Dissociation of cytokinesis initiation from mitotic control in a eukaryote

A limitation lifted: A conditional knockdown system reveals essential roles for Polo-like kinase and Aurora kinase 1 in Trypanosoma cruzi cell division

While advances in genome editing technologies have simplified gene disruption in many organisms, the study of essential genes requires development of conditional disruption or knockdown systems that are not available in most organisms. Such is the case for Trypanosoma cruzi, a parasite that causes Chagas disease, a severely neglected tropical disease endemic to Latin America that is often fatal. Our knowledge of the identity of essential genes and their functions in T. cruzi has been severely constrained by historical challenges in very basic genetic manipulation and the absence of RNA interference machinery. Here, we describe the development and use of self-cleaving RNA sequences to conditionally regulate essential gene expression in T. cruzi. Using these tools, we identified essential roles for Polo-like and Aurora kinases in T. cruzi cell division, mirroring their functions in Trypanosoma brucei. Importantly, we demonstrate conditional knockdown of essential genes in intracellular amastigotes, the disease-causing stage of the parasite in its human host. This conditional knockdown system enables the efficient and scalable functional characterization of essential genes in T. cruzi and provides a framework for the development of conditional gene knockdown systems for other nonmodel organisms.

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.

The microtubule quartet protein SNAP1 in Trypanosoma brucei facilitates flagellum and cell division plane positioning by promoting basal body segregation

The unicellular protozoan Trypanosoma brucei has a single flagellum that is involved in cell motility, cell morphogenesis, and cell division. Inheritance of the newly assembled flagellum during the cell cycle requires its correct positioning, which depends on the faithful duplication or segregation of multiple flagellum-associated cytoskeletal structures, including the basal body, the flagellum attachment zone, and the hook complex. Along the flagellum attachment zone sites a set of four microtubules termed the microtubule quartet (MtQ), whose molecular function remains enigmatic. We recently reported that the MtQ-localized protein NHL1 interacts with the microtubule-binding protein TbSpef1 and regulates flagellum inheritance by promoting basal body rotation and segregation. Here, we identified a TbSpef1- and NHL1-associated protein named SNAP1, which co-localizes with NHL1 and TbSpef1 at the proximal portion of the MtQ, depends on TbSpef1 for localization and is required for NHL1 localization to the MtQ. Knockdown of SNAP1 impairs the rotation and segregation of the basal body, the elongation of the flagellum attachment zone filament, and the positioning of the newly assembled flagellum, thereby causing mis-placement of the cell division plane, a halt in cleavage furrow ingression, and an inhibition of cytokinesis completion. Together, these findings uncover a coordinating role of SNAP1 with TbSpef1 and NHL1 in facilitating flagellum positioning and cell division plane placement for the completion of cytokinesis.

Polo-like kinase and Aurora B kinase phosphorylate and cooperate with the CIF1-CIF2 complex to promote cytokinesis initiation in Trypanosoma brucei

Cytokinesis in eukaryotes is regulated by a Polo-like kinase-mediated and Aurora B kinase-mediated signalling pathway that promotes the assembly of the actomyosin contractile ring, a cytokinesis machinery conserved across evolution from yeast to humans. Trypanosoma brucei, an early divergent parasitic protozoan, employs an actomyosin-independent mechanism for its unusual cytokinesis that is controlled by a regulatory pathway comprising the Polo-like kinase TbPLK, the Aurora B kinase TbAUK1 and multiple trypanosomatid-specific regulators. However, whether any of these trypanosomatid-specific regulators function as substrates of TbPLK and/or TbAUK1 and how they cooperate with TbPLK and TbAUK1 to promote cytokinesis remain unknown. Here, we demonstrate that TbPLK and TbAUK1 phosphorylate the cytokinesis regulators CIF1 and CIF2 on multiple sites within their intrinsically disordered regions. We further show that TbPLK localization depends on its interaction with CIF1 from S/G2 phases, that TbPLK maintains CIF1 and CIF2 localization from G2 phase until early mitosis, and that TbAUK1 maintains CIF1 and CIF2 localization from late mitosis. Finally, we demonstrate that the cytokinesis regulators CIF4 and FPRC are not substrates of TbPLK and TbAUK1, and that they function upstream of TbPLK and TbAUK1 in the cytokinesis regulatory pathway. Together, these results provide insights into the functional interplay and the order of actions between the two protein kinases and the trypanosomatid-specific cytokinesis regulators in T. brucei.

Genome-scale RNA interference profiling of Trypanosoma brucei cell cycle progression defects

Trypanosomatids, which include major pathogens of humans and livestock, are flagellated protozoa for which cell cycle controls and the underlying mechanisms are not completely understood. Here, we describe a genome-wide RNA-interference library screen for cell cycle defects in Trypanosoma brucei. We induced massive parallel knockdown, sorted the perturbed population using high-throughput flow cytometry, deep-sequenced RNAi-targets from each stage and digitally reconstructed cell cycle profiles at a genomic scale; also enabling data visualisation using an online tool ( https://tryp-cycle.pages.dev/ ). Analysis of several hundred genes that impact cell cycle progression reveals >100 flagellar component knockdowns linked to genome endoreduplication, evidence for metabolic control of the G₁-S transition, surface antigen regulatory mRNA-binding protein knockdowns linked to G₂M accumulation, and a putative nucleoredoxin required for both mitochondrial genome segregation and for mitosis. The outputs provide comprehensive functional genomic evidence for the known and novel machineries, pathways and regulators that coordinate trypanosome cell cycle progression.

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.

Structural Domains of CIF3 Required for Interaction with Cytokinesis Regulatory Proteins and for Cytokinesis Initiation in Trypanosoma brucei

Cytokinesis in Trypanosoma brucei occurs unidirectionally from the anterior toward the posterior through mechanisms distinct from those of its human host and is controlled by a signaling pathway comprising evolutionarily conserved and trypanosome-specific regulatory proteins. The mechanistic roles and the functional interplay of these cytokinesis regulators remain poorly understood. Here, we investigate the requirement of the structural motifs in the trypanosome-specific cytokinesis regulator CIF3 for the initiation of cytokinesis, the interaction with other cytokinesis regulators, and the recruitment of CIF3-interacting proteins to the cytokinesis initiation site. We demonstrate that the internal and C-terminal coiled-coil motifs, but not the N-terminal coiled-coil motif, of CIF3 play essential roles in cytokinesis and interact with distinct cytokinesis regulators. CIF3 interacts with TbPLK, CIF1, CIF4, and FPRC through the N-terminal and C-terminal coiled-coil motifs and with KAT80 through all three coiled-coil motifs. The C-terminal coiled-coil motif of CIF3 is required for the localization of CIF3 and all of its interacting proteins, and additionally, the internal coiled-coil motif of CIF3 is required for KAT80 localization. Conversely, all the CIF3-interacting proteins are required to maintain CIF3 at the cytokinesis initiation site at different cell cycle stages. These results demonstrate that CIF3 cooperates with multiple interacting partner proteins to promote cytokinesis in T. brucei.

IMPORTANCE Cytokinesis is the final stage of cell division and is regulated by a signaling pathway conserved from yeast to humans. Cytokinesis in Trypanosoma brucei, an early-branching protozoan parasite causing human sleeping sickness, is regulated by mechanisms that are distinct from those of its human host, employing a number of trypanosome-specific regulatory proteins to cooperate with evolutionarily conserved regulators. The functional interplay of these cytokinesis regulators is still poorly understood. In this work, we investigated the structural requirement of the trypanosome-specific cytokinesis regulator CIF3 for the initiation of cytokinesis, the interaction with other cytokinesis regulatory proteins, and the recruitment of CIF3-interacting proteins. We demonstrated that different structural motifs of CIF3 played distinct roles in cytokinesis, interacted with distinct cytokinesis regulatory proteins, and were required for the recruitment of distinct cytokinesis regulatory proteins. These findings provided novel insights into the cooperative roles of cytokinesis regulators in promoting cytokinesis in T. brucei.

