The Trypanosoma brucei subpellicular microtubule array is organized into functionally discrete subdomains defined by microtubule associated proteins

Microtubule reorganization during mitotic cell division in the dinoflagellate Ostreospis cf. ovata

Dinoflagellates are marine organisms that undergo seasonal proliferation events known as algal blooms. Vegetative cell proliferation is a main contributing factor in these events. However, mechanistical understanding of mitosis and cytokinesis in dinoflagellates remains rudimentary. Using an optimized immunofluorescence protocol, we analysed changes in microtubule organization occurring during the mitotic cycle of the toxic dinoflagellate Ostreopsis cf. ovata. We find that the flagella and the cortical microtubule array persist throughout the mitotic cycle. Two cytoplasmic microtubule bundles originate from the ventral area, where the basal bodies are located - a cortical bundle and a cytoplasmic bundle. The latter associates with the nucleus in the cell centre before mitosis and with the acentrosomal extranuclear spindle during mitosis. Analysis of tubulin post-translational modifications identifies two populations of spindle microtubules - polar acetylated microtubules, whose length is constant, and central tyrosinated microtubules, which elongate during chromosome segregation. During cell division a microtubule-rich structure forms along the dorsal-ventral axis, associated with the site of cytokinesis, consistent with a cytokinetic mechanism that is independent of the actomyosin ring typical of animal and yeast cells.

A kinesin-13 family kinesin in Trypanosoma brucei regulates cytokinesis and cytoskeleton morphogenesis by promoting microtubule bundling

The early branching eukaryote Trypanosoma brucei divides uni-directionally along the longitudinal cell axis from the cell anterior toward the cell posterior, and the cleavage furrow ingresses along the cell division plane between the new and the old flagella of a dividing bi-flagellated cell. Regulation of cytokinesis in T. brucei involves actomyosin-independent machineries and trypanosome-specific signaling pathways, but the molecular mechanisms underlying cell division plane positioning remain poorly understood. Here we report a kinesin-13 family protein, KIN13-5, that functions downstream of FPRC in the cytokinesis regulatory pathway and determines cell division plane placement. KIN13-5 localizes to multiple cytoskeletal structures, interacts with FPRC, and depends on FPRC for localization to the site of cytokinesis initiation. Knockdown of KIN13-5 causes loss of microtubule bundling at both ends of the cell division plane, leading to mis-placement of the cleavage furrow and unequal cytokinesis, and at the posterior cell tip, causing the formation of a blunt posterior. In vitro biochemical assays demonstrate that KIN13-5 bundles microtubules, providing mechanistic insights into the role of KIN13-5 in cytokinesis and posterior morphogenesis. Altogether, KIN13-5 promotes microtubule bundle formation to ensure cleavage furrow placement and to maintain posterior cytoskeleton morphology in T. brucei.

Microtubule polyglutamylation is an essential regulator of cytoskeletal integrity in Trypanosoma brucei

Tubulin polyglutamylation, catalysed by members of the tubulin tyrosine ligase-like (TTLL) protein family, is an evolutionarily highly conserved mechanism involved in the regulation of microtubule dynamics and function in eukaryotes. In the protozoan parasite Trypanosoma brucei, the microtubule cytoskeleton is essential for cell motility and maintaining cell shape. In a previous study, we showed that T. brucei TTLL6A and TTLL12B are required to regulate microtubule dynamics at the posterior cell pole. Here, using gene deletion, we show that the polyglutamylase TTLL1 is essential for the integrity of the highly organised microtubule structure at the cell pole, with a phenotype distinct from that observed in TTLL6A- and TTLL12B-depleted cells. Reduced polyglutamylation in TTLL1-deficient cells also leads to increased levels in tubulin tyrosination, providing new evidence for an interplay between the tubulin tyrosination and detyrosination cycle and polyglutamylation. We also show that TTLL1 acts differentially on specific microtubule doublets of the flagellar axoneme, although the absence of TTLL1 appears to have no measurable effect on cell motility.

