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

Novel axonemal protein ZMYND12 interacts with TTC29 and DNAH1, and is required for male fertility and flagellum function

Male infertility is common and complex, presenting a wide range of heterogeneous phenotypes. Although about 50% of cases are estimated to have a genetic component, the underlying cause often remains undetermined. Here, from whole-exome sequencing on samples from 168 infertile men with asthenoteratozoospermia due to severe sperm flagellum, we identified homozygous ZMYND12 variants in four unrelated patients. In sperm cells from these individuals, immunofluorescence revealed altered localization of DNAH1, DNALI1, WDR66, and TTC29. Axonemal localization of ZMYND12 ortholog TbTAX-1 was confirmed using the Trypanosoma brucei model. RNAi knock-down of TbTAX-1 dramatically affected flagellar motility, with a phenotype similar to the sperm from men bearing homozygous ZMYND12 variants. Co-immunoprecipitation and ultrastructure expansion microscopy in T. brucei revealed TbTAX-1 to form a complex with TTC29. Comparative proteomics with samples from Trypanosoma and Ttc29 KO mice identified a third member of this complex: DNAH1. The data presented revealed that ZMYND12 is part of the same axonemal complex as TTC29 and DNAH1, which is critical for flagellum function and assembly in humans, and Trypanosoma. ZMYND12 is thus a new asthenoteratozoospermia-associated gene, bi-allelic variants of which cause severe flagellum malformations and primary male infertility.

Characterisation of TbSmee1 suggests endocytosis allows surface-bound cargo to enter the trypanosome flagellar pocket

All endocytosis and exocytosis in the African trypanosome Trypanosoma brucei occurs at a single subdomain of the plasma membrane. This subdomain, the flagellar pocket, is a small vase-shaped invagination containing the root of the single flagellum of the cell. Several cytoskeleton-associated multiprotein complexes are coiled around the neck of the flagellar pocket on its cytoplasmic face. One of these, the hook complex, was proposed to affect macromolecule entry into the flagellar pocket lumen. In previous work, knockdown of T. brucei (Tb)MORN1, a hook complex component, resulted in larger cargo being unable to enter the flagellar pocket. In this study, the hook complex component TbSmee1 was characterised in bloodstream form T. brucei and found to be essential for cell viability. TbSmee1 knockdown resulted in flagellar pocket enlargement and impaired access to the flagellar pocket membrane by surface-bound cargo, similar to depletion of TbMORN1. Unexpectedly, inhibition of endocytosis by knockdown of clathrin phenocopied TbSmee1 knockdown, suggesting that endocytic activity itself is a prerequisite for the entry of surface-bound cargo into the flagellar pocket.

FAZ assembly in bloodstream form Trypanosoma brucei requires kinesin KIN-E

Trypanosoma brucei, the causative agent of African sleeping sickness, uses its flagellum for movement, cell division, and signaling. The flagellum is anchored to the cell body membrane via the flagellum attachment zone (FAZ), a complex of proteins, filaments, and microtubules that spans two membranes with elements on both flagellum and cell body sides. How FAZ components are carried into place to form this complex is poorly understood. Here, we show that the trypanosome-specific kinesin KIN-E is required for building the FAZ in bloodstream-form parasites. KIN-E is localized along the flagellum with a concentration at its distal tip. Depletion of KIN-E by RNAi rapidly inhibits flagellum attachment and leads to cell death. A detailed analysis reveals that KIN-E depletion phenotypes include failure in cytokinesis completion, kinetoplast DNA missegregation, and transport vesicle accumulation. Together with previously published results in procyclic form parasites, these data suggest KIN-E plays a critical role in FAZ assembly in T. brucei.