Revealing spatio-temporal dynamics with long-term trypanosomatid live-cell imaging

Trypanosoma brucei, the causative agent of human African trypanosomiasis, is highly motile and must be able to move in all three dimensions for reliable cell division. These characteristics make long-term microscopic imaging of live T. brucei cells challenging, which has limited our understanding of important cellular events. To address this issue, we devised an imaging approach that confines cells in small volumes within cast agarose microwells that can be imaged continuously for up to 24 h. Individual T. brucei cells were imaged through multiple rounds of cell division with high spatial and temporal resolution. We developed a strategy that employs in-well “sentinel” cells to monitor potential imaging toxicity during loss-of-function experiments such as small-molecule inhibition and RNAi. Using our approach, we show that the asymmetric daughter cells produced during T. brucei division subsequently divide at different rates, with the old-flagellum daughter cell dividing first. The flagellar detachment phenotype that appears during inhibition of the Polo-like kinase homolog TbPLK occurs in a stepwise fashion, with the new flagellum initially linked by its tip to the old, attached flagellum. We probe the feasibility of a previously proposed “back-up” cytokinetic mechanism and show that cells that initiate this process do not appear to complete cell division. This live-cell imaging method will provide a novel avenue for studying a wide variety of cellular events in trypanosomatids that have previously been inaccessible.

Drug Target Validation of the Protein Kinase AEK1, Essential for Proliferation, Host Cell Invasion, and Intracellular Replication of the Human Pathogen Trypanosoma cruzi

Protein phosphorylation is involved in several key biological roles in the complex life cycle of Trypanosoma cruzi, the etiological agent of Chagas disease, and protein kinases are potential drug targets. Here, we report that the AGC essential kinase 1 (TcAEK1) exhibits a cytosolic localization and a higher level of expression in the replicative stages of the parasite. A CRISPR/Cas9 editing technique was used to generate ATP analog-sensitive TcAEK1 gatekeeper residue mutants that were selectively and acutely inhibited by bumped kinase inhibitors (BKIs). Analysis of a single allele deletion cell line (TcAEK1-SKO), and gatekeeper mutants upon treatment with inhibitor, showed that epimastigote forms exhibited a severe defect in cytokinesis. Moreover, we also demonstrated that TcAEK1 is essential for epimastigote proliferation, trypomastigote host cell invasion, and amastigote replication. We suggest that TcAEK1 is a pleiotropic player involved in cytokinesis regulation in T. cruzi and thus validate TcAEK1 as a drug target for further exploration. The gene editing strategy we applied to construct the ATP analog-sensitive enzyme could be appropriate for the study of other proteins of the T. cruzi kinome. IMPORTANCE Chagas disease affects 6 to 7 million people in the Americas, and its treatment has been limited to drugs with relatively high toxicity and low efficacy in the chronic phase of the infection. New validated targets are needed to combat this disease. In this work, we report the chemical and genetic validation of the protein kinase AEK1, which is essential for cytokinesis and infectivity, using a novel gene editing strategy.

Alternate histories of cytokinesis: lessons from the trypanosomatids

Popular culture has recently produced several “alternate histories” that describe worlds where key historical events had different outcomes. Beyond entertainment, asking “could this have happened a different way?” and “what would the consequences be?” are valuable approaches for exploring molecular mechanisms in many areas of research, including cell biology. Analogous to alternate histories, studying how the evolutionary trajectories of related organisms have been selected to provide a range of outcomes can tell us about the plasticity and potential contained within the genome of the ancestral cell. Among eukaryotes, a group of model organisms has been employed with great success to identify a core, conserved framework of proteins that segregate the duplicated cellular organelles into two daughter cells during cell division, a process known as cytokinesis. However, these organisms provide relatively sparse sampling across the broad evolutionary distances that exist, which has limited our understanding of the true potential of the ancestral eukaryotic toolkit. Recent work on the trypanosomatids, a group of eukaryotic parasites, exemplifies alternate historical routes for cytokinesis that illustrate the range of eukaryotic diversity, especially among unicellular organisms.

The kinetoplastid-specific phosphatase KPP1 attenuates PLK activity to facilitate flagellum inheritance in Trypanosoma brucei

Trypanosoma brucei, an important human parasite, has a flagellum that controls cell motility, morphogenesis, proliferation, and cell-cell communication. Inheritance of the newly assembled flagellum during the cell cycle requires the Polo-like kinase homolog TbPLK and the kinetoplastid-specific protein phosphatase KPP1, although whether TbPLK acts on KPP1 or vice versa has been unclear. Here, we showed that dephosphorylation of TbPLK on Thr¹²⁵ by KPP1 maintained low TbPLK activity in the flagellum-associated hook complex structure, thereby ensuring proper flagellum positioning and attachment. This dephosphorylation event required the recognition of phosphorylated Thr¹⁹⁸ in the activation loop of TbPLK by the N-terminal Plus3 domain of KPP1 and the dephosphorylation of phosphorylated Thr¹²⁵ in TbPLK by the C-terminal catalytic domain of KPP1. Dephosphorylation of TbPLK by KPP1 prevented hyperphosphorylation of the hook complex protein TbCentrin2, thereby allowing timely dephosphorylation of phosphorylated TbCentrin2 for hook complex duplication and flagellum positioning and attachment. Thus, KPP1 attenuates TbPLK activity by dephosphorylating TbPLK to facilitate flagellum inheritance.

Polo-like kinase in trypanosomes: an odd member out of the Polo family

Polo-like kinases (Plks) are evolutionarily conserved serine/threonine protein kinases playing crucial roles during multiple stages of mitosis and cytokinesis in yeast and animals. Plks are characterized by a unique Polo-box domain, which plays regulatory roles in controlling Plk activation, interacting with substrates and targeting Plk to specific subcellular locations. Plk activity and protein abundance are subject to temporal and spatial control through transcription, phosphorylation and proteolysis. In the early branching protists, Plk orthologues are present in some taxa, such as kinetoplastids and Giardia, but are lost in apicomplexans, such as Plasmodium. Works from characterizing a Plk orthologue in Trypanosoma brucei, a kinetoplastid protozoan, discover its essential roles in regulating the inheritance of flagellum-associated cytoskeleton and the initiation of cytokinesis, but not any stage of mitosis. These studies reveal evolutionarily conserved and species-specific features in the control of Plk activation, substrate recognition and protein abundance, and suggest the divergence of Plk function and regulation for specialized needs in this flagellated unicellular eukaryote.

Trypanosomes have divergent kinesin-2 proteins that function differentially in flagellum biosynthesis and cell viability

Trypanosoma brucei, the causative agent of African sleeping sickness, has a flagellum that is crucial for motility, pathogenicity, and viability. In most eukaryotes, the intraflagellar transport (IFT) machinery drives flagellum biogenesis, and anterograde IFT requires kinesin-2 motor proteins. In this study, we investigated the function of the two T. brucei kinesin-2 proteins, TbKin2a and TbKin2b, in bloodstream form trypanosomes. We found that, compared to kinesin-2 proteins across other phyla, TbKin2a and TbKin2b show greater variation in neck, stalk and tail domain sequences. Both kinesins contributed additively to flagellar lengthening. Silencing TbKin2a inhibited cell proliferation, cytokinesis and motility, whereas silencing TbKin2b did not. TbKin2a was localized on the flagellum and colocalized with IFT components near the basal body, consistent with it performing a role in IFT. TbKin2a was also detected on the flagellar attachment zone, a specialized structure that connects the flagellum to the cell body. Our results indicate that kinesin-2 proteins in trypanosomes play conserved roles in flagellar biosynthesis and exhibit a specialized localization, emphasizing the evolutionary flexibility of motor protein function in an organism with a large complement of kinesins.

