Clathrin-dependent targeting of receptors to the flagellar pocket of procyclic-form Trypanosoma 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 First Non-LRV RNA Virus in Leishmania

In this work, we describe the first Leishmania-infecting leishbunyavirus-the first virus other than Leishmania RNA virus (LRV) found in trypanosomatid parasites. Its host is Leishmania martiniquensis, a human pathogen causing infections with a wide range of manifestations from asymptomatic to severe visceral disease. This virus (LmarLBV1) possesses many characteristic features of leishbunyaviruses, such as tripartite organization of its RNA genome, with ORFs encoding RNA-dependent RNA polymerase, surface glycoprotein, and nucleoprotein on L, M, and S segments, respectively. Our phylogenetic analyses suggest that LmarLBV1 originated from leishbunyaviruses of monoxenous trypanosomatids and, probably, is a result of genomic re-assortment. The LmarLBV1 facilitates parasites' infectivity in vitro in primary murine macrophages model. The discovery of a virus in L. martiniquensis poses the question of whether it influences pathogenicity of this parasite in vivo, similarly to the LRV in other Leishmania species.

The endocytic activity of the flagellar pocket in Trypanosoma brucei is regulated by an adjacent phosphatidylinositol phosphate kinase

Phosphoinositides are spatially restricted membrane signaling molecules. Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]--a phosphoinositide that is highly enriched in, and present throughout, the plasma membrane--has been implicated in endocytosis. Trypanosoma brucei has one of the highest known rates of endocytosis, a process it uses to evade the immune system. To determine whether phosphoinositides play a role in endocytosis in this organism, we have identified and characterized one of the enzymes that is responsible for generating PI(4,5)P2. Surprisingly, this phosphoinositide was found to be highly concentrated in the flagellar pocket, the only site of endocytosis and exocytosis in this organism. The enzyme (designated TbPIPKA, annotated as Tb927.10.1620) was present at the neck of the pocket, towards the anterior-end of the parasite. Depletion of TbPIPKA led to depletion of PI(4,5)P2 and enlargement of the pocket, the result of impaired endocytosis. Taken together, these data suggest that TbPIPKA and its product PI(4,5)P2 are important for endocytosis and, consequently, for homeostasis of the flagellar pocket.

Rab28 function in trypanosomes: interactions with retromer and ESCRT pathways

Early endosomal cargo is typically targeted to either a degradative or recycling pathway. Despite established functions for the retromer and ESCRT complexes at late endosomes/multivesicular bodies, the mechanisms integrating and coordinating these functions remain largely unknown. Rab family GTPases are key membrane trafficking organizers and could contribute. Here, in the unicellular organism Trypanosoma brucei, we demonstrate that Rab28 locates to the endosomal pathway and partially colocalizes with Vps23, an ESCRT I component. Rab28 is required for turnover of endocytosed proteins and for lysosomal delivery of protein cargo. Using RNA interference we find that in Rab28-depleted cells, protein levels of ESCRT I (Vps23/28) and retromer (Vps26) are also decreased, suggesting that Rab28 is an important regulator of these factors. We suggest that Rab28 coordinates the activity of retromer-dependent trafficking and ESCRT-mediated degradative pathways.

The AP-1 clathrin adaptor facilitates cilium formation and functions with RAB-8 in C. elegans ciliary membrane transport

Clathrin adaptor (AP) complexes facilitate membrane trafficking between subcellular compartments. One such compartment is the cilium, whose dysfunction underlies disorders classified as ciliopathies. Although AP-1mu subunit (UNC-101) is linked to cilium formation and targeting of transmembrane proteins (ODR-10) to nematode sensory cilia at distal dendrite tips, these functions remain poorly understood. Here, using Caenorhabditis elegans sensory neurons and mammalian cell culture models, we find conservation of AP-1 function in facilitating cilium morphology, positioning and orientation, and microtubule stability and acetylation. These defects appear to be independent of IFT, because AP-1-depleted cells possess normal IFT protein localisation and motility. By contrast, disruption of chc-1 (clathrin) or rab-8 phenocopies unc-101 worms, preventing ODR-10 vesicle formation and causing misrouting of ODR-10 to all plasma membrane destinations. Finally, ODR-10 colocalises with RAB-8 in cell soma and they cotranslocate along dendrites, whereas ODR-10 and UNC-101 signals do not overlap. Together, these data implicate conserved roles for metazoan AP-1 in facilitating cilium structure and function, and suggest cooperation with RAB-8 to coordinate distinct early steps in neuronal ciliary membrane sorting and trafficking.

