The two cytoplasmic dynein-2 isoforms in Leishmania mexicana perform separate functions

The Dictyostelium Centrosome

The centrosome of Dictyostelium amoebae contains no centrioles and consists of a cylindrical layered core structure surrounded by a corona harboring microtubule-nucleating γ-tubulin complexes. It is the major centrosomal model beyond animals and yeasts. Proteomics, protein interaction studies by BioID and superresolution microscopy methods led to considerable progress in our understanding of the composition, structure and function of this centrosome type. We discuss all currently known components of the Dictyostelium centrosome in comparison to other centrosomes of animals and yeasts.

Genome mutations of the Turkish strain Leishmania infantum_TR01

PURPOSE

Today, almost 15 years have passed since the whole genome sequence (WGS) of a Leishmania parasite was completed for the first time. However, information on the genetics of these parasites remains to be elucidated. Genome-based studies may contribute to control strategies for leishmaniasis. The increase in genetically based studies, particularly whole-genome sequencing studies on Leishmania, will contribute to the control of leishmaniasis, which is a common and neglected disease worldwide. Our previous study obtained the Leishmania infantum_TR01 (Lin_TR01) genome sequence (Guldemir et al., 2020). In the present study, we focused on the mutations detected in the genome and aimed to investigate the effects of these mutations.

METHODS

In our previous study, the whole-genome sequence of the L. infantum_TR01 strain was obtained (Guldemir et al., 2020). In the present study, 3153 polymorphisms were detected in bioinformatics analysis performed on the Geneious 11.0.5. (www.geneious.com) platform. Herein, the L. infantum JPCM5 strain was used as the reference genome for genome mapping. Polymorphic regions were determined using the Find Variations/SNPs program on the Geneious platform. We further analyzed these polymorphisms detected in the previous study. Additionally, a literature review was performed by searching the PubMed database for proteins with polymorphisms.

RESULTS

In our previous study (Guldemir et al., 2020), the genomic DNA sequence was submitted to the NCBI GenBank (www.ncbi.nlm.nih.gov) database and registered under the name Leishmania infantum_TR01 (Lin_TR01) and project accession number PRJNA437593. As a result of the annotation of the genome, 3153 polymorphisms were identified. In this study, 166 protein-coding polymorphisms were found among the 3153 polymorphisms, affecting 63 different proteins. Fourteen of them were studied, and the remaining 49 proteins were not studied. The 14 proteins examined in terms of the mutations detected in this study were related to virulence (n = 5), vaccine candidates (n = 2), diagnosis/typing (n = 4), drug resistance (n = 2), drug targets (n = 3) and vital function (n = 1).

CONCLUSION

As mentioned previously, the acquisition of the Lin_TR01 genome was described in our previous study (Guldemir et al., 2020). The present meta-analytical study is the first comparison report of whole-genome sequence-based polymorphisms between the Turkish strain Leishmania infantum_TR01 and reference Leishmania infantum JPCM5 strain and evaluated polymorphisms and proteins. In this study, we focused on the mutations detected in the genome, and the effects of these mutations were investigated and evaluated together with the current literature. In our previous study, a high-quality WGS of Leishmania infantum was successfully obtained for the first time in Turkey (1). In this study, the comparison of both genomes will contribute to providing the scientific community with a solid infrastructure for postgenomic investigations of the parasite.