Who Needs a Contractile Actomyosin Ring? The Plethora of Alternative Ways to Divide a Protozoan Parasite

Cytokinesis, or the division of the cytoplasm, following the end of mitosis or meiosis, is accomplished in animal cells, fungi, and amoebae, by the constriction of an actomyosin contractile ring, comprising filamentous actin, myosin II, and associated proteins. However, despite this being the best-studied mode of cytokinesis, it is restricted to the Opisthokonta and Amoebozoa, since members of other evolutionary supergroups lack myosin II and must, therefore, employ different mechanisms. In particular, parasitic protozoa, many of which cause significant morbidity and mortality in humans and animals as well as considerable economic losses, employ a wide diversity of mechanisms to divide, few, if any, of which involve myosin II. In some cases, cell division is not only myosin II-independent, but actin-independent too. Mechanisms employed range from primitive mechanical cell rupture (cytofission), to motility- and/or microtubule remodeling-dependent mechanisms, to budding involving the constriction of divergent contractile rings, to hijacking host cell division machinery, with some species able to utilize multiple mechanisms. Here, I review current knowledge of cytokinesis mechanisms and their molecular control in mammalian-infective parasitic protozoa from the Excavata, Alveolata, and Amoebozoa supergroups, highlighting their often-underappreciated diversity and complexity. Billions of people and animals across the world are at risk from these pathogens, for which vaccines and/or optimal treatments are often not available. Exploiting the divergent cell division machinery in these parasites may provide new avenues for the treatment of protozoal disease.

The trypanosome-specific proteins FPRC and CIF4 regulate cytokinesis initiation by recruiting CIF1 to the cytokinesis initiation site

The evolutionarily early divergent human parasite Trypanosoma brucei proliferates through binary cell fission in both its tsetse fly vector and mammalian host. The parasite divides unidirectionally along the longitudinal cell axis from the anterior cell tip toward the posterior cell tip through a mechanism distinct from that in the cells of its human host. Initiation of cytokinesis in T. brucei is regulated by two evolutionarily conserved protein kinases, the Polo-like kinase TbPLK and the Aurora B kinase TbAUK1, and a cohort of trypanosome-specific proteins, including the three cytokinesis initiation factors CIF1, CIF2, and CIF3. Here, using RNAi, in situ epitope tagging of proteins, GST pulldown, and coimmunoprecipitation assays, and immunofluorescence and scanning electron microscopy analyses, we report the identification and functional characterization of two trypanosome-specific proteins, flagellum attachment zone tip-localizing protein required for cytokinesis (FPRC) and CIF4. We found that the two proteins colocalize to the distal tips of the new and the old flagellum attachment zones and are required for cytokinesis initiation. Knockdown of FPRC or CIF4 disrupted the localization of CIF1, suggesting that they function upstream of CIF1. Moreover, depletion of CIF4 abolished FPRC localization, indicating that CIF4 acts upstream of FPRC. Together, these results identify two new cytokinesis regulators in T. brucei and integrate them into the CIF1-mediated cytokinesis regulatory pathway. These findings highlight the existence of a cytokinesis pathway in T. brucei that is different from that of its mammalian host and therefore suggest that cytokinesis in T. brucei could potentially be exploited as a new drug target.

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.

BOH1 cooperates with Polo-like kinase to regulate flagellum inheritance and cytokinesis initiation in Trypanosoma brucei

Trypanosoma brucei possesses a motile flagellum that determines cell morphology and the cell division plane. Inheritance of the newly assembled flagellum during the cell cycle is controlled by the Polo-like kinase homolog TbPLK, which also regulates cytokinesis initiation. How TbPLK is targeted to its subcellular locations remains poorly understood. Here we report the trypanosome-specific protein BOH1 that cooperates with TbPLK to regulate flagellum inheritance and cytokinesis initiation in the procyclic form of T. brucei BOH1 localizes to an unusual sub-domain in the flagellum-associated hook complex, bridging the hook complex, the centrin arm and the flagellum attachment zone. Depletion of BOH1 disrupts hook-complex morphology, inhibits centrin-arm elongation and abolishes flagellum attachment zone assembly, leading to flagellum mis-positioning and detachment. Further, BOH1 deficiency impairs the localization of TbPLK and the cytokinesis regulator CIF1 to the cytokinesis initiation site, providing a molecular mechanism for its role in cytokinesis initiation. These findings reveal the roles of BOH1 in maintaining hook-complex morphology and regulating flagellum inheritance, and establish BOH1 as an upstream regulator of the TbPLK-mediated cytokinesis regulatory pathway.

Functional Analyses of Cytokinesis Regulators in Bloodstream Stage Trypanosoma brucei Parasites Identify Functions and Regulations Specific to the Life Cycle Stage

The early divergent protozoan parasite Trypanosoma brucei is the causative agent of sleeping sickness in humans and nagana in cattle in sub-Saharan Africa. This parasite has a complex life cycle by alternating between the insect vector and the mammalian hosts and proliferates by binary cell fission. The control of cell division in trypanosomes appears to be distinct from that in its human host and differs substantially between two life cycle stages, the procyclic (insect) form and the bloodstream form. Cytokinesis, the final step of binary cell fission, is regulated by a novel signaling cascade consisting of two evolutionarily conserved protein kinases and a cohort of trypanosome-specific regulators in the procyclic form, but whether this signaling pathway operates in a similar manner in the bloodstream form is unclear. In this report, we performed a functional analysis of multiple cytokinesis regulators and discovered their distinct functions and regulations in the bloodstream form.

The early divergent protozoan parasite Trypanosoma brucei alternates between the insect vector and the mammalian hosts during its life cycle and proliferates through binary cell fission. The cell cycle control system in T. brucei differs substantially from that in its mammalian hosts and possesses distinct mitosis-cytokinesis checkpoint controls between two life cycle stages, the procyclic form and the bloodstream form. T. brucei undergoes an unusual mode of cytokinesis, which is controlled by a novel signaling cascade consisting of evolutionarily conserved protein kinases and trypanosome-specific regulatory proteins in the procyclic form. However, given the distinct mitosis-cytokinesis checkpoints between the two forms, it is unclear whether the cytokinesis regulatory pathway discovered in the procyclic form also operates in a similar manner in the bloodstream form. Here, we showed that the three regulators of cytokinesis initiation, cytokinesis initiation factor 1 (CIF1), CIF2, and CIF3, are interdependent for subcellular localization but not for protein stability as in the procyclic form. Further, we demonstrated that KLIF, a regulator of cytokinesis completion in the procyclic form, plays limited roles in cytokinesis in the bloodstream form. Finally, we showed that the cleavage furrow-localizing protein FRW1 is required for cytokinesis initiation in the bloodstream form but is nonessential for cytokinesis in the procyclic form. Together, these results identify conserved and life cycle-specific functions of cytokinesis regulators, highlighting the distinction in the regulation of cytokinesis between different life cycle stages of T. brucei.

IMPORTANCE The early divergent protozoan parasite Trypanosoma brucei is the causative agent of sleeping sickness in humans and nagana in cattle in sub-Saharan Africa. This parasite has a complex life cycle by alternating between the insect vector and the mammalian hosts and proliferates by binary cell fission. The control of cell division in trypanosomes appears to be distinct from that in its human host and differs substantially between two life cycle stages, the procyclic (insect) form and the bloodstream form. Cytokinesis, the final step of binary cell fission, is regulated by a novel signaling cascade consisting of two evolutionarily conserved protein kinases and a cohort of trypanosome-specific regulators in the procyclic form, but whether this signaling pathway operates in a similar manner in the bloodstream form is unclear. In this report, we performed a functional analysis of multiple cytokinesis regulators and discovered their distinct functions and regulations in the bloodstream form.