Pseudokinase NRP1 facilitates endocytosis of transferrin in the African trypanosome

Trypanosoma brucei causes human African trypanosomiasis (HAT) and nagana in cattle. During infection of a vertebrate, endocytosis of host transferrin (Tf) is important for viability of the parasite. The majority of proteins involved in trypanosome endocytosis of Tf are unknown. Here we identify pseudokinase NRP1 (Tb427tmp.160.4770) as a regulator of Tf endocytosis. Genetic knockdown of NRP1 inhibited endocytosis of Tf without blocking uptake of bovine serum albumin. Binding of Tf to the flagellar pocket was not affected by knockdown of NRP1. However the quantity of Tf per endosome dropped significantly, consistent with NRP1 promoting robust capture and/or retention of Tf in vesicles. NRP1 is involved in motility of Tf-laden vesicles since distances between endosomes and the kinetoplast were reduced after knockdown of the gene. In search of possible mediators of NRP1 modulation of Tf endocytosis, the gene was knocked down and the phosphoproteome analyzed. Phosphorylation of protein kinases forkhead, NEK6, and MAPK10 was altered, in addition to EpsinR, synaptobrevin and other vesicle-associated proteins predicted to be involved in endocytosis. These candidate proteins may link NRP1 functionally either to protein kinases or to vesicle-associated proteins.

Kinetoplastid-specific X2-family kinesins interact with a kinesin-like pleckstrin homology domain protein that localizes to the trypanosomal microtubule quartet

Kinesins are motor proteins found in all eukaryotic lineages that move along microtubules to mediate cellular processes such as mitosis and intracellular transport. In trypanosomatids, the kinesin superfamily has undergone a prominent expansion, resulting in one of the most diverse kinesin repertoires that includes the two kinetoplastid-restricted families X1 and X2. Here, we characterize in Trypanosoma brucei TbKifX2A, an orphaned X2 kinesin. TbKifX2A tightly interacts with TbPH1, a kinesin-like protein with a likely inactive motor domain, a rarely reported occurrence. Both TbKifX2A and TbPH1 localize to the microtubule quartet (MtQ), a characteristic but poorly understood cytoskeletal structure that wraps around the flagellar pocket as it extends to the cell body anterior. The proximal proteome of TbPH1 revealed two other interacting proteins, the flagellar pocket protein FP45 and intriguingly another X2 kinesin, TbKifX2C. Simultaneous ablation of TbKifX2A/TbPH1 results in the depletion of FP45 and TbKifX2C and also an expansion of the flagellar pocket, among other morphological defects. TbKifX2A is the first motor protein to be localized to the MtQ. The observation that TbKifX2C also associates with the MtQ suggests that the X2 kinesin family may have co-evolved with the MtQ, both kinetoplastid-specific traits.

TFK1, a basal body transition fibre protein that is essential for cytokinesis in Trypanosoma brucei

In Trypanosoma brucei, transition fibres (TF) form a nine-bladed pattern-like structure connecting the base of the flagellum to the flagellar pocket membrane. Despite the characterization of two TF proteins, CEP164C and TbRP2, little is known about the organization of these fibres. Here, we report the identification and characterization of the first kinetoplastid-specific TF protein named TFK1 (Tb927.6.1180). Bioinformatics and functional domain analysis identified three TFK1 distinct domains: an N-terminal domain of an unpredicted function, a coiled-coil domain involved in TFK1-TFK1 interaction and a C-terminal intrinsically disordered region potentially involved in protein interaction. Cellular immuno-localization showed that TFK1 is a newly identified basal body maturation marker. Further, using ultrastructure expansion and immuno-electron microscopies we localized CEP164C and TbRP2 at the TF and TFK1 on the distal appendage matrix of the TF. Importantly, RNAi knockdown of TFK1 in bloodstream form cells induced misplacement of basal bodies, a defect in the furrow or fold generation and eventually cell death. We hypothesize that TFK1 is a basal body positioning specific actor and a key regulator of cytokinesis in the bloodstream form Trypanosoma brucei.