Leishmania infantum chagasi: a genome-based approach to identification of excreted/secreted proteins

The parasitic protozoan, Leishmania, survives in harsh environments within its mammalian and sand fly hosts. Secreted proteins likely play critical roles in the parasite's interactions with its environment. As a preliminary identification of the spectrum of potential excreted/secreted (ES) proteins of Leishmania infantum chagasi (Lic), a causative agent of visceral leishmaniasis, we used standard algorithms to screen the annotated L. infantum genome for genes whose predicted protein products have an N-terminal signal peptide and lack transmembrane domains and membrane anchors. A suite of 181 candidate ES proteins were identified. These included several that were documented in the literature to be released by other Leishmania spp. Six candidate ES proteins were selected for further validation of their expression and release by different parasite stages. We found both amastigote-specific and promastigote-specific released proteins. The ES proteins of Lic are candidates for future studies of parasite virulence determinants and host protective immunity.

Role of AP-1 in developmentally regulated lysosomal trafficking in Trypanosoma brucei

African trypanosomes are the causative agents of human trypanosomiasis (sleeping sickness). The pathogenic stage of the parasite has unique adaptations to life in the bloodstream of the mammalian host, including upregulation of endocytic and lysosomal activities. We investigated stage-specific requirements for cytoplasmic adaptor/clathrin machinery in post-Golgi apparatus biosynthetic sorting to the lysosome using RNA interference silencing of the Tbmu1 subunit of adaptor complex 1 (AP-1), in conjunction with immunolocalization, kinetic analyses of reporter transport, and quantitative endocytosis assays. Tbmu1 silencing was lethal in both stages, indicating a critical function(s) for the AP-1 machinery. Transport of soluble and membrane-bound secretory cargoes was Tbmu1 independent in both stages. In procyclic parasites, trafficking of the lysosomal membrane protein, p67, was disrupted, leading to cell surface mislocalization. The lysosomal protease trypanopain was also secreted, suggesting a transmembrane-sorting receptor for this soluble hydrolase. In bloodstream trypanosomes, both p67 and trypanopain trafficking were unaffected by Tbmu1 silencing, suggesting that AP-1 is not necessary for biosynthetic lysosomal trafficking. Endocytosis in bloodstream cells was also unaffected, indicating that AP-1 does not function at the flagellar pocket. These results indicate that post-Golgi apparatus sorting to the lysosome is critically dependent on the AP-1/clathrin machinery in procyclic trypanosomes but that this machinery is not necessary in bloodstream parasites. We propose a simple model for stage-specific default secretory trafficking in trypanosomes that is consistent with the behavior of other soluble and glycosylphosphatidylinositol-anchored cargos and which is influenced by upregulation of endocytosis in bloodstream parasites as an adaptation to life in the mammalian bloodstream.

Ancient Leishmania coronin (CRN12) is involved in microtubule remodeling during cytokinesis

In general, coronins play an important role in actin-based processes, and are expressed in a variety of eukaryotic cells, including Leishmania. Here, we show that Leishmania coronin preferentially distributes to the distal tip during cytokinesis, and interacts with microtubules through a microtubule-based motor, kinesin K39. We further show that reduction in coronin levels by 40-50% in heterozygous coronin mutants results in generation of bipolar cells (25-30%), specifically in the log phase, owing to unregulated growth of the corset microtubules. Further analysis of bipolar cells revealed that the main cause of generation of bipolar cell morphology is the intrusion of the persistently growing corset microtubules into the other daughter cell corset from the opposite direction. This defect in cytokinesis, however, disappears upon episomal gene complementation. Additionally, our attempts to prepare homozygous mutants were unsuccessful, as only the aneuploid cells survive the selection process. These results indicate that coronin regulates microtubule remodeling during Leishmania cytokinesis and is essentially required for survival of these parasites in culture.

Electron microscopy and cytochemistry analysis of the endocytic pathway of pathogenic protozoa

Endocytosis is essential for eukaryotic cell survival and has been well characterized in mammal and yeast cells. Among protozoa it is also important for evading from host immune defenses and to support intense proliferation characteristic of some life cycle stages. Here we focused on the contribution of morphological and cytochemical studies to the understanding of endocytosis in Trichomonas, Giardia, Entamoeba, Plasmodium, and trypanosomatids, mainly Trypanosoma cruzi, and also Trypanosoma brucei and Leishmania.