Overexpression of Trypanosoma cruzi High Mobility Group B protein (TcHMGB) alters the nuclear structure, impairs cytokinesis and reduces the parasite infectivity

Kinetoplastid parasites, included Trypanosoma cruzi, the causal agent of Chagas disease, present a unique genome organization and gene expression. Although they control gene expression mainly post-transcriptionally, chromatin accessibility plays a fundamental role in transcription initiation control. We have previously shown that High Mobility Group B protein from Trypanosoma cruzi (TcHMGB) can bind DNA in vitro. Here, we show that TcHMGB also acts as an architectural protein in vivo, since the overexpression of this protein induces changes in the nuclear structure, mainly the reduction of the nucleolus and a decrease in the heterochromatin:euchromatin ratio. Epimastigote replication rate was markedly reduced presumably due to a delayed cell cycle progression with accumulation of parasites in G2/M phase and impaired cytokinesis. Some functions involved in pathogenesis were also altered in TcHMGB-overexpressing parasites, like the decreased efficiency of trypomastigotes to infect cells in vitro, the reduction of intracellular amastigotes replication and the number of released trypomastigotes. Taken together, our results suggest that the TcHMGB protein is a pleiotropic player that controls cell phenotype and it is involved in key cellular processes.

The CIF1 protein is a master orchestrator of trypanosome cytokinesis that recruits several cytokinesis regulators to the cytokinesis initiation site

To proliferate, the parasitic protozoan Trypanosoma brucei undergoes binary fission in a unidirectional manner along the cell's longitudinal axis from the cell anterior toward the cell posterior. This unusual mode of cell division is controlled by a regulatory pathway composed of two evolutionarily conserved protein kinases, Polo-like kinase and Aurora B kinase, and three trypanosome-specific proteins, CIF1, CIF2, and CIF3, which act in concert at the cytokinesis initiation site located at the distal tip of the newly assembled flagellum attachment zone (FAZ). However, additional regulators that function in this cytokinesis signaling cascade remain to be identified and characterized. Using proximity biotinylation, co-immunofluorescence microscopy, and co-immunoprecipitation, we identified 52 CIF1-associated proteins and validated six CIF1-interacting proteins, including the putative protein phosphatase KPP1, the katanin p80 subunit KAT80, the cleavage furrow-localized proteins KLIF and FRW1, and the FAZ tip-localized proteins FAZ20 and FPRC. Further analyses of the functional interplay between CIF1 and its associated proteins revealed a requirement of CIF1 for localization of a set of CIF1-associated proteins, an interdependence between KPP1 and CIF1, and an essential role of katanin in the completion of cleavage furrow ingression. Together, these results suggest that CIF1 acts as a master regulator of cytokinesis in T. brucei by recruiting a cohort of cytokinesis regulatory proteins to the cytokinesis initiation site.

Identification of TOEFAZ1-interacting proteins reveals key regulators of Trypanosoma brucei cytokinesis

The protist parasite Trypanosoma brucei is an obligate extracellular pathogen that retains its highly polarized morphology during cell division and has evolved a novel cytokinetic process independent of non-muscle myosin II. The polo-like kinase homolog TbPLK is essential for transmission of cell polarity during division and for cytokinesis. We previously identified a putative TbPLK substrate named Tip of the Extending FAZ 1 (TOEFAZ1) as an essential kinetoplastid-specific component of the T. brucei cytokinetic machinery. We performed a proximity-dependent biotinylation identification (BioID) screen using TOEFAZ1 as a means to identify additional proteins that are involved in cytokinesis. Using quantitative proteomic methods, we identified nearly 500 TOEFAZ1-proximal proteins and characterized 59 in further detail. Among the candidates, we identified an essential putative phosphatase that regulates the expression level and localization of both TOEFAZ1 and TbPLK, a previously uncharacterized protein that is necessary for the assembly of a new cell posterior, and a microtubule plus-end directed orphan kinesin that is required for completing cleavage furrow ingression. The identification of these proteins provides new insight into T. brucei cytokinesis and establishes TOEFAZ1 as a key component of this essential and uniquely configured process in kinetoplastids.

The trypanosome-specific protein CIF3 cooperates with the CIF1 protein to promote cytokinesis in Trypanosoma brucei

Cytokinesis, the terminal step in cell division, in the protist human pathogen Trypanosoma brucei occurs along the longitudinal axis from the anterior tip of the new flagellum attachment zone (FAZ) toward the posterior cell tip. This process is regulated by a signaling cascade composed of the Polo-like kinase homolog TbPLK, the Aurora B kinase homolog TbAUK1, and the trypanosome-specific CIF1-CIF2 protein complex. However, the regulatory mechanism and the signaling pathway for this unusual mode of cytokinesis remain poorly understood. Here, we report another trypanosome-specific protein assembly, the CIF1-CIF3 complex, and its essential role in cytokinesis initiation. Through biochemical and genetic approaches, we demonstrate that CIF3 interacts with CIF1 in a TbPLK-dependent manner and maintains CIF1 localization at the new FAZ tip. Conversely, CIF1 maintains CIF3 stability at the new FAZ tip. We further show that TbPLK is required for CIF3 localization and that CIF3 is necessary for targeting TbAUK1 to the new FAZ tip during anaphase. These results suggest that two trypanosome-specific CIF1-containing protein complexes cooperate with the evolutionarily conserved Polo-like kinase and Aurora B kinase to promote cytokinesis in T. brucei.

Interaction between the flagellar pocket collar and the hook complex via a novel microtubule-binding protein in Trypanosoma brucei

Trypanosoma brucei belongs to a group of unicellular, flagellated parasites that are responsible for human African trypanosomiasis. An essential aspect of parasite pathogenicity is cytoskeleton remodelling, which occurs during the life cycle of the parasite and is accompanied by major changes in morphology and organelle positioning. The flagellum originates from the basal bodies and exits the cell body through the flagellar pocket (FP) but remains attached to the cell body via the flagellum attachment zone (FAZ). The FP is an invagination of the pellicular membrane and is the sole site for endo- and exocytosis. The FAZ is a large complex of cytoskeletal proteins, plus an intracellular set of four specialised microtubules (MtQ) that elongate from the basal bodies to the anterior end of the cell. At the distal end of the FP, an essential, intracellular, cytoskeletal structure called the flagellar pocket collar (FPC) circumvents the flagellum. Overlapping the FPC is the hook complex (HC) (a sub-structure of the previously named bilobe) that is also essential and is thought to be involved in protein FP entry. BILBO1 is the only functionally characterised FPC protein and is necessary for FPC and FP biogenesis. Here, we used a combination of in vitro and in vivo approaches to identify and characterize a new BILBO1 partner protein-FPC4. We demonstrate that FPC4 localises to the FPC, the HC, and possibly to a proximal portion of the MtQ. We found that the C-terminal domain of FPC4 interacts with the BILBO1 N-terminal domain, and we identified the key amino acids required for this interaction. Interestingly, the FPC4 N-terminal domain was found to bind microtubules. Over-expression studies highlight the role of FPC4 in its association with the FPC, HC and FPC segregation. Our data suggest a tripartite association between the FPC, the HC and the MtQ.