TbKINX1B: a novel BILBO1 partner and an essential protein in bloodstream form Trypanosoma brucei

The flagellar pocket (FP) of the pathogen Trypanosoma brucei is an important single copy structure that is formed by the invagination of the pellicular membrane. It is the unique site of endo- and exocytosis and is required for parasite pathogenicity. The FP consists of distinct structural sub-domains with the least explored being the flagellar pocket collar (FPC). TbBILBO1 is the first-described FPC protein of Trypanosoma brucei. It is essential for parasite survival, FP and FPC biogenesis. In this work, we characterize TbKINX1B, a novel TbBILBO1 partner. We demonstrate that TbKINX1B is located on the basal bodies, the microtubule quartet (a set of four microtubules) and the FPC in T. brucei. Down-regulation of TbKINX1B by RNA interference in bloodstream forms is lethal, inducing an overall disturbance in the endomembrane network. In procyclic forms, the RNAi knockdown of TbKINX1B leads to a minor phenotype with a small number of cells displaying epimastigote-like morphologies, with a misplaced kinetoplast. Our results characterize TbKINX1B as the first putative kinesin to be localized both at the basal bodies and the FPC with a potential role in transporting cargo along with the microtubule quartet.

Bhalin, an Essential Cytoskeleton-Associated Protein of Trypanosoma brucei Linking TbBILBO1 of the Flagellar Pocket Collar with the Hook Complex

Background: In most trypanosomes, endo and exocytosis only occur at a unique organelle called the flagellar pocket (FP) and the flagellum exits the cell via the FP. Investigations of essential cytoskeleton-associated structures located at this site have revealed a number of essential proteins. The protein TbBILBO1 is located at the neck of the FP in a structure called the flagellar pocket collar (FPC) and is essential for biogenesis of the FPC and parasite survival. TbMORN1 is a protein that is present on a closely linked structure called the hook complex (HC) and is located anterior to and overlapping the collar. TbMORN1 is essential in the bloodstream form of T. brucei. We now describe the location and function of BHALIN, an essential, new FPC-HC protein. Methodology/Principal Findings: Here, we show that a newly characterised protein, BHALIN (BILBO1 Hook Associated LINker protein), is localised to both the FPC and HC and has a TbBILBO1 binding domain, which was confirmed in vitro. Knockdown of BHALIN by RNAi in the bloodstream form parasites led to cell death, indicating an essential role in cell viability. Conclusions/Significance: Our results demonstrate the essential role of a newly characterised hook complex protein, BHALIN, that influences flagellar pocket organisation and function in bloodstream form T. brucei parasites.

Intrabody-Induced Cell Death by Targeting the T. brucei Cytoskeletal Protein TbBILBO1

Trypanosoma brucei belongs to a genus of protists that cause life-threatening and economically important diseases of human and animal populations in Sub-Saharan Africa. T. brucei cells are covered in surface glycoproteins, some of which are used to escape the host immune system. Exo-/endocytotic trafficking of these and other molecules occurs via a single copy organelle called the flagellar pocket (FP). The FP is maintained and enclosed around the flagellum by the flagellar pocket collar (FPC). To date, the most important cytoskeletal component of the FPC is an essential calcium-binding, polymer-forming protein called TbBILBO1. In searching for novel tools to study this protein, we raised nanobodies (Nb) against purified, full-length TbBILBO1. Nanobodies were selected according to their binding properties to TbBILBO1, tested as immunofluorescence tools, and expressed as intrabodies (INb). One of them, Nb48, proved to be the most robust nanobody and intrabody. We further demonstrate that inducible, cytoplasmic expression of INb48 was lethal to these parasites, producing abnormal phenotypes resembling those of TbBILBO1 RNA interference (RNAi) knockdown. Our results validate the feasibility of generating functional single-domain antibody-derived intrabodies to target trypanosome cytoskeleton proteins.