Three-dimensional cellular architecture of the flagellar pocket and associated cytoskeleton in trypanosomes revealed by electron microscope tomography

This study uses electron tomography linked to a variety of other EM methods to provide an integrated view of the flagellar pocket and basal body area of the African trypanosome procyclic trypomastigote. We reveal the pocket as an asymmetric membranous 'balloon' with two boundary structures. One of these - the collar - defines the flagellum exit point. The other defines the entry point of the flagellum into the pocket and consists of both an internal transitional fibre array and an external membrane collarette. A novel set of nine radial fibres is described in the basal body proximal zone. The pocket asymmetry is invariably correlated with the position of the probasal body and Golgi. The neck region, just distal to the flagellum exit site, is a specialised area of membrane associated with the start of the flagellum attachment zone and signifies the point where a special set of four microtubules, nucleated close to the basal bodies, joins the subpellicular array. The neck region is also associated with the single Golgi apparatus of the cell. The flagellar exit point interrupts the subpellicular microtubule array with discrete endings of microtubules at the posterior side. Overall, our studies reveal a highly organised, yet dynamic, area of cytoplasm and will be informative in understanding its function.

Multiple motifs regulate trafficking of the LAMP-like protein p67 in the ancient eukaryote Trypanosoma brucei

p67 is a lysosome-associated membrane protein-like lysosomal type I transmembrane glycoprotein in African trypanosomes. The p67 cytoplasmic domain (CD) is both necessary and sufficient for lysosomal targeting in procyclic insect-stage parasites. The p67CD contains two [DE]XXXL[LI]-type dileucine motifs, which function as lysosomal targeting signals in mammalian cells. Using a green fluorescent protein fusion to the p67 transmembrane and cytoplasmic domains as a reporter system, we investigated the role of these motifs in lysosomal targeting in procyclic trypanosomes. Pulse-chase turnover studies, steady-state immunolocalization and quantitative flow cytometry all gave consistent results. Mutagenesis of the membrane-distal dileucine motif impairs lysosomal trafficking leading to partial appearance of the reporter on the cell surface. Mutagenesis of the membrane-proximal motif has little effect on proper targeting. Simultaneous mutagenesis of both motifs results in quantitative delivery to the cell surface. Thus, the distal motif plays a dominant role, but both dileucine motifs are necessary for maximal lysosomal targeting. Additional studies suggest that the upstream acidic residues in each motif influence lysosomal targeting and may also affect forward trafficking in the early secretory pathway. These results strongly suggest an evolutionary conservation in lysosomal trafficking mechanisms in the ancient eukaryote Trypanosoma brucei.

Phosphatidylinositol 4-kinase III-beta is required for Golgi maintenance and cytokinesis in Trypanosoma brucei

The parasitic protozoan Trypanosoma brucei contains two type III phosphatidylinositol 4-kinases (alpha and beta). We have cloned the gene encoding the T. brucei type III phosphatidylinositol 4-kinase beta (TbPI4KIII-beta), expressed the protein in COS-7 cells, and confirmed that the protein catalyzes the phosphorylation of phosphatidylinositol. Depletion of TbPI4KIII-beta in procyclic T. brucei by RNA interference (RNAi) resulted in inhibition of cell growth and a distorted cellular morphology. RNAi cells had a distorted Golgi apparatus, and lysosomal and flagellar pocket proteins were mislocalized. Ultrastructural analysis revealed the internal accumulation of a heterogeneous population of vesicles, abnormal positioning of organelles, and a loss of cell polarity. Scanning electron microcopy revealed a twisted phenotype, and dividing cells often exhibited a detached daughter flagellum and lacked a cleavage furrow. Cell cycle analysis confirmed that cells depleted of TbPI4KIII-beta have a postmitotic cytokinesis block that occurs after a single round of mitosis, suggestive of a specific cell cycle block. In summary, TbPI4KIII-beta is an essential protein in procyclic T. brucei, required for maintenance of Golgi structure, protein trafficking, normal cellular shape, and cytokinesis.