Functional analyses of the CIF1-CIF2 complex in trypanosomes identify the structural motifs required for cytokinesis

Cytokinesis in trypanosomes occurs uni-directionally along the longitudinal axis from the cell anterior towards the cell posterior and requires a trypanosome-specific CIF1-CIF2 protein complex. However, little is known about the contribution of the structural motifs in CIF1 and CIF2 to complex assembly and cytokinesis. Here, we demonstrate that the two zinc-finger motifs but not the coiled-coil motif in CIF1 are required for interaction with the EF-hand motifs in CIF2. We further show that localization of CIF1 depends on the coiled-coil motif and the first zinc-finger motif and that localization of CIF2 depends on the EF-hand motifs. Deletion of the coiled-coil motif and mutation of either zinc-finger motif in CIF1 disrupts cytokinesis. Furthermore, mutation of either zinc-finger motif in CIF1 mislocalizes CIF2 to the cytosol and destabilizes CIF2, whereas deletion of the coiled-coil motif in CIF1 spreads CIF2 over to the new flagellum attachment zone and stabilizes CIF2. Together, these results uncover the requirement of the coiled-coil and zinc-finger motifs for CIF1 function in cytokinesis and for CIF2 localization and stability, providing structural insights into the functional interplay between the two cytokinesis regulators.

CRL4WDR1 Controls Polo-like Kinase Protein Abundance to Promote Bilobe Duplication, Basal Body Segregation and Flagellum Attachment in Trypanosoma brucei

The Polo-like kinase homolog in Trypanosoma brucei, TbPLK, plays essential roles in basal body segregation, flagellum attachment and cytokinesis. The level of TbPLK protein is tightly controlled, but the underlying mechanism remains elusive. Here, we report a Cullin-RING ubiquitin ligase composed of Cullin4, the DNA damage-binding protein 1 homolog TbDDB1 and a WD40-repeat protein WDR1 that controls TbPLK abundance in the basal body and the bilobe. WDR1, through its C-terminal domain, interacts with the PEST motif in TbPLK and, through its N-terminal WD40 motif, binds to TbDDB1. Depletion of WDR1 inhibits bilobe duplication and basal body segregation, disrupts the assembly of the new flagellum attachment zone filament and detaches the new flagellum. Consistent with its role in TbPLK degradation, depletion of WDR1 causes excessive accumulation of TbPLK in the basal body and the bilobe, leading to continuous phosphorylation of TbCentrin2 in the bilobe at late cell cycle stages. Together, these results identify a novel WD40-repeat protein as a TbPLK receptor in the Cullin4-DDB1 ubiquitin ligase complex for degrading TbPLK in the basal body and the bilobe after the G1/S cell cycle transition, thereby promoting bilobe duplication, basal body separation and flagellum-cell body adhesion.

A unified approach towards Trypanosoma brucei functional genomics using Gibson assembly

Trypanosoma brucei is the causative agent of human African trypanosomiasis and nagana in cattle. Recent advances in high throughput phenotypic and interaction screens have identified a wealth of novel candidate proteins for diverse functions such as drug resistance, life cycle progression, and cytoskeletal biogenesis. Characterization of these proteins will allow a more mechanistic understanding of the biology of this important pathogen and could identify novel drug targets. However, methods for rapidly validating and prioritizing these potential targets are still being developed. While gene tagging via homologous recombination and RNA interference are available in T. brucei, a general strategy for creating the most effective constructs for these approaches is lacking. Here, we adapt Gibson assembly, a one-step isothermal process that rapidly assembles multiple DNA segments in a single reaction, to create endogenous tagging, overexpression, and long hairpin RNAi constructs that are compatible with well-established T. brucei vectors. The generality of the Gibson approach has several advantages over current methodologies and substantially increases the speed and ease with which these constructs can be assembled.

An EF-hand-containing Protein in Trypanosoma brucei Regulates Cytokinesis Initiation by Maintaining the Stability of the Cytokinesis Initiation Factor CIF1

Trypanosoma brucei undergoes cytokinesis uni-directionally from the anterior tip of the new flagellum attachment zone (FAZ) toward the posterior end of the cell. We recently delineated a novel signaling pathway composed of polo-like kinase, cytokinesis initiation factor 1 (CIF1), and aurora B kinase that acts in concert at the new FAZ tip to regulate cytokinesis initiation. To identify new cytokinesis regulators, we carried out proximity-dependent biotin identification and identified many CIF1 binding partners and near neighbors. Here we report a novel CIF1-binding protein, named CIF2, and its mechanistic role in cytokinesis initiation. CIF2 interacts with CIF1 in vivo and co-localizes with CIF1 at the new FAZ tip during early cell cycle stages. RNAi of CIF2 inhibited the normal, anterior-to-posterior cytokinesis but activated an alternative, posterior-to-anterior cytokinesis. CIF2 depletion destabilized CIF1 and disrupted the localization of polo-like kinase and aurora B kinase to the new FAZ tip, thus revealing the mechanistic role of CIF2 in cytokinesis initiation. Surprisingly, overexpression of CIF2 also inhibited the normal, anterior-to-posterior cytokinesis and triggered the alternative, posterior-to-anterior cytokinesis, suggesting a tight control of CIF2 protein abundance. These results identified a new regulator in the cytokinesis regulatory pathway and reiterated that a backup cytokinesis pathway is activated by inhibiting the normal cytokinesis pathway.

Divergent polo box domains underpin the unique kinetoplastid kinetochore

Kinetochores are macromolecular machines that drive eukaryotic chromosome segregation by interacting with centromeric DNA and spindle microtubules. While most eukaryotes possess conventional kinetochore proteins, evolutionarily distant kinetoplastid species have unconventional kinetochore proteins, composed of at least 19 proteins (KKT1-19). Polo-like kinase (PLK) is not a structural kinetochore component in either system. Here, we report the identification of an additional kinetochore protein, KKT20, in Trypanosoma brucei. KKT20 has sequence similarity with KKT2 and KKT3 in the Cys-rich region, and all three proteins have weak but significant similarity to the polo box domain (PBD) of PLK. These divergent PBDs of KKT2 and KKT20 are sufficient for kinetochore localization in vivo. We propose that the ancestral PLK acquired a Cys-rich region and then underwent gene duplication events to give rise to three structural kinetochore proteins in kinetoplastids.

Two distinct cytokinesis pathways drive trypanosome cell division initiation from opposite cell ends

Cytokinesis in Trypanosoma brucei, an early branching protozoan, occurs along its longitudinal axis uni-directionally from the anterior tip of the new flagellum attachment zone filament toward the cell's posterior end. However, the underlying mechanisms remain elusive. Here we report that cytokinesis in T. brucei is regulated by a concerted action of Polo-like kinase, Aurora B kinase, and a trypanosome-specific protein CIF1. Phosphorylation of CIF1 by Polo-like kinase targets it to the anterior tip of the new flagellum attachment zone filament, where it subsequently recruits Aurora B kinase to initiate cytokinesis. Consistent with its role, CIF1 depletion inhibits cytokinesis initiation from the anterior end of the cell, but, surprisingly, triggers cytokinesis initiation from the posterior end of the cell, suggesting the activation of an alternative cytokinesis from the opposite cell end. Our results reveal the mechanistic roles of CIF1 and Polo-like kinase in cytokinesis initiation and elucidate the mechanism underlying the recruitment of Aurora B kinase to the cytokinesis initiation site at late anaphase. These findings also delineate a signaling cascade controlling cytokinesis initiation from the anterior end of the cell and uncover a backup cytokinesis that is initiated from the posterior end of the cell when the typical anterior-to-posterior cytokinesis is compromised.