IMPORTANCETrypanosoma brucei belongs to a group of important zoonotic parasites. We investigated how these organisms develop their cytoskeleton (the internal skeleton that controls cell shape) and focused on an essential protein (BILBO1) first described in T. brucei. To develop our analysis, we used purified BILBO1 protein to immunize an alpaca to make nanobodies (Nb). Nanobodies are derived from the antigen-binding portion of a novel antibody type found only in the camel and shark families of animals. Anti-BILBO1 nanobodies were obtained, and their encoding genes were inducibly expressed within the cytoplasm of T. brucei as intrabodies (INb). Importantly, INb48 expression rapidly killed parasites producing phenotypes normally observed after RNA knockdown, providing clear proof of principle. The importance of this study is derived from this novel approach, which can be used to study neglected and emerging pathogens as well as new model organisms, especially those that do not have the RNAi system.

Glycerol suppresses glucose consumption in trypanosomes through metabolic contest

Microorganisms must make the right choice for nutrient consumption to adapt to their changing environment. As a consequence, bacteria and yeasts have developed regulatory mechanisms involving nutrient sensing and signaling, known as "catabolite repression," allowing redirection of cell metabolism to maximize the consumption of an energy-efficient carbon source. Here, we report a new mechanism named "metabolic contest" for regulating the use of carbon sources without nutrient sensing and signaling. Trypanosoma brucei is a unicellular eukaryote transmitted by tsetse flies and causing human African trypanosomiasis, or sleeping sickness. We showed that, in contrast to most microorganisms, the insect stages of this parasite developed a preference for glycerol over glucose, with glucose consumption beginning after the depletion of glycerol present in the medium. This "metabolic contest" depends on the combination of 3 conditions: (i) the sequestration of both metabolic pathways in the same subcellular compartment, here in the peroxisomal-related organelles named glycosomes; (ii) the competition for the same substrate, here ATP, with the first enzymatic step of the glycerol and glucose metabolic pathways both being ATP-dependent (glycerol kinase and hexokinase, respectively); and (iii) an unbalanced activity between the competing enzymes, here the glycerol kinase activity being approximately 80-fold higher than the hexokinase activity. As predicted by our model, an approximately 50-fold down-regulation of the GK expression abolished the preference for glycerol over glucose, with glucose and glycerol being metabolized concomitantly. In theory, a metabolic contest could be found in any organism provided that the 3 conditions listed above are met.

Structural and functional studies of the first tripartite protein complex at the Trypanosoma brucei flagellar pocket collar

The flagellar pocket (FP) is the only endo- and exocytic organelle in most trypanosomes and, as such, is essential throughout the life cycle of the parasite. The neck of the FP is maintained enclosed around the flagellum via the flagellar pocket collar (FPC). The FPC is a macromolecular cytoskeletal structure and is essential for the formation of the FP and cytokinesis. FPC biogenesis and structure are poorly understood, mainly due to the lack of information on FPC composition. To date, only two FPC proteins, BILBO1 and FPC4, have been characterized. BILBO1 forms a molecular skeleton upon which other FPC proteins can, theoretically, dock onto. We previously identified FPC4 as the first BILBO1 interacting partner and demonstrated that its C-terminal domain interacts with the BILBO1 N-terminal domain (NTD). Here, we report by yeast two-hybrid, bioinformatics, functional and structural studies the characterization of a new FPC component and BILBO1 partner protein, BILBO2 (Tb927.6.3240). Further, we demonstrate that BILBO1 and BILBO2 share a homologous NTD and that both domains interact with FPC4. We have determined a 1.9 Å resolution crystal structure of the BILBO2 NTD in complex with the FPC4 BILBO1-binding domain. Together with mutational analyses, our studies reveal key residues for the function of the BILBO2 NTD and its interaction with FPC4 and evidenced a tripartite interaction between BILBO1, BILBO2, and FPC4. Our work sheds light on the first atomic structure of an FPC protein complex and represents a significant step in deciphering the FPC function in Trypanosoma brucei and other pathogenic kinetoplastids.