Emerging Functions of Actins and Actin Binding Proteins in Trypanosomatids

Actin is the major protein constituent of the cytoskeleton that performs wide range of cellular functions. It exists in monomeric and filamentous forms, dynamics of which is regulated by a large repertoire of actin binding proteins. However, not much was known about existence of these proteins in trypanosomatids, till the genome sequence data of three important organisms of this class, viz. Trypanosoma brucei, Trypanosoma cruzi and Leishmania major, became available. Here, we have reviewed most of the findings reported to date on the intracellular distribution, structure and functions of these proteins and based on them, we have hypothesized some of their functions. The major findings are as follows: (1) All the three organisms encode at least a set of ten actin binding proteins (profilin, twinfilin, ADF/cofilin, CAP/srv2, CAPz, coronin, two myosins, two formins) and one isoform of actin, except that T. cruzi encodes for three formins and several myosins along with four actins. (2) Actin 1 and a few actin binding proteins (ADF/cofilin, profilin, twinfilin, coronin and myosin13 in L. donovani; ADF/cofilin, profilin and myosin1 in T. brucei; profilin and myosin-F in T.cruzi) have been identified and characterized. (3) In all the three organisms, actin cytoskeleton has been shown to regulate endocytosis and intracellular trafficking. (4) Leishmania actin1 has been the most characterized protein among trypanosomatid actins. (5) This protein is localized to the cytoplasm as well as in the flagellum, nucleus and kinetoplast, and in vitro, it binds to DNA and displays scDNA relaxing and kDNA nicking activities. (6) The pure protein prefers to form bundles instead of thin filaments, and does not bind DNase1 or phalloidin. (7) Myosin13, myosin1 and myosin-F regulate endocytosis and intracellular trafficking, respectively, in Leishmania, T. brucei and T. cruzi. (8) Actin-dependent myosin13 motor is involved in dynamics and assembly of Leishmania flagellum. (9) Leishmania twinfilin localizes mostly to the nucleolus and coordinates karyokinesis by effecting splindle elongation and DNA synthesis. (10) Leishmania coronin binds and promotes actin filament formation and exists in tetrameric form rather than trimeric form, like other coronins. (11) Trypanosomatid profilins are essential for survival of all the three parasites.

Deciphering the role of UBA-like domains in intraflagellar distribution and functions of myosin XXI in Leishmania

Myosin XXI (Myo21) is a novel class of myosin present in all kinetoplastid parasites, such as Trypanosoma and Leishmania. This protein in Leishmania promastigotes is predominantly localized to the proximal region of the flagellum, and is involved in the flagellum assembly, cell motility and intracellular vesicle transport. As Myo21 contains two ubiquitin associated (UBA)-like domains (UBLD) in its amino acid sequence, we considered it of interest to analyze the role of these domains in the intracellular distribution and functions of this protein in Leishmania cells. In this context, we created green fluorescent protein (GFP)-conjugates of Myo21 constructs lacking one of the two UBLDs at a time or both the UBLDs as well as GFP-conjugates of only the two UBLDs and Myo21 tail lacking the two UBLDs and separately expressed them in the Leishmania cells. Our results show that unlike Myo21-GFP, Myo21-GFP constructs lacking either one or both the UBLDs failed to concentrate and co-distribute with actin in the proximal region of the flagellum. Nevertheless, the GFP conjugate of only the two UBLDs was found to predominantly localize to the flagellum base. Additionally, the cells that expressed only one or both the UBLDs-deleted Myo21-GFP constructs possessed shorter flagellum and displayed slower motility, compared to Myo21-GFP expressing cells. Further, the intracellular vesicle transport and cell growth were severely impaired in the cells that expressed both the UBLDs deleted Myo21-GFP construct, but in contrast, virtually no effect was observed on the intracellular vesicle transport and growth in the cells that expressed single UBLD deleted mutant proteins. Moreover, the observed slower growth of both the UBLDs-deleted Myo21-GFP expressing cells was primarily due to delayed G2/M phase caused by aberrant nuclear and daughter cell segregation during their cell division process. These results taken together clearly reveal that the presence of UBLDs in Myo21 are essentially required for its predominant localization to the flagellum base, and perhaps also in its involvement in the flagellum assembly and cell division. Possible role of UBLDs in involvement of Myo21 during Leishmania flagellum assembly and cell cycle is discussed.

Cytoplasmic dynein-2 at a glance

Cytoplasmic dynein-2 is a motor protein complex that drives the movement of cargoes along microtubules within cilia, facilitating the assembly of these organelles on the surface of nearly all mammalian cells. Dynein-2 is crucial for ciliary function, as evidenced by deleterious mutations in patients with skeletal abnormalities. Long-standing questions include how the dynein-2 complex is assembled, regulated, and switched between active and inactive states. A combination of model organisms, in vitro cell biology, live-cell imaging, structural biology and biochemistry has advanced our understanding of the dynein-2 motor. In this Cell Science at a Glance article and the accompanying poster, we discuss the current understanding of dynein-2 and its roles in ciliary assembly and function.