Parasite microtubule arrays

Microtubules are a key component of eukaryotic cell architecture. Regulation of the dynamic growth and shrinkage of microtubules gives cells their shape, allows cells to swim, and drives the separation of chromosomes. Parasites have developed intriguingly divergent biology, seemingly expanding upon and reinventing microtubule use in fascinating ways. These organisms affect life on the planet at scales that are often overlooked: there are likely more parasitic than free-living organisms on Earth, and they have a sizeable influence across ecosystems. As parasites can cause devastating diseases, this in turn drives evolutionary adaptations and species diversity. Parasites are varied, living in all environments and at all scales - from the tiny 2 μm single-celled Plasmodium merozoite that invades red blood cells to the 40 m long Tetragonoporus, a large intestinal tapeworm of whales. To survive in their various niches, parasites have undergone striking adaptations and developed complex life cycles, often involving two or more host species. This diversity is reflected at the cellular level, where unique molecular mechanisms, cytoskeletal structures and organellar compositions are found. Hence, the study of parasite cell biology provides a biological playground for understanding diversity and species diversification. It also facilitates the identification of specific targets to develop urgently needed therapeutics: for example, drugs targeting microtubules are used at large scale to treat intestinal worms and parasites that form tissue cysts in our livers and brains. Here, we discuss some of the curious microtubule arrays found in a small, select number of human-infecting, single-celled parasites of medical importance (Table 1). Our aim is to put a spotlight on distinctive molecular features in a field that promises exciting cell-biological discoveries with the potential for therapeutic breakthroughs.

TrypTag.org: from images to discoveries using genome-wide protein localisation in Trypanosoma brucei

TrypTag was a 4-year project to tag the N- and C-termini of almost all Trypanosoma brucei proteins with a fluorescent protein and record the subcellular localisation through images and manual annotation. We highlight the new routes to cell biological discovery this transformative resource is enabling for parasitologists and cell biologists.

CRK2 controls cytoskeleton morphogenesis in Trypanosoma brucei by phosphorylating β-tubulin to regulate microtubule dynamics

Microtubules constitute a vital part of the cytoskeleton in eukaryotes by mediating cell morphogenesis, cell motility, cell division, and intracellular transport. The cytoskeleton of the parasite Trypanosoma brucei contains an array of subpellicular microtubules with their plus-ends positioned toward the posterior cell tip, where extensive microtubule growth and cytoskeleton remodeling take place during early cell cycle stages. However, the control mechanism underlying microtubule dynamics at the posterior cell tip remains elusive. Here, we report that the S-phase cyclin-dependent kinase-cyclin complex CRK2-CYC13 in T. brucei regulates microtubule dynamics by phosphorylating β-tubulin on multiple evolutionarily conserved serine and threonine residues to inhibit its incorporation into cytoskeletal microtubules and promote its degradation in the cytosol. Consequently, knockdown of CRK2 or CYC13 causes excessive microtubule extension and loss of microtubule convergence at the posterior cell tip, leading to cytoskeleton elongation and branching. These findings uncover a control mechanism for cytoskeletal microtubule dynamics by which CRK2 phosphorylates β-tubulin and fine-tunes cellular β-tubulin protein abundance to restrict excess microtubule extension for the maintenance of cytoskeleton architecture.

Genome-wide subcellular protein map for the flagellate parasite Trypanosoma brucei

Trypanosoma brucei is a model trypanosomatid, an important group of human, animal and plant unicellular parasites. Understanding their complex cell architecture and life cycle is challenging because, as with most eukaryotic microbes, ~50% of genome-encoded proteins have completely unknown functions. Here, using fluorescence microscopy and cell lines expressing endogenously tagged proteins, we mapped the subcellular localization of 89% of the T. brucei proteome, a resource we call TrypTag. We provide clues to function and define lineage-specific organelle adaptations for parasitism, mapping the ultraconserved cellular architecture of eukaryotes, including the first comprehensive 'cartographic' analysis of the eukaryotic flagellum, which is vital for morphogenesis and pathology. To demonstrate the power of this resource, we identify novel organelle subdomains and changes in molecular composition through the cell cycle. TrypTag is a transformative resource, important for hypothesis generation for both eukaryotic evolutionary molecular cell biology and fundamental parasite cell biology.