SAS-4 Protein in Trypanosoma brucei Controls Life Cycle Transitions by Modulating the Length of the Flagellum Attachment Zone Filament

The evolutionarily conserved centriole/basal body protein SAS-4 regulates centriole duplication in metazoa and basal body duplication in flagellated and ciliated organisms. Here, we report that the SAS-4 homolog in the flagellated protozoan Trypanosoma brucei, TbSAS-4, plays an unusual role in controlling life cycle transitions by regulating the length of the flagellum attachment zone (FAZ) filament, a specialized cytoskeletal structure required for flagellum adhesion and cell morphogenesis. TbSAS-4 is concentrated at the distal tip of the FAZ filament, and depletion of TbSAS-4 in the trypomastigote form disrupts the elongation of the new FAZ filament, generating cells with a shorter FAZ associated with a longer unattached flagellum and repositioned kinetoplast and basal body, reminiscent of epimastigote-like morphology. Further, we show that TbSAS-4 associates with six additional FAZ tip proteins, and depletion of TbSAS-4 disrupts the enrichment of these FAZ tip proteins at the new FAZ tip, suggesting a role of TbSAS-4 in maintaining the integrity of this FAZ tip protein complex. Together, these results uncover a novel function of TbSAS-4 in regulating the length of the FAZ filament to control basal body positioning and life cycle transitions in T. brucei.

A Novel Basal Body Protein That Is a Polo-like Kinase Substrate Is Required for Basal Body Segregation and Flagellum Adhesion in Trypanosoma brucei

The Polo-like kinase (PLK) in Trypanosoma brucei plays multiple roles in basal body segregation, flagellum attachment, and cytokinesis. However, the mechanistic role of TbPLK remains elusive, mainly because most of its substrates are not known. Here, we report a new substrate of TbPLK, SPBB1, and its essential roles in T. brucei. SPBB1 was identified through yeast two-hybrid screening with the kinase-dead TbPLK as the bait. It interacts with TbPLK in vitro and in vivo, and is phosphorylated by TbPLK in vitro. SPBB1 localizes to both the mature basal body and the probasal body throughout the cell cycle, and co-localizes with TbPLK at the basal body during early cell cycle stages. RNAi against SPBB1 in procyclic trypanosomes inhibited basal body segregation, disrupted the new flagellum attachment zone filament, detached the new flagellum, and caused defective cytokinesis. Moreover, RNAi of SPBB1 confined TbPLK at the basal body and the bilobe structure, resulting in constitutive phosphorylation of TbCentrin2 at the bilobe. Altogether, these results identified a basal body protein as a TbPLK substrate and its essential role in promoting basal body segregation and flagellum attachment zone filament assembly for flagellum adhesion and cytokinesis initiation.

Sphingosine Kinase Regulates Microtubule Dynamics and Organelle Positioning Necessary for Proper G1/S Cell Cycle Transition in Trypanosoma brucei

Sphingolipids are important constituents of cell membranes and also serve as mediators of cell signaling and cell recognition. Sphingolipid metabolites such as sphingosine-1-phosphate and ceramide regulate signaling cascades involved in cell proliferation and differentiation, autophagy, inflammation, and apoptosis. Little is known about how sphingolipids and their metabolites function in single-celled eukaryotes. In the present study, we investigated the role of sphingosine kinase (SPHK) in the biology of the protozoan parasite Trypanosoma brucei, the agent of African sleeping sickness. T. brucei SPHK (TbSPHK) is constitutively but differentially expressed during the life cycle of T. brucei. Depletion of TbSPHK in procyclic-form T. brucei causes impaired growth and attenuation in the G1/S phase of the cell cycle. TbSPHK-depleted cells also develop organelle positioning defects and an accumulation of tyrosinated α-tubulin at the elongated posterior end of the cell, known as the "nozzle" phenotype, caused by other molecular perturbations in this organism. Our studies indicate that TbSPHK is involved in G1-to-S cell cycle progression, organelle positioning, and maintenance of cell morphology. Cytotoxicity assays using TbSPHK inhibitors revealed a favorable therapeutic index between T. brucei and human cells, suggesting TbSPHK to be a novel drug target.

IMPORTANCE

Trypanosoma brucei is a single-celled parasite that is transmitted between humans and other animals by the tsetse fly. T. brucei is endemic in sub-Saharan Africa, where over 70 million people and countless livestock are at risk of developing T. brucei infection, called African sleeping sickness, resulting in economic losses of ~$35 million from the loss of cattle alone. New drugs for this infection are sorely needed and scientists are trying to identify essential enzymes in the parasite that can be targets for new therapies. One possible enzyme target is sphingosine kinase, an enzyme involved in the synthesis of lipids important for cell surface integrity and regulation of cell functions. In this study, we found that sphingosine kinase is essential for normal growth and structure of the parasite, raising the possibility that it could be a good target for new chemotherapy for sleeping sickness.

Proteomic identification of novel cytoskeletal proteins associated with TbPLK, an essential regulator of cell morphogenesis in Trypanosoma brucei

Trypanosoma brucei is the causative agent of African sleeping sickness, a devastating disease endemic to sub-Saharan Africa with few effective treatment options. The parasite is highly polarized, including a single flagellum that is nucleated at the posterior of the cell and adhered along the cell surface. These features are essential and must be transmitted to the daughter cells during division. Recently we identified the T. brucei homologue of polo-like kinase (TbPLK) as an essential morphogenic regulator. In the present work, we conduct proteomic screens to identify potential TbPLK binding partners and substrates to better understand the molecular mechanisms of kinase function. These screens identify a cohort of proteins, most of which are completely uncharacterized, which localize to key cytoskeletal organelles involved in establishing cell morphology, including the flagella connector, flagellum attachment zone, and bilobe structure. Depletion of these proteins causes substantial changes in cell division, including mispositioning of the kinetoplast, loss of flagellar connection, and prevention of cytokinesis. The proteins identified in these screens provide the foundation for establishing the molecular networks through which TbPLK directs cell morphogenesis in T. brucei.

Depletion of the SR-Related Protein TbRRM1 Leads to Cell Cycle Arrest and Apoptosis-Like Death in Trypanosoma brucei

Arginine-Serine (RS) domain-containing proteins are RNA binding proteins with multiple functions in RNA metabolism. In mammalian cells this group of proteins is also implicated in regulation and coordination of cell cycle and apoptosis. In trypanosomes, an early branching group within the eukaryotic lineage, this group of proteins is represented by 3 members, two of them are SR proteins and have been recently shown to be involved in rRNA processing as well as in pre-mRNA splicing and stability. Here we report our findings on the 3rd member, the SR-related protein TbRRM1. In the present study, we showed that TbRRM1 ablation by RNA-interference in T. brucei procyclic cells leads to cell-cycle block, abnormal cell elongation compatible with the nozzle phenotype and cell death by an apoptosis-like mechanism. Our results expand the role of the trypanosomal RS-domain containing proteins in key cellular processes such as cell cycle and apoptosis-like death, roles also carried out by the mammalian SR proteins, and thus suggesting a conserved function in this phylogenetically conserved protein family.

The Centriole Cartwheel Protein SAS-6 in Trypanosoma brucei Is Required for Probasal Body Biogenesis and Flagellum Assembly

The centriole in eukaryotes functions as the cell's microtubule-organizing center (MTOC) to nucleate spindle assembly, and its biogenesis requires an evolutionarily conserved protein, SAS-6, which assembles the centriole cartwheel. Trypanosoma brucei, an early branching protozoan, possesses the basal body as its MTOC to nucleate flagellum biogenesis. However, little is known about the components of the basal body and their roles in basal body biogenesis and flagellum assembly. Here, we report that the T. brucei SAS-6 homolog, TbSAS-6, is localized to the mature basal body and the probasal body throughout the cell cycle. RNA interference (RNAi) of TbSAS-6 inhibited probasal body biogenesis, compromised flagellum assembly, and caused cytokinesis arrest. Surprisingly, overexpression of TbSAS-6 in T. brucei also impaired probasal body duplication and flagellum assembly, contrary to SAS-6 overexpression in humans, which produces supernumerary centrioles. Furthermore, we showed that depletion of T. brucei Polo-like kinase, TbPLK, or inhibition of TbPLK activity did not abolish TbSAS-6 localization to the basal body, in contrast to the essential role of Polo-like kinase in recruiting SAS-6 to centrioles in animals. Altogether, these results identified the essential role of TbSAS-6 in probasal body biogenesis and flagellum assembly and suggest the presence of a TbPLK-independent pathway governing basal body duplication in T. brucei.