Trypanosomes belong to a group of zoonotic, protist, parasites that are found in Africa, Asia, South America, and Europe and are responsible for severe human and animal diseases. They all have a common structure called the flagellar pocket (FP). In most trypanosomes, all macromolecular exchanges between the trypanosome and the environment occur via the FP. The FP is thus essential for cell viability and evading the host immune response. We have been studying the flagellar pocket collar (FPC), an enigmatic macromolecular structure at the neck of the FP, and demonstrated its essentiality for the biogenesis of the FP. We demonstrated that BILBO1 is an essential protein of the FPC that interacts with other proteins including a microtubule-binding protein FPC4.

Here we identify another bona fide FPC protein, BILBO2, so named because of close similarity with BILBO1 in protein localization and functional domains. We demonstrate that BILBO1 and BILBO2 share a common N-terminal domain involved in the interaction with FPC4, and illustrate a tripartite interaction between BILBO1, BILBO2, and FPC4. Our study also provides the first atomic view of two FPC components. These data represent an additional step in deciphering the FPC structure and function in T. brucei.

Basic Biology of Trypanosoma brucei with Reference to the Development of Chemotherapies

Trypanosoma brucei are protozoan parasites that cause the lethal human disease African sleeping sickness and the economically devastating disease of cattle, Nagana. African sleeping sickness, also known as Human African Trypanosomiasis (HAT), threatens 65 million people and animal trypanosomiasis makes large areas of farmland unusable. There is no vaccine and licensed therapies against the most severe, late-stage disease are toxic, impractical and ineffective. Trypanosomes are transmitted by tsetse flies, and HAT is therefore predominantly confined to the tsetse fly belt in sub-Saharan Africa. They are exclusively extracellular and they differentiate between at least seven developmental forms that are highly adapted to host and vector niches. In the mammalian (human) host they inhabit the blood, cerebrospinal fluid (late-stage disease), skin, and adipose fat. In the tsetse fly vector they travel from the tsetse midgut to the salivary glands via the ectoperitrophic space and proventriculus. Trypanosomes are evolutionarily divergent compared with most branches of eukaryotic life. Perhaps most famous for their extraordinary mechanisms of monoallelic gene expression and antigenic variation, they have also been investigated because much of their biology is either highly unconventional or extreme. Moreover, in addition to their importance as pathogens, many researchers have been attracted to the field because trypanosomes have some of the most advanced molecular genetic tools and database resources of any model system. The following will cover just some aspects of trypanosome biology and how its divergent biochemistry has been leveraged to develop drugs to treat African sleeping sickness. This is by no means intended to be a comprehensive survey of trypanosome features. Rather, I hope to present trypanosomes as one of the most fascinating and tractable systems to do discovery biology.

The kinesin of the flagellum attachment zone in Leishmania is required for cell morphogenesis, cell division and virulence in the mammalian host

Leishmania parasites possess a unique and complex cytoskeletal structure termed flagellum attachment zone (FAZ) connecting the base of the flagellum to one side of the flagellar pocket (FP), an invagination of the cell body membrane and the sole site for endocytosis and exocytosis. This structure is involved in FP architecture and cell morphogenesis, but its precise role and molecular composition remain enigmatic. Here, we characterized Leishmania FAZ7, the only known FAZ protein containing a kinesin motor domain, and part of a clade of trypanosomatid-specific kinesins with unknown functions. The two paralogs of FAZ7, FAZ7A and FAZ7B, display different localizations and functions. FAZ7A localizes at the basal body, while FAZ7B localizes at the distal part of the FP, where the FAZ structure is present in Leishmania. While null mutants of FAZ7A displayed normal growth rates, the deletion of FAZ7B impaired cell growth in both promastigotes and amastigotes of Leishmania. The kinesin activity is crucial for its function. Deletion of FAZ7B resulted in altered cell division, cell morphogenesis (including flagellum length), and FP structure and function. Furthermore, knocking out FAZ7B induced a mis-localization of two of the FAZ proteins, and disrupted the molecular organization of the FP collar, affecting the localization of its components. Loss of the kinesin FAZ7B has important consequences in the insect vector and mammalian host by reducing proliferation in the sand fly and pathogenicity in mice. Our findings reveal the pivotal role of the only FAZ kinesin as part of the factors important for a successful life cycle of Leishmania.