Coordination of the Cell Cycle in Trypanosomes

Trypanosomes have complex life cycles within which there are both proliferative and differentiation cell divisions. The coordination of the cell cycle to achieve these different divisions is critical for the parasite to infect both host and vector. From studying the regulation of the proliferative cell cycle of the Trypanosoma brucei procyclic life cycle stage, three subcycles emerge that control the duplication and segregation of (a) the nucleus, (b) the kinetoplast, and (c) a set of cytoskeletal structures. We discuss how the clear dependency relationships within these subcycles, and the potential for cross talk between them, are likely required for overall cell cycle coordination. Finally, we look at the implications this interdependence has for proliferative and differentiation divisions through the T. brucei life cycle and in related parasitic trypanosomatid species.

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.

Cellular landmarks of Trypanosoma brucei and Leishmania mexicana

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Trypanosoma and Leishmania are single cell eukaryotic parasites.

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The cell organisation of these human pathogens is complex and highly structured.

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This describes an inventory of reliable reference markers for 32 cell structures.

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These light microscopy landmarks are a valuable resource for researchers.

The kinetoplastids Trypanosoma brucei and Leishmania mexicana are eukaryotes with a highly structured cellular organisation that is reproduced with great fidelity in each generation. The pattern of signal from a fluorescently tagged protein can define the specific structure/organelle that this protein localises to, and can be extremely informative in phenotype analysis in experimental perturbations, life cycle tracking, post-genomic assays and functional analysis of organelles. Using the vast coverage of protein subcellular localisations provided by the TrypTag project, an ongoing project to determine the localisation of every protein encoded in the T. brucei genome, we have generated an inventory of reliable reference organelle markers for both parasites that combines epifluorescence images with a detailed description of the key features of each localisation. We believe this will be a useful comparative resource that will enable researchers to quickly and accurately pinpoint the localisation of their proteins of interest and will provide cellular markers for many types of cell biology studies. We see this as another important step in the post-genomic era analyses of these parasites, in which ever expanding datasets generate numerous candidates to analyse. Adoption of these reference proteins by the community is likely to enhance research studies and enable better comparison of data.

Dependency relationships between IFT-dependent flagellum elongation and cell morphogenesis in Leishmania

Flagella have multiple functions that are associated with different axonemal structures. Motile flagella typically have a 9 + 2 arrangement of microtubules, whereas sensory flagella normally have a 9 + 0 arrangement. Leishmania exhibits both of these flagellum forms and differentiation between these two flagellum forms is associated with cytoskeletal and cell shape changes. We disrupted flagellum elongation in Leishmania by deleting the intraflagellar transport (IFT) protein IFT140 and examined the effects on cell morphogenesis. Δift140 cells have no external flagellum, having only a very short flagellum within the flagellar pocket. This short flagellum had a collapsed 9 + 0 (9v) axoneme configuration reminiscent of that in the amastigote and was not attached to the pocket membrane. Although amastigote-like changes occurred in the flagellar cytoskeleton, the cytoskeletal structures of Δift140 cells retained their promastigote configurations, as examined by fluorescence microscopy of tagged proteins and serial electron tomography. Thus, Leishmania promastigote cell morphogenesis does not depend on the formation of a long flagellum attached at the neck. Furthermore, our data show that disruption of the IFT system is sufficient to produce a switch from the 9 + 2 to the collapsed 9 + 0 (9v) axonemal structure, echoing the process that occurs during the promastigote to amastigote differentiation.