Sorting signals required for trafficking of the cysteine-rich acidic repetitive transmembrane protein in Trypanosoma brucei

In trypanosomatids, endocytosis and exocytosis are restricted to the flagellar pocket (FP). The cysteine-rich acidic repetitive transmembrane (CRAM) protein is located at the FP of Trypanosoma brucei and potentially functions as a receptor or an essential component for lipoprotein uptake. We characterized sorting determinants involved in efficient trafficking of CRAM to and from the FP of T. brucei. Previous studies indicated the presence of signals in the CRAM C terminus, specific for its localization to the FP and for efficient endocytosis (H. Yang, D. G. Russell, B. Zeng, M. Eiki, and M.G.-S. Lee, Mol. Cell. Biol. 20:5149-5163, 2000.) To delineate functional domains of putative sorting signals, we performed a mutagenesis series of the CRAM C terminus. Subcellular localization of CRAM mutants demonstrated that the amino acid sequence between -5 and -14 (referred to as a transport signal) is essential for exporting CRAM from the endoplasmic reticulum to the FP, and mutations of amino acids at -12 (V), -10 (V), or -5 (D) led to retention of CRAM in the endoplasmic reticulum. Comparison of the endocytosis efficiency of CRAM mutants demonstrated that the sequence from amino acid -5 to -23 (referred to as a putative endocytosis signal) is required for efficient endocytosis and overlaps with the transport signal. Apparently the CRAM-derived sorting signal can efficiently interact with the T. brucei micro1 adaptin, and mutations at amino acids essential for the function of the transport signal abolished the interaction of the signal with T. brucei micro1, strengthening the hypothesis of the involvement of the clathrin- and adaptor-dependent pathway in trafficking of CRAM via the FP.

Ablation of the single dynamin of T. brucei blocks mitochondrial fission and endocytosis and leads to a precise cytokinesis arrest

Mitochondrial fission is mediated by dynamin-like proteins (DLPs). Trypanosoma brucei contains a single DLP, which is the only member of the dynamin superfamily. We have previously shown that expression of the human proapoptotic Bax in T. brucei induces extensive mitochondrial fragmentation. Here we report that Baxinduced mitochondrial fission is abolished in cell lines lacking functional DLP suggesting that the protein is also required for mitochondrial division during the cell cycle. Furthermore, DLP-ablated cells are deficient for endocytosis and as a consequence accumulate enlarged flagellar pockets. Thus, besides its expected role in mitochondrial fission the trypanosomal DLP is required for endocytosis, a function thought to be restricted to classical dynamins. In agreement with its dual function, the DLP localizes to both the mitochondrion and the flagellar pocket, the site where endocytosis occurs. Unexpectedly, ablation of DLP also causes an arrest of cytokinesis. The fact that no multinucleation is observed in the arrested cells argues for a precise cell-cycle block. Furthermore, analysis of a clathrin-knockdown cell line suggests that the cytokinesis arrest is not due to the endocytosis defect. Thus, our results support a working model in which mitochondrial fission triggers a checkpoint for cytokinesis.

Interfering with the brain: use of RNA interference for understanding the pathophysiology of psychiatric and neurological disorders

Psychiatric and neurological disorders are among the most complex, poorly understood, and debilitating diseases in medicine. The burgeoning advances in functional genomic technologies have led to the identification of a vast number of novel genes that are potentially implicated in the pathophysiology of such disorders. However, many of these candidate genes have not yet been functionalized and require validation in vivo. Traditionally, abrogating gene function is one of the primary means of examining the physiological significance of a given gene product. Several methods have been developed for gene ablation or knockdown, however, with limited levels of success. The recent discovery of RNA interference (RNAi), as a highly efficient method for gene knockdown, has been one of the major breakthroughs in molecular medicine. In vivo application of RNAi is further demonstrating the promise of this technology. Recent efforts have focused on applying RNAi-based knockdown to understand the genes implicated in neuropsychiatric disorders. However, the greatest challenge with this approach is translating the success of RNAi from mammalian cell cultures to the brain in animal models of disease and, subsequently, in patients. In this review, we describe the various methods that are being developed to deliver RNAi into the brain for down-regulating gene expression and subsequent phenotyping of genes in vivo. We illustrate the utility of various approaches with a few successful examples and also discuss the potential benefits and pitfalls associated with the use of each delivery approach. Appropriate tailoring of tools that deliver RNAi in the brain may not only aid our understanding of the complex pathophysiology of neuropsychiatric disorders, but may also serve as a valuable therapy for disorders, where there is an immense unmet medical need.

The developmental cell biology of Trypanosoma brucei

Trypanosoma brucei provides an excellent system for studies of many aspects of cell biology, including cell structure and morphology, organelle positioning, cell division and protein trafficking. However, the trypanosome has a complex life cycle in which it must adapt either to the mammalian bloodstream or to different compartments within the tsetse fly. