Flagellum Structure and Function in Trypanosomes

Extensible membrane nanotubules mediate attachment of Trypanosoma cruzi epimastigotes under flow

Trypanosoma cruzi is the etiological agent of Chagas disease, an important cause of infectious chronic myocardiopathy in Latin America. The life cycle of the parasite involves two main hosts: a triatomine (arthropod hematophagous vector) and a mammal. Epimastigotes are flagellated forms inside the triatomine gut; they mature in its intestine into metacyclic trypomastigotes, the infective form for humans. Parasites attach despite the shear stress generated by fluid flow in the intestines of the host, but little is known about the mechanisms that stabilize attachment in these conditions. Here, we describe the effect of varying levels of shear stress on attached T. cruzi epimastigotes using a parallel plate flow chamber. When flow is applied, parasites are partially dragged but maintain a connection to the surface via ~40 nm wide filaments (nanotubules) and the activity of flagella is reduced. When flow stops, parasites return near their original position and flagellar motion resumes. Nanotubule elongation increases with increasing shear stress and is consistent with a model of membrane tether extension under force. Fluorescent probes used to confirm membrane composition also show micron-wide anchoring pads at the distal end of the nanotubules. Multiple tethering accounts for more resistance to large shear stresses and for reduced flagellar movement when flow is stopped. The formation of membrane nanotubules is a possible mechanism to enhance adherence to host cells under shear stress, favoring the continuity of the parasite´s life cycle.

STEM tomography analysis of the trypanosome transition zone

The protist Trypanosoma brucei is an emerging model for the study of cilia and flagella. Here, we used scanning transmission electron microscopy (STEM) tomography to describe the structure of the trypanosome transition zone (TZ). At the base of the TZ, nine transition fibres irradiate from the B microtubule of each doublet towards the membrane. The TZ adopts a 9 + 0 structure throughout its length of ∼300 nm and its lumen contains an electron-dense structure. The proximal portion of the TZ has an invariant length of 150 nm and is characterised by a collarette surrounding the membrane and the presence of electron-dense material between the membrane and the doublets. The distal portion exhibits more length variation (from 55 to 235 nm) and contains typical Y-links. STEM analysis revealed a more complex organisation of the Y-links compared to what was reported by conventional transmission electron microscopy. Observation of the very early phase of flagellum assembly demonstrated that the proximal portion and the collarette are assembled early during construction. The presence of the flagella connector that maintains the tip of the new flagellum to the side of the old was confirmed and additional filamentous structures making contact with the membrane of the flagellar pocket were also detected. The structure and potential functions of the TZ in trypanosomes are discussed, as well as its mode of assembly.

Flagellar membrane proteins in kinetoplastid parasites

All kinetoplastid parasites, including protozoa such as Leishmania species, Trypanosoma brucei, and Trypanosoma cruzi that cause devastating diseases in humans and animals, are flagellated throughout their life cycles. Although flagella were originally thought of primarily as motility organelles, flagellar functions in other critical processes, especially in sensing and signal transduction, have become more fully appreciated in the recent past. The flagellar membrane is a highly specialized subdomain of the surface membrane, and flagellar membrane proteins are likely to be critical components for all the biologically important roles of flagella. In this review, we summarize recent discoveries relevant to flagellar membrane proteins in these parasites, including the identification of such proteins, investigation of their biological functions, and mechanisms of selective trafficking to the flagellar membrane. Prospects for future investigations and current unsolved problems are highlighted.

[Elongation of the axoneme and dynamics of intraflagellar transport]

Cilia and flagella are essential organelles in most eukaryotes including human beings. In this review, we will discuss the mode of assembly of these complex organelles that depends on a dynamic process called intraflagellar transport or IFT. IFT delivers structural elements at the distal end of the cilium where assembly takes place, thereby allowing the growth of the organelle. We next discuss the different models for control of cilium length and their alterations in ciliopathies, genetic diseases associated to ciliary defects.

Getting to the heart of intraflagellar transport using Trypanosoma and Chlamydomonas models: the strength is in their differences

Cilia and flagella perform diverse roles in motility and sensory perception, and defects in their construction or their function are responsible for human genetic diseases termed ciliopathies. Cilia and flagella construction relies on intraflagellar transport (IFT), the bi-directional movement of ‘trains’ composed of protein complexes found between axoneme microtubules and the flagellum membrane. Although extensive information about IFT components and their mode of action were discovered in the green algae Chlamydomonas reinhardtii, other model organisms have revealed further insights about IFT. This is the case of Trypanosoma brucei, a flagellated protist responsible for sleeping sickness that is turning out to be an emerging model for studying IFT. In this article, we review different aspects of IFT, based on studies of Chlamydomonas and Trypanosoma. Data available from both models are examined to ask challenging questions about IFT such as the initiation of flagellum construction, the setting-up of IFT and the mode of formation of IFT trains, and their remodeling at the tip as well as their recycling at the base. Another outstanding question is the individual role played by the multiple IFT proteins. The use of different models, bringing their specific biological and experimental advantages, will be invaluable in order to obtain a global understanding of IFT.

A new asymmetric division contributes to the continuous production of infective trypanosomes in the tsetse fly

African trypanosomes are flagellated protozoan parasites that cause sleeping sickness and are transmitted by the bite of the tsetse fly. To complete their life cycle in the insect, trypanosomes reach the salivary glands and transform into the metacyclic infective form. The latter are expelled with the saliva at each blood meal during the whole life of the insect. Here, we reveal a means by which the continuous production of infective parasites could be ensured. Dividing trypanosomes present in the salivary glands of infected tsetse flies were monitored by live video-microscopy and by quantitative immunofluorescence analysis using molecular markers for the cytoskeleton and for surface antigens. This revealed the existence of two distinct modes of trypanosome proliferation occurring simultaneously in the salivary glands. The first cycle produces two equivalent cells that are not competent for infection and are attached to the epithelium. This mode of proliferation is predominant at the early steps of infection, ensuring a rapid colonization of the glands. The second mode is more frequent at later stages of infection and involves an asymmetric division. It produces a daughter cell that matures into the infective metacyclic form that is released in the saliva, as demonstrated by the expression of specific molecular markers - the calflagins. The levels of these calcium-binding proteins increase exclusively in the new flagellum during the asymmetric division, showing the commitment of the future daughter cell to differentiation. The coordination of these two alternative cell cycles contributes to the continuous production of infective parasites, turning the tsetse fly into an efficient and long-lasting vector for African trypanosomes.