Emerging mechanisms of dynein transport in the cytoplasm versus the cilium

Two classes of dynein power long-distance cargo transport in different cellular contexts. Cytoplasmic dynein-1 is responsible for the majority of transport toward microtubule minus ends in the cell interior. Dynein-2, also known as intraflagellar transport dynein, moves cargoes along the axoneme of eukaryotic cilia and flagella. Both dyneins operate as large ATP-driven motor complexes, whose dysfunction is associated with a group of human disorders. But how similar are their mechanisms of action and regulation? To examine this question, this review focuses on recent advances in dynein-1 and -2 research, and probes to what extent the emerging principles of dynein-1 transport could apply to or differ from those of the less well-understood dynein-2 mechanoenzyme.

Shape, form, function and Leishmania pathogenicity: from textbook descriptions to biological understanding

The shape and form of protozoan parasites are inextricably linked to their pathogenicity. The evolutionary pressure associated with establishing and maintaining an infection and transmission to vector or host has shaped parasite morphology. However, there is not a 'one size fits all' morphological solution to these different pressures, and parasites exhibit a range of different morphologies, reflecting the diversity of their complex life cycles. In this review, we will focus on the shape and form of Leishmania spp., a group of very successful protozoan parasites that cause a range of diseases from self-healing cutaneous leishmaniasis to visceral leishmaniasis, which is fatal if left untreated.

The intraflagellar transport dynein complex of trypanosomes is made of a heterodimer of dynein heavy chains and of light and intermediate chains of distinct functions

Cilia and flagella are assembled by intraflagellar transport (IFT) of protein complexes that bring tubulin and other precursors to the incorporation site at their distal tip. Anterograde transport is driven by kinesin, whereas retrograde transport is ensured by a specific dynein. In the protist Trypanosoma brucei, two distinct genes encode fairly different dynein heavy chains (DHCs; ∼40% identity) termed DHC2.1 and DHC2.2, which form a heterodimer and are both essential for retrograde IFT. The stability of each heavy chain relies on the presence of a dynein light intermediate chain (DLI1; also known as XBX-1/D1bLIC). The presence of both heavy chains and of DLI1 at the base of the flagellum depends on the intermediate dynein chain DIC5 (FAP133/WDR34). In the IFT140(RNAi) mutant, an IFT-A protein essential for retrograde transport, the IFT dynein components are found at high concentration at the flagellar base but fail to penetrate the flagellar compartment. We propose a model by which the IFT dynein particle is assembled in the cytoplasm, reaches the base of the flagellum, and associates with the IFT machinery in a manner dependent on the IFT-A complex.

Biology of human pathogenic trypanosomatids: epidemiology, lifecycle and ultrastructure

Leishmania and Trypanosoma belong to the Trypanosomatidae family and cause important human infections such as leishmaniasis, Chagas disease, and sleeping sickness. Leishmaniasis, caused by protozoa belonging to Leishmania, affects about 12 million people worldwide and can present different clinical manifestations, i.e., visceral leishmaniasis (VL), cutaneous leishmaniasis (CL), mucocutaneous leishmaniasis (MCL), diffuse cutaneous leishmaniasis (DCL), and post-kala-azar dermal leishmaniasis (PKDL). Chagas disease, also known as American trypanosomiasis, is caused by Trypanosoma cruzi and is mainly prevalent in Latin America but is increasingly occurring in the United States, Canada, and Europe. Sleeping sickness or human African trypanosomiasis (HAT), caused by two sub-species of Trypanosoma brucei (i.e., T. b. rhodesiense and T. b. gambiense), occurs only in sub-Saharan Africa countries. These pathogenic trypanosomatids alternate between invertebrate and vertebrate hosts throughout their lifecycles, and different developmental stages can live inside the host cells and circulate in the bloodstream or in the insect gut. Trypanosomatids have a classical eukaryotic ultrastructural organization with some of the same main organelles found in mammalian host cells, while also containing special structures and organelles that are absent in other eukaryotic organisms. For example, the mitochondrion is ramified and contains a region known as the kinetoplast, which houses the mitochondrial DNA. Also, the glycosomes are specialized peroxisomes containing glycolytic pathway enzymes. Moreover, a layer of subpellicular microtubules confers mechanic rigidity to the cell. Some of these structures have been investigated to determine their function and identify potential enzymes and metabolic pathways that may constitute targets for new chemotherapeutic drugs.

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.

