eukaryotes.

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.

Approaches for functional analysis of flagellar proteins in African trypanosomes

The eukaryotic flagellum is a highly conserved organelle serving motility, sensory, and transport functions. Although genetic, genomic, and proteomic studies have led to the identification of hundreds of flagellar and putative flagellar proteins, precisely how these proteins function individually and collectively to drive flagellum motility and other functions remains to be determined. In this chapter we provide an overview of tools and approaches available for studying flagellum protein function in the protozoan parasite Trypanosoma brucei. We begin by outlining techniques for in vitro cultivation of both T. brucei life cycle stages, as well as transfection protocols for the delivery of DNA constructs. We then describe specific assays used to assess flagellum function including flagellum preparation and quantitative motility assays. We conclude the chapter with a description of molecular genetic approaches for manipulating gene function. In summary, the availability of potent molecular tools, as well as the health and economic relevance of T. brucei as a pathogen, combine to make the parasite an attractive and integral experimental system for the functional analysis of flagellar proteins.

High-speed digital imaging of ependymal cilia in the murine brain

The development and health of mammals requires proper ciliary motility. Ciliated epithelia are found in the airways, the uterus and Fallopian tubes, the efferent ducts of the testes, and the ventricular system of the brain. A technique is described for the motion analysis of ependymal cilia in the murine brain. Vibratome sections of the brain are imaged by differential interference contrast microscopy and recorded by high-speed digital imaging. Side views of individual cilia are traced to establish their bending pattern. Tracking of individual cilia recorded in top view allows determination of bend planarity and beat direction. Ciliary beat frequency is determined from line scans of image sequences. The capacity of the epithelium to move fluid and objects is revealed by analyzing the velocity of polystyrene beads added to brain sections. The technique is useful for detailed assessment of how various conditions or mutations affect the fidelity of ciliary motility at the ependyma. The methods are also applicable to other ciliated epithelia, for example, in airways.