Leishmania are flagellated trypanosomatid parasites causing worldwide human and animal diseases. As ’divergent eukaryotes’, their biology presents unique features and structures, of which the specific functions constitute potential drug targets. Among others, they possess a unique cytoskeletal structure termed the flagellum attachment zone (FAZ) attaching the base of their flagellum to one side of the flagellar pocket (FP), which is the sole site for endocytosis and exocytosis. The FP together with other unique flagellum-associated structures are crucial for parasite survival, but the functioning of this whole remains largely enigmatic. Leishmania also possess an expanded repertoire of kinesins (>55), including two trypanosomatid-specific families. Here, we show that the deletion of the sole kinesin among FAZ proteins disrupts cell morphogenesis, FP organisation and cell division. Furthermore, the ability to proliferate in the insect vector and mammalian host is reduced in parasites lacking the kinesin FAZ7B. This study helps elucidate the factors contributing to the successful lifecycle and pathogenicity of the parasite. It also highlights the functional diversification of motor proteins during evolution.

The Trypanosome UDP-Glucose Pyrophosphorylase Is Imported by Piggybacking into Glycosomes, Where Unconventional Sugar Nucleotide Synthesis Takes Place

Glycosomes are peroxisome-related organelles of trypanosomatid parasites containing metabolic pathways, such as glycolysis and biosynthesis of sugar nucleotides, usually present in the cytosol of other eukaryotes. UDP-glucose pyrophosphorylase (UGP), the enzyme responsible for the synthesis of the sugar nucleotide UDP-glucose, is localized in the cytosol and glycosomes of the bloodstream and procyclic trypanosomes, despite the absence of any known peroxisome-targeting signal (PTS1 and PTS2). The questions that we address here are (i) is the unusual glycosomal biosynthetic pathway of sugar nucleotides functional and (ii) how is the PTS-free UGP imported into glycosomes? We showed that UGP is imported into glycosomes by piggybacking on the glycosomal PTS1-containing phosphoenolpyruvate carboxykinase (PEPCK) and identified the domains involved in the UGP/PEPCK interaction. Proximity ligation assays revealed that this interaction occurs in 3 to 10% of glycosomes, suggesting that these correspond to organelles competent for protein import. We also showed that UGP is essential for the growth of trypanosomes and that both the glycosomal and cytosolic metabolic pathways involving UGP are functional, since the lethality of the knockdown UGP mutant cell line (^RNAiUGP, where RNAi indicates RNA interference) was rescued by expressing a recoded UGP (rUGP) in the organelle (^RNAiUGP/^EXPrUGP-GPDH, where GPDH is glycerol-3-phosphate dehydrogenase). Our conclusion was supported by targeted metabolomic analyses (ion chromatography–high-resolution mass spectrometry [IC-HRMS]) showing that UDP-glucose is no longer detectable in the ^RNAiUGP mutant, while it is still produced in cells expressing UGP exclusively in the cytosol (PEPCK null mutant) or glycosomes (^RNAiUGP/^EXPrUGP-GPDH). Trypanosomatids are the only known organisms to have selected functional peroxisomal (glycosomal) sugar nucleotide biosynthetic pathways in addition to the canonical cytosolic ones.

Trypanosomatid Flagellar Pocket from Structure to Function

The trypanosomatids Trypanosoma brucei, Trypanosoma cruzi, and Leishmania spp. are flagellate eukaryotic parasites that cause serious diseases in humans and animals. These parasites have cell shapes defined by a subpellicular microtubule array and all share a number of important cellular features. One of these is the flagellar pocket, an invagination of the cell membrane around the proximal end of the flagellum, which is an important organelle for endo/exocytosis. The flagellar pocket plays a crucial role in parasite pathogenicity and persistence in the host and has a great influence on cell morphogenesis and cell division. Here, we compare the morphology and function of the flagellar pockets between different trypanosomatids, with their life cycles and ecological niches likely influencing these differences.