Carcinogens induce loss of the primary cilium in human renal proximal tubular epithelial cells independently of effects on the cell cycle

The primary cilium is an immotile sensory and signaling organelle found on the majority of mammalian cell types. Of the multitude of roles that the primary cilium performs, perhaps some of the most important include maintenance of differentiation, quiescence, and cellular polarity. Given that the progression of cancer requires disruption of all of these processes, we have investigated the effects of several carcinogens on the primary cilium of the RPTEC/TERT1 human proximal tubular epithelial cell line. Using both scanning electron microscopy and immunofluorescent labeling of the ciliary markers acetylated tubulin and Arl13b, we confirmed that RPTEC/TERT1 cells express primary cilium upon reaching confluence. Treatment with the carcinogens ochratoxin A (OTA) and potassium bromate (KBrO(3)) caused a significant reduction in the number of ciliated cells, while exposure to nifedipine, a noncarcinogenic renal toxin, had no effect on primary cilium expression. Flow cytometric analysis of the effects of all three compounds on the cell cycle revealed that only KBrO(3) resulted in an increase in the proportion of cells entering the cell cycle. Microarray analysis revealed dysregulation of multiple pathways affecting ciliogenesis and ciliary maintenance following OTA and KBrO(3) exposure, which were unaffected by nifedipine exposure. The primary cilium represents a unique physical checkpoint with relevance to carcinogenesis. We have shown that the renal carcinogens OTA and KBrO(3) cause significant deciliation in a model of the proximal tubule. With KBrO(3), this was followed by reentry into the cell cycle; however, deciliation was not found to be associated with reentry into the cell cycle following OTA exposure. Transcriptomic analysis identified dysregulation of Wnt signaling and ciliary trafficking in response to OTA and KBrO(3) exposure.

Overexpression of S4D mutant of Leishmania donovani ADF/cofilin impairs flagellum assembly by affecting actin dynamics

Leishmania, like other eukaryotes, contains large amounts of actin and a number of actin-related and actin binding proteins. Our earlier studies have shown that deletion of the gene corresponding to Leishmania actin-depolymerizing protein (ADF/cofilin) adversely affects flagellum assembly, intracellular trafficking, and cell division. To further analyze this, we have now created ADF/cofilin site-specific point mutants and then examined (i) the actin-depolymerizing, G-actin binding, and actin-bound nucleotide exchange activities of the mutant proteins and (ii) the effect of overexpression of these proteins in wild-type cells. Here we show that S4D mutant protein failed to depolymerize F-actin but weakly bound G-actin and inhibited the exchange of G-actin-bound nucleotide. We further observed that overexpression of this protein impaired flagellum assembly and consequently cell motility by severely impairing the assembly of the paraflagellar rod, without significantly affecting vesicular trafficking or cell growth. Taken together, these results indicate that dynamic actin is essentially required in assembly of the eukaryotic flagellum.

A dysflagellar mutant of Leishmania (Viannia) braziliensis isolated from a cutaneous leishmaniasis patient

Background

Parasites of the Leishmania genus alternate between the flagellated extracellular promastigote stage and intracellular amastigotes. Here we report the characterization of a Leishmania isolate, obtained from a cutaneous leishmaniasis patient, which presents peculiar morphological features.

Methods

The parasite was cultured in vitro and characterized morphologically using optical and electron microscopy. Identification was performed based on monoclonal antibodies and internal ribosomal spacer typing. In vitro macrophage cultures, murine experimental models and sand fly infections were used to evaluate infectivity in vitro and in vivo.

Results

The isolate was identified as Leishmania (Viannia) braziliensis. In the atypical promastigotes grown in culture, a short flagellum surrounded or interrupted by a protuberance of disorganized material was observed. A normal axoneme was present close to the basal body but without elongation much further outside the flagellar pocket. A disorganized swelling at the precocious end of the axoneme coincided with the lack of a paraflagellar rod structure. The isolate was able to infect macrophages in vitro, induce lesions in BALB/c mice and infect Lutzomyia longipalpis.

Conclusions

Notwithstanding the lack of an extracellular flagellum, this isolate infects macrophages in vitro and produces lesions when inoculated into mice. Moreover, it is able to colonize phlebotomine sand flies. Considering the importance attributed to the flagellum in the successful infection and survival of Leishmania in the insect midgut and in the invasion of macrophages, these findings may bring new light into the infectious mechanisms of L. (V.) braziliensis.

Flagellum assembly and function during the Leishmania life cycle

During a complex digenetic life cycle flagellated Leishmania parasites alternate between promastigote and amastigote forms which differ significantly in cellular morphology and flagellum length. Recent studies have provided important new insights into mechanisms by which Leishmania regulate expression of genes required for flagellum assembly, and mechanisms used to modify flagellum length. While the critical role of the promastigote flagellum in parasite biology has long been appreciated, the importance of the amastigote flagellum has often been disregarded. However, recent work suggests that the 'rudimentary' amastigote flagellum may serve indispensable roles in cellular organisation, and/or sensory perception, which are critical for intracellular survival of Leishmania within host macrophages.

ADF/cofilin-driven actin dynamics in early events of Leishmania cell division

ADF/cofilin is an actin-dynamics-regulating protein that is required for several actin-based cellular processes such as cell motility and cytokinesis. A homologue of this protein has recently been identified in the protozoan parasite Leishmania, which has been shown to be essentially required in flagellum assembly and cell motility. However, the role of this protein in cytokinesis remains largely unknown. We show here that deletion of the gene encoding ADF/cofilin in these organisms results in several aberrations in the process of cell division. These aberrations include delay in basal body and kinetoplast separation, cleavage furrow progression and flagellar pocket division. In addition to these changes, the intracellular trafficking and actin dynamics are also adversely affected. All these abnormalities are, however, reversed by episomal complementation. Together, these results indicate that actin dynamics regulates early events in Leishmania cell division.

Trafficking activity of myosin XXI is required in assembly of Leishmania flagellum

Actin-based myosin motors have a pivotal role in intracellular trafficking in eukaryotic cells. The parasitic protozoan organism Leishmania expresses a novel class of myosin, myosin XXI (Myo21), which is preferentially localized at the proximal region of the flagellum. However, its function in this organism remains largely unknown. Here, we show that Myo21 interacts with actin, and its expression is dependent of the growth stage. We further reveal that depletion of Myo21 levels results in impairment of the flagellar assembly and intracellular trafficking. These defects are, however, reversed by episomal complementation. Additionally, it is shown that deletion of the Myo21 gene leads to generation of ploidy, suggesting an essential role of Myo21 in survival of Leishmania cells. Together, these results indicate that actin-dependent trafficking activity of Myo21 is essentially required during assembly of the Leishmania flagellum.

Characterization of a subunit of the outer dynein arm docking complex necessary for correct flagellar assembly in Leishmania donovani

Background

In order to proceed through their life cycle, Leishmania parasites switch between sandflies and mammals. The flagellated promastigote cells transmitted by the insect vector are phagocytized by macrophages within the mammalian host and convert into the amastigote stage, which possesses a rudimentary flagellum only. During an earlier proteomic study of the stage differentiation of the parasite we identified a component of the outer dynein arm docking complex, a structure of the flagellar axoneme. The 70 kDa subunit of the outer dynein arm docking complex consists of three subunits altogether and is essential for the assembly of the outer dynein arm onto the doublet microtubule of the flagella. According to the nomenclature of the well-studied Chlamydomonas reinhardtii complex we named the Leishmania protein LdDC2.

Methodology/Principal Findings

This study features a characterization of the protein over the life cycle of the parasite. It is synthesized exclusively in the promastigote stage and localizes to the flagellum. Gene replacement mutants of lddc2 show reduced growth rates and diminished flagellar length. Additionally, the normally spindle-shaped promastigote parasites reveal a more spherical cell shape giving them an amastigote-like appearance. The mutants lose their motility and wiggle in place. Ultrastructural analyses reveal that the outer dynein arm is missing. Furthermore, expression of the amastigote-specific A2 gene family was detected in the deletion mutants in the absence of a stage conversion stimulus. In vitro infectivity is slightly increased in the mutant cell line compared to wild-type Leishmania donovani parasites.