Comparative analysis of chromosome segregation in human, yeasts and trypanosome

Chromosome segregation is a tightly regulated process through which duplicated genetic materials are equally partitioned into daughter cells. During the past decades, tremendous efforts have been made to understand the molecular mechanism of chromosome segregation using animals and yeasts as model systems. Recently, new insights into chromosome segregation have gradually emerged using trypanosome, an early branching parasitic protozoan, as a model organism. To uncover the unique aspects of chromosome segregation in trypanosome, which potentially could serve as new drug targets for anti-trypanosome chemotherapy, it is necessary to perform a comparative analysis of the chromosome segregation machinery between trypanosome and its human host. Here, we briefly review the current knowledge about chromosome segregation in human and Trypanosoma brucei, with a focus on the regulation of cohesin and securin degradation triggered by the activation of the anaphase promoting complex/cyclosome (APC/C). We also include yeasts in our comparative analysis since some of the original discoveries were made using budding and fission yeasts as the model organisms and, therefore, these could provide hints about the evolution of the machinery. We highlight both common and unique features in these model systems and also provide perspectives for future research in trypanosome.

New insights into the molecular mechanisms of mitosis and cytokinesis in trypanosomes

Trypanosoma brucei, a unicellular eukaryote and the causative agent of human sleeping sickness, possesses multiple single-copy organelles that all need to be duplicated and segregated during cell division. Trypanosomes undergo a closed mitosis in which the mitotic spindle is anchored on the nuclear envelope and connects the kinetochores made of novel protein components. Cytokinesis in trypanosomes is initiated from the anterior tip of the new flagellum attachment zone, and proceeds along the longitudinal axis without the involvement of the actomyosin contractile ring, the well-recognized cytokinesis machinery conserved from yeast to humans. Trypanosome appears to employ both evolutionarily conserved and trypanosome-specific proteins to regulate its cell cycle, and has evolved certain cell cycle regulatory pathways that are either distinct between its life cycle stages or different from its human host. Understanding the mechanisms of mitosis and cytokinesis in trypanosomes not only would shed novel light on the evolution of cell cycle control, but also could provide new drug targets for chemotherapy.

The Aurora B kinase in Trypanosoma brucei undergoes post-translational modifications and is targeted to various subcellular locations through binding to TbCPC1

The chromosomal passenger complex (CPC) in animals, consisting of Aurora B kinase and three evolutionarily conserved proteins, plays crucial roles in mitosis and cytokinesis. However, Trypanosoma brucei expresses an unusual CPC consisting of an Aurora-like kinase, TbAUK1, and two kinetoplastid-specific proteins, TbCPC1 and TbCPC2. Despite their essential functions, little is known about the regulation of TbAUK1 and the roles of TbCPC1 and TbCPC2. Here, we investigate the effect of post-translational modification on the activity and spatiotemporal control of TbAUK1, and demonstrate that phosphorylation of two conserved threonine residues in the activation loop of the kinase domain contributes to TbAUK1 activation and function. TbAUK1 is SUMOylated in vivo, and mutation of the SUMO-conjugation site compromises TbAUK1 function. Degradation of TbAUK1 requires two destruction boxes and is mediated by the anaphase-promoting complex/cyclosome (APC/C), whereas degradation of TbCPC1 and TbCPC2 is not dependent on the predicted destruction boxes and is APC/C-independent. Moreover, we determine the domains in CPC subunits that mediate the pairwise interactions, and show that disruption of the interaction impairs the localization of TbAUK1 and TbCPC2 but not TbCPC1. Our results demonstrate the requirement of post-translational modifications for TbAUK1 function and a crucial role of TbCPC1 in mediating TbAUK1 localization.

More than meets the eye: understanding Trypanosoma brucei morphology in the tsetse

T. brucei, the causative parasite for African trypanosomiasis, faces an interesting dilemma in its life cycle. It has to successfully complete its infection cycle in the tsetse vector to be able to infect other vertebrate hosts. T. brucei has to undergo multiple morphological changes as it invades the alimentary canal of the tsetse to finally achieve infectivity in the salivary glands. In this review, we attempt to elucidate how these morphological changes are possible for a parasite that has evolved a highly robust cell structure to survive the chemically and physically diverse environments it finds itself in. To achieve this, we juxtaposed the experimental evidence that has been collected from T. brucei forms that are cultured in vitro with the observations that have been carried out on tsetse-infective forms in vivo. Although the accumulated knowledge on T. brucei biology is by no means trivial, several outstanding questions remain for how the parasite mechanistically changes its morphology as it traverses the tsetse and how those changes are triggered. However, we conclude that with recent breakthroughs allowing for the replication of the tsetse-infection process of T. brucei in vitro, these outstanding questions can finally be addressed.

Distinct roles of a mitogen-activated protein kinase in cytokinesis between different life cycle forms of Trypanosoma brucei

Mitogen-activated protein kinase (MAPK) modules are evolutionarily conserved signaling cascades that function in response to the environment and play crucial roles in intracellular signal transduction in eukaryotes. The involvement of a MAP kinase in regulating cytokinesis in yeast, animals, and plants has been reported, but the requirement for a MAP kinase for cytokinesis in the early-branching protozoa is not documented. Here, we show that a MAP kinase homolog (TbMAPK6) from Trypanosoma brucei plays distinct roles in cytokinesis in two life cycle forms of T. brucei. TbMAPK6 is distributed throughout the cytosol in the procyclic form but is localized in both the cytosol and the nucleus in the bloodstream form. RNA interference (RNAi) of TbMAPK6 results in moderate growth inhibition in the procyclic form but severe growth defects and rapid cell death in the bloodstream form. Moreover, TbMAPK6 appears to be implicated in furrow ingression and cytokinesis completion in the procyclic form but is essential for cytokinesis initiation in the bloodstream form. Despite the distinct defects in cytokinesis in the two forms, RNAi of TbMAPK6 also caused defective basal body duplication/segregation in a small cell population in both life cycle forms. Altogether, our results demonstrate the involvement of the TbMAPK6-mediated pathway in regulating cytokinesis in trypanosomes and suggest distinct roles of TbMAPK6 in cytokinesis between different life cycle stages of T. brucei.

Evolutionary cell biology of chromosome segregation: insights from trypanosomes

Faithful transmission of genetic material is essential for the survival of all organisms. Eukaryotic chromosome segregation is driven by the kinetochore that assembles onto centromeric DNA to capture spindle microtubules and govern the movement of chromosomes. Its molecular mechanism has been actively studied in conventional model eukaryotes, such as yeasts, worms, flies and human. However, these organisms are closely related in the evolutionary time scale and it therefore remains unclear whether all eukaryotes use a similar mechanism. The evolutionary origins of the segregation apparatus also remain enigmatic. To gain insights into these questions, it is critical to perform comparative studies. Here, we review our current understanding of the mitotic mechanism in Trypanosoma brucei, an experimentally tractable kinetoplastid parasite that branched early in eukaryotic history. No canonical kinetochore component has been identified, and the design principle of kinetochores might be fundamentally different in kinetoplastids. Furthermore, these organisms do not appear to possess a functional spindle checkpoint that monitors kinetochore-microtubule attachments. With these unique features and the long evolutionary distance from other eukaryotes, understanding the mechanism of chromosome segregation in T. brucei should reveal fundamental requirements for the eukaryotic segregation machinery, and may also provide hints about the origin and evolution of the segregation apparatus.

A minimal anaphase promoting complex/cyclosome (APC/C) in Trypanosoma brucei

The anaphase-promoting complex/cyclosome (APC/C) is a multi-subunit E3 ubiquitin ligase that initiates chromosome segregation and mitotic exit by targeting critical cell-cycle regulators for proteolytic destruction. Previously, seven APC/C subunit homologues were identified in the genome of Trypanosoma brucei. In the present study, we tested five of them in yeast complementation studies and found none of them capable of complementing the yeast mutants lacking the corresponding subunits, suggesting significant discrepancies between the two APC/C’s. Subunit homologues of mitotic checkpoint complex (MCC) have not yet been identified in T. brucei, raising the possibility that a MCC-APC/C complex equivalent may not exist in T. brucei. We performed tandem affinity purification of the protein complex containing a APC1 fusion protein expressed in the cells enriched in different phases of the cell cycle of procyclic form T. brucei, and compared their protein profiles using LC-MS/MS analyses. The seven putative APC/C subunits were identified in the protein complex throughout the cell cycle together with three additional proteins designated the associated proteins (AP) AP1, AP2 and AP3. Abundance of the 10 proteins remained relatively unchanged throughout the cell cycle, suggesting that they are the core subunits of APC/C. AP1 turned out to be a homologue of APC4. An RNAi knockdown of APC4 and AP3 showed no detectable cellular phenotype, whereas an AP2 knockdown enriched the cells in G2/M phase. The AP2-depleted cells showed stabilized mitotic cyclin B. An accumulation of poly-ubiquitinated cyclin B was indicated in the cells treated with the proteasome inhibitor MG132, demonstrating the involvement of proteasome in degrading poly-ubiquitinated cyclin B. In all, a 10-subunit APC/C machinery with a conserved function is identified in T. brucei without linking to a MCC-like complex, thus indicating a unique T. brucei APC/C.

An analogue-sensitive approach identifies basal body rotation and flagellum attachment zone elongation as key functions of PLK in Trypanosoma brucei

The Polo-like kinase homologue in Trypanosoma brucei (TbPLK) regulates the assembly of a series of organelles necessary for positioning the parasite's flagellum. An analogue-sensitive strategy is used to acutely and specifically inhibit the kinase with a small molecule, making it possible to identify novel TbPLK functions.

Polo-like kinases are important regulators of cell division, playing diverse roles in mitosis and cytoskeletal inheritance. In the parasite Trypanosoma brucei, the single PLK homologue TbPLK is necessary for the assembly of a series of essential organelles that position and adhere the flagellum to the cell surface. Previous work relied on RNA interference or inhibitors of undefined specificity to inhibit TbPLK, both of which have significant experimental limitations. Here we use an analogue-sensitive approach to selectively and acutely inhibit TbPLK. T. brucei cells expressing only analogue-sensitive TbPLK (TbPLK^as) grow normally, but upon treatment with inhibitor develop defects in flagellar attachment and cytokinesis. TbPLK cannot migrate effectively when inhibited and remains trapped in the posterior of the cell throughout the cell cycle. Using synchronized cells, we show that active TbPLK is a direct requirement for the assembly and extension of the flagellum attachment zone, which adheres the flagellum to the cell surface, and for the rotation of the duplicated basal bodies, which positions the new flagellum so that it can extend without impinging on the old flagellum. This approach should be applicable to the many kinases found in the T. brucei genome that lack an ascribed function.

Trypanosoma brucei FKBP12 differentially controls motility and cytokinesis in procyclic and bloodstream forms

FKBP12 proteins are able to inhibit TOR kinases or calcineurin phosphatases upon binding of rapamycin or FK506 drugs, respectively. The Trypanosoma brucei FKBP12 homologue (TbFKBP12) was found to be a cytoskeleton-associated protein with specific localization in the flagellar pocket area of the bloodstream form. In the insect procyclic form, RNA interference-mediated knockdown of TbFKBP12 affected motility. In bloodstream cells, depletion of TbFKBP12 affected cytokinesis and cytoskeleton architecture. These last effects were associated with the presence of internal translucent cavities limited by an inside-out configuration of the normal cell surface, with a luminal variant surface glycoprotein coat lined up by microtubules. These cavities, which recreated the streamlined shape of the normal trypanosome cytoskeleton, might represent unsuccessful attempts for cell abscission. We propose that TbFKBP12 differentially affects stage-specific processes through association with the cytoskeleton.

Role of centrins 2 and 3 in organelle segregation and cytokinesis in Trypanosoma brucei

Centrins are calcium binding proteins involved in cell division in eukaryotes. Previously, we have shown that depletion of centrin1 in Trypanosoma brucei (T. brucei) displayed arrested organelle segregation resulting in loss of cytokinesis. In this study we analyzed the role of T. brucei centrin2 (TbCen2) and T. brucei 3 (TbCen3) in the early events of T. brucei procyclic cell cycle. Both the immunofluorescence assay and electron microscopy showed that TbCen2 and 3-deficient cells were enlarged in size with duplicated basal bodies, multinuclei and new flagella that are detached along the length of the cell body. In both TbCen2 and TbCen3 depleted cells segregation of the organelles i.e. basal bodies, kinetoplast and nucleus was disrupted. Further analysis of the cells with defective organelle segregation identified three different sub configurations of organelle mis-segregations (Type 1-3). In addition, in majority of the TbCen2 depleted cells and in nearly half of the TbCen3 depleted cells, the kinetoplasts were enlarged and undivided. The abnormal segregations ultimately led to aborted cytokinesis and hence affected growth in these cells. Therefore, both centrin2 and 3 are involved in organelle segregation similar to centrin1 as was previously observed. In addition, we identified their role in kinetoplast division which may be also linked to overall mis-segregation.

Regulation of the cell division cycle in Trypanosoma brucei

The cell division cycle is tightly regulated by the activation and inactivation of a series of proteins that control the replication and segregation of organelles to the daughter cells. During the past decade, we have witnessed significant advances in our understanding of the cell cycle in Trypanosoma brucei and how the cycle is regulated by various regulatory proteins. However, many other regulators, especially those unique to trypanosomes, remain to be identified, and we are just beginning to delineate the signaling pathways that drive the transitions through different cell cycle stages, such as the G(1)/S transition, G(2)/M transition, and mitosis-cytokinesis transition. Trypanosomes appear to employ both evolutionarily conserved and trypanosome-specific molecules to regulate the various stages of its cell cycle, including DNA replication initiation, spindle assembly, chromosome segregation, and cytokinesis initiation and completion. Strikingly, trypanosomes lack some crucial regulators that are well conserved across evolution, such as Cdc6 and Cdt1, which are involved in DNA replication licensing, the spindle motor kinesin-5, which is required for spindle assembly, the central spindlin complex, which has been implicated in cytokinesis initiation, and the actomyosin contractile ring, which is located at the cleavage furrow. Conversely, trypanosomes possess certain regulators, such as cyclins, cyclin-dependent kinases, and mitotic centromere-associated kinesins, that are greatly expanded and likely play diverse cellular functions. Overall, trypanosomes apparently have integrated unique regulators into the evolutionarily conserved pathways to compensate for the absence of those conserved molecules and, additionally, have evolved certain cell cycle regulatory pathways that are either different from its human host or distinct between its own life cycle forms.