A rapid approach for in locus overexpression of Trypanosoma brucei proteins

Altering amounts of a protein in a cell has become a crucial tool for understanding its function. In many organisms, including the protozoan parasite Trypanosoma brucei, protein overexpression has been achieved by inserting a protein-coding sequence into an overexpression vector. Here, we have adapted the PCR only based system for tagging trypanosome proteins at their endogenous loci such that it in addition enables a tetracycline-inducible T7 RNA polymerase-mediated protein overexpression. Hence, this approach bypasses the need for molecular cloning, making it rapid and cost effective. We validated the approach for ten flagellum-associated proteins with molecular weights ranging from 40 to over 500 kDa. For a majority of the recombinant proteins a significant (3-50 fold) increase in the cellular amount was achieved upon induction of overexpression. Two of the largest proteins studied, the dynein heavy chains, were significantly overexpressed, while two were not. Our data suggest that this may reflect the extent of the T7 RNA polymerase processivity on the trypanosome genomic DNA. We further show that the overexpression is informative as to cellular functions of the studied proteins, and that these cultures can serve as an excellent source for purification of the overexpressed proteins. We believe that this rapid in locus overexpression system will become a valuable tool to interrogate cellular functions and biochemical activities of trypanosome proteins.

Basal Body Protein TbSAF1 Is Required for Microtubule Quartet Anchorage to the Basal Bodies in Trypanosoma brucei

Trypanosoma brucei contains a large array of single-copied organelles and structures. Through extensive interorganelle connections, these structures replicate and divide following a strict temporal and spatial order. A microtubule quartet (MtQ) originates from the basal bodies and extends toward the anterior end of the cell, stringing several cytoskeletal structures together along its path. In this study, we examined the interaction network of TbSpef1, the only protein specifically located to the MtQ. We identified an interaction between TbSpef1 and a basal body protein TbSAF1, which is required for MtQ anchorage to the basal bodies. This study thus provides the first molecular description of MtQ association with the basal bodies, since the discovery of this association ∼30 years ago. The results also reveal a general mechanism of the evolutionarily conserved Spef1/CLAMP, which achieves specific cellular functions via their conserved microtubule functions and their diverse molecular interaction networks.

Sperm flagellar protein 1 (Spef1, also known as CLAMP) is a microtubule-associated protein involved in various microtubule-related functions from ciliary motility to polarized cell movement and planar cell polarity. In Trypanosoma brucei, the causative agent of trypanosomiasis, a single Spef1 ortholog (TbSpef1) is associated with a microtubule quartet (MtQ), which is in close association with several single-copied organelles and is required for their coordinated biogenesis during the cell cycle. Here, we investigated the interaction network of TbSpef1 using BioID, a proximity-dependent protein-protein interaction screening method. Characterization of selected candidates provided a molecular description of TbSpef1-MtQ interactions with nearby cytoskeletal structures. Of particular interest, we identified a new basal body protein TbSAF1, which is required for TbSpef1-MtQ anchorage to the basal bodies. The results demonstrate that MtQ-basal body anchorage is critical for the spatial organization of cytoskeletal organelles, as well as the morphology of the membrane-bound flagellar pocket where endocytosis takes place in this parasite.

Biallelic variants in MAATS1 encoding CFAP91, a calmodulin-associated and spoke-associated complex protein, cause severe astheno-teratozoospermia and male infertility

BACKGROUND

Multiple morphological abnormalities of the flagella (MMAF) consistently lead to male infertility due to a reduced or absent sperm motility defined as asthenozoospermia. Despite numerous genes recently described to be recurrently associated with MMAF, more than half of the cases analysed remain unresolved, suggesting that many yet uncharacterised gene defects account for this phenotype METHODS: Exome sequencing was performed on 167 infertile men with an MMAF phenotype. Immunostaining and transmission electron microscopy (TEM) in sperm cells from affected individuals were performed to characterise the ultrastructural sperm defects. Gene inactivation using RNA interference (RNAi) was subsequently performed in Trypanosoma.