Conclusions/Significance

Our results indicate that the correct assembly of the flagellum has a great influence on the investigated characteristics of Leishmania parasites. The lack of a single flagellar protein causes an aberrant morphology, impaired growth and altered infectiousness of the parasite.

Leishmania parasites are responsible for the disease leishmaniasis. They are spread through sandflies. The primary hosts are mammals, including humans. They occur in two different morphological forms. The flagellated promastigotes live in the gut of the sandfly vector. After transmission to the mammalian host they get phagocytized by macrophages and convert into the amastigote form, which is able to survive within the phagolysosome. The molecular mechanisms underlying this transformation process from promastigote to amastigote are poorly understood so far. A striking difference of the life cycle stages is a long flagellum in the promastigote compared to only a rudimentary flagellum in the mammalian stage amastigote. During an earlier study of the stage differentiation of Leishmania donovani we identified a flagellar protein, a subunit of the outer dynein arm docking complex (ODA-DC2). This protein is part of a flagellar structure called the axoneme. Here we have further characterized the protein regarding its role within the life cycle of the parasite. Mutant promastigotes lacking DC2 protein show reduced flagellar length and a more amastigote-like appearance overall. In addition, the motility is heavily retrenched and transmission electron microscopy indicated that the flagellar ultrastructure is affected. Furthermore, the mutants express amastigote-specific genes and show increased in vitro infectiousness towards macrophages. Therefore, we conclude that the correct assembly of the flagellum is vital for maintenance of the promastigote stage of the parasite.

The Trypanosoma brucei flagellum: moving parasites in new directions

African trypanosomes are devastating human and animal pathogens. Trypanosoma brucei rhodesiense and T. b. gambiense subspecies cause the fatal human disease known as African sleeping sickness. It is estimated that several hundred thousand new infections occur annually and the disease is fatal if untreated. T. brucei is transmitted by the tsetse fly and alternates between bloodstream-form and insect-form life cycle stages that are adapted to survive in the mammalian host and the insect vector, respectively. The importance of the flagellum for parasite motility and attachment to the tsetse fly salivary gland epithelium has been appreciated for many years. Recent studies have revealed both conserved and novel features of T. brucei flagellum structure and composition, as well as surprising new functions that are outlined here. These discoveries are important from the standpoint of understanding trypanosome biology and identifying novel drug targets, as well as for advancing our understanding of fundamental aspects of eukaryotic flagellum structure and function.

Dynein-2 affects the regulation of ciliary length but is not required for ciliogenesis in Tetrahymena thermophila

Eukaryotic cilia and flagella are assembled and maintained by the bidirectional intraflagellar transport (IFT). Studies in alga, nematode, and mouse have shown that the heavy chain (Dyh2) and the light intermediate chain (D2LIC) of the cytoplasmic dynein-2 complex are essential for retrograde intraflagellar transport. In these organisms, disruption of either dynein-2 component results in short cilia/flagella with bulbous tips in which excess IFT particles have accumulated. In Tetrahymena, the expression of the DYH2 and D2LIC genes increases during reciliation, consistent with their roles in IFT. However, the targeted elimination of either DYH2 or D2LIC gene resulted in only a mild phenotype. Both knockout cell lines assembled motile cilia, but the cilia were of more variable lengths and less numerous than wild-type controls. Electron microscopy revealed normally shaped cilia with no swelling and no obvious accumulations of material in the distal ciliary tip. These results demonstrate that dynein-2 contributes to the regulation of ciliary length but is not required for ciliogenesis in Tetrahymena.