3D FIB-SEM structural insights into the architecture of sub-pellicular microtubules of Trypanosoma cruzi epimastigotes

BACKGROUND INFORMATION

Trypanosomatidae, which includes eukaryotic species agents of diseases like leishmaniasis, sleeping sickness, and Chagas disease, have special structures and organelles not found in mammalian cells. They present a layer of microtubules, known as subpellicular microtubules (SPMT), located underneath the plasma membrane and responsible for preserving cell morphology, cell polarity, the position of single copy organelles, and morphological changes that occur throughout the protozoan life cycle. Even though a lot of knowledge about the SPMT is available, we still do not know exactly how each microtubule in the system is organized in three dimensions. Here, we use focused ion beam scanning electron microscopy (FIB-SEM) to analyze the tridimensional organization of epimastigotes SPMT.

RESULTS

The high-resolution 3D analyses revealed that certain microtubules of the SPMT end more prematurely than the neighboring ones.

CONCLUSIONS

These microtubules could (1) be shorter or (2) have the same length as the neighboring ones, assuming that those end up earlier at their other end, might be treadmilling/catastrophe events that have not yet been described in trypanosomatids.

KLIF-associated cytoskeletal proteins in Trypanosoma brucei regulate cytokinesis by promoting cleavage furrow positioning and ingression

Cytokinesis in the early divergent protozoan Trypanosoma brucei occurs from the anterior cell tip of the new-flagellum daughter toward the nascent posterior end of the old-flagellum daughter of a dividing biflagellated cell. The cleavage furrow ingresses unidirectionally along the preformed cell division fold and is regulated by an orphan kinesin named kinesin localized to the ingressing furrow (KLIF) that localizes to the leading edge of the ingressing furrow. Little is known about how furrow ingression is controlled by KLIF and whether KLIF interacts with and cooperates with other cytokinesis regulatory proteins to promote furrow ingression. Here, we investigated the roles of KLIF in cleavage furrow ingression and identified a cohort of KLIF-associated cytoskeletal proteins as essential cytokinesis regulators. By genetic complementation, we demonstrated the requirement of the kinesin motor activity, but not the putative tropomyosin domain, of KLIF in promoting furrow ingression. We further showed that depletion of KLIF impaired the resolution of the nascent posterior of the old-flagellar daughter cell, thereby stalking cleavage furrow ingression at late stages of cytokinesis. Through proximity biotinylation, we identified a subset of cytoskeleton-associated proteins (CAPs) as KLIF-proximal proteins, and functional characterization of these cytoskeletal proteins revealed the essential roles of CAP46 and CAP52 in positioning the cleavage furrow and the crucial roles of CAP42 and CAP50 in promoting cleavage furrow ingression. Together, these results identified multiple cytoskeletal proteins as cytokinesis regulators and uncovered their essential and distinct roles in cytokinesis.

Trypanin Disruption Affects the Motility and Infectivity of the Protozoan Trypanosoma cruzi

The flagellum of Trypanosomatids is an organelle that contributes to multiple functions, including motility, cell division, and host–pathogen interaction. Trypanin was first described in Trypanosoma brucei and is part of the dynein regulatory complex. TbTrypanin knockdown parasites showed motility defects in procyclic forms; however, silencing in bloodstream forms was lethal. Since TbTrypanin mutants show drastic phenotypic changes in mammalian stages, we decided to evaluate if the Trypanosoma cruzi ortholog plays a similar role by using the CRISPR-Cas9 system to generate null mutants. A ribonucleoprotein complex of SaCas9 and sgRNA plus donor oligonucleotide were used to edit both alleles of TcTrypanin without any selectable marker. TcTrypanin −/− epimastigotes showed a lower growth rate, partially detached flagella, normal numbers of nuclei and kinetoplasts, and motility defects such as reduced displacement and speed and increased tumbling propensity. The epimastigote mutant also showed decreased efficiency of in-vitro metacyclogenesis. Mutant parasites were able to complete the entire life cycle in vitro; however, they showed a reduction in their infection capacity compared with WT and addback cultures. Our data show that T. cruzi life cycle stages have differing sensitivities to TcTrypanin deletion. In conclusion, additional work is needed to dissect the motility components of T. cruzi and to identify essential molecules for mammalian stages.