RESULTS

We identified six unrelated affected patients carrying a homozygous deleterious variants in MAATS1, a gene encoding CFAP91, a calmodulin-associated and spoke-associated complex (CSC) protein. TEM and immunostaining experiments in sperm cells showed severe central pair complex (CPC) and radial spokes defects. Moreover, we confirmed that the WDR66 protein is a physical and functional partner of CFAP91 into the CSC. Study of Trypanosoma MAATS1's orthologue (TbCFAP91) highlighted high sequence and structural analogies with the human protein and confirmed the axonemal localisation of the protein. Knockdown of TbCFAP91 using RNAi impaired flagellar movement led to CPC defects in Trypanosoma as observed in humans.

CONCLUSIONS

We showed that CFAP91 is essential for normal sperm flagellum structure and function in human and Trypanosoma and that biallelic variants in this gene lead to severe flagellum malformations resulting in astheno-teratozoospermia and primary male infertility.

Crystal structure of the N-terminal domain of the trypanosome flagellar protein BILBO1 reveals a ubiquitin fold with a long structured loop for protein binding

Trypanosoma brucei is a protist parasite causing sleeping sickness and nagana in sub-Saharan Africa. T. brucei has a single flagellum whose base contains a bulblike invagination of the plasma membrane called the flagellar pocket (FP). Around the neck of the FP on its cytoplasmic face is a structure called the flagellar pocket collar (FPC), which is essential for FP biogenesis. BILBO1 was the first characterized component of the FPC in trypanosomes. BILBO1's N-terminal domain (NTD) plays an essential role in T. brucei FPC biogenesis and is thus vital for the parasite's survival. Here, we report a 1.6-Å resolution crystal structure of TbBILBO1-NTD, which revealed a conserved horseshoe-like hydrophobic pocket formed by an unusually long loop. Results from mutagenesis experiments suggested that another FPC protein, FPC4, interacts with TbBILBO1 by mainly contacting its three conserved aromatic residues Trp-71, Tyr-87, and Phe-89 at the center of this pocket. Our findings disclose the binding site of TbFPC4 on TbBILBO1-NTD, which may provide a basis for rational drug design targeting BILBO1 to combat T. brucei infections.

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.

Mutations in CFAP43 and CFAP44 cause male infertility and flagellum defects in Trypanosoma and human

Spermatogenesis defects concern millions of men worldwide, yet the vast majority remains undiagnosed. Here we report men with primary infertility due to multiple morphological abnormalities of the sperm flagella with severe disorganization of the sperm axoneme, a microtubule-based structure highly conserved throughout evolution. Whole-exome sequencing was performed on 78 patients allowing the identification of 22 men with bi-allelic mutations in DNAH1 (n = 6), CFAP43 (n = 10), and CFAP44 (n = 6). CRISPR/Cas9 created homozygous CFAP43/44 male mice that were infertile and presented severe flagellar defects confirming the human genetic results. Immunoelectron and stimulated-emission-depletion microscopy performed on CFAP43 and CFAP44 orthologs in Trypanosoma brucei evidenced that both proteins are located between the doublet microtubules 5 and 6 and the paraflagellar rod. Overall, we demonstrate that CFAP43 and CFAP44 have a similar structure with a unique axonemal localization and are necessary to produce functional flagella in species ranging from Trypanosoma to human.

Asthenozoospermia is a major cause of male infertility, and multiple morphological abnormalities of the flagella (MMAF) is a particularly severe form. Here, using whole-exome sequencing of 78 MMAF patients, the authors identify mutations in two WDR proteins, CFAP43 and CFAP44, and confirm that these proteins are required for flagellogenesis in mouse and Trypanosoma brucei.