Actin-depolymerizing factor, ADF/cofilin, is essentially required in assembly of Leishmania flagellum

ADF/cofilins are ubiquitous actin dynamics-regulating proteins that have been mainly implicated in actin-based cell motility. Trypanosomatids, e.g. Leishmania and Trypanosoma, which mediate their motility through flagellum, also contain a putative ADF/cofilin homologue, but its role in flagellar motility remains largely unexplored. We have investigated the role of this protein in assembly and motility of the Leishmania flagellum after knocking out the ADF/cofilin gene by targeted gene replacement. The resultant mutants were completely immotile, short and stumpy, and had reduced flagellar length and severely impaired beat. In addition, the assembly of the paraflagellar rod was lost, vesicle-like structures were seen throughout the length of the flagellum and the state and distribution of actin were altered. However, episomal complementation of the gene restored normal morphology and flagellar function. These results for the first time indicate that the actin dynamics-regulating protein ADF/cofilin plays a critical role in assembly and motility of the eukaryotic flagellum.

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

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

Intraflagellar transport and functional analysis of genes required for flagellum formation in trypanosomes

Intraflagellar transport (IFT) is the bidirectional movement of protein complexes required for cilia and flagella formation. We investigated IFT by analyzing nine conventional IFT genes and five novel putative IFT genes (PIFT) in Trypanosoma brucei that maintain its existing flagellum while assembling a new flagellum. Immunostaining against IFT172 or expression of tagged IFT20 or green fluorescent protein GFP::IFT52 revealed the presence of IFT proteins along the axoneme and at the basal body and probasal body regions of both old and new flagella. IFT particles were detected by electron microscopy and exhibited a strict localization to axonemal microtubules 3-4 and 7-8, suggesting the existence of specific IFT tracks. Rapid (>3 microm/s) bidirectional intraflagellar movement of GFP::IFT52 was observed in old and new flagella. RNA interference silencing demonstrated that all individual IFT and PIFT genes are essential for new flagellum construction but the old flagellum remained present. Inhibition of IFTB proteins completely blocked axoneme construction. Absence of IFTA proteins (IFT122 and IFT140) led to formation of short flagella filled with IFT172, indicative of defects in retrograde transport. Two PIFT proteins turned out to be required for retrograde transport and three for anterograde transport. Finally, flagellum membrane elongation continues despite the absence of axonemal microtubules in all IFT/PIFT mutant.

Basal body positioning is controlled by flagellum formation in Trypanosoma brucei

To perform their multiple functions, cilia and flagella are precisely positioned at the cell surface by mechanisms that remain poorly understood. The protist Trypanosoma brucei possesses a single flagellum that adheres to the cell body where a specific cytoskeletal structure is localised, the flagellum attachment zone (FAZ). Trypanosomes build a new flagellum whose distal tip is connected to the side of the old flagellum by a discrete structure, the flagella connector. During this process, the basal body of the new flagellum migrates towards the posterior end of the cell. We show that separate inhibition of flagellum assembly, base-to-tip motility or flagella connection leads to reduced basal body migration, demonstrating that the flagellum contributes to its own positioning. We propose a model where pressure applied by movements of the growing new flagellum on the flagella connector leads to a reacting force that in turn contributes to migration of the basal body at the proximal end of the flagellum.

The flagellar attachment zone of Trypanosoma cruzi epimastigote forms

The flagellar attachment zone (FAZ) is an adhesion region of Trypanosoma cruzi epimastigote forms where the flagellum emerges from the flagellar pocket and remains attached to the cell body. This region shows a junctional complex which is formed by a linear series of apposed macular structures that are separated by amorphous material and clusters of intramembranous particles. Two protein groups appear to be important in the FAZ region: a membrane glycoprotein of 72kDa and several high molecular weight proteins. To gain a better understanding of the FAZ region, we compared wild-type Y strain T. cruzi epimastigotes with a mutant cell in which the 72-kDa surface glycoprotein (Gp72), involved in cell body-flagellum adhesion, had been deleted by target gene replacement. Using immunofluorescence confocal microscopy and electron microscopy techniques to analyze the FAZ region the results suggest that, in the absence of Gp72, other proteins involved in the formation of FAZ remain concentrated in the flagellar pocket region. The analysis of a 3-D reconstruction model of wild-type epimastigotes showed that the endoplasmic reticulum and mitochondrion are in intimate association with FAZ, in contrast to the null mutant cells where the endoplasmic reticulum was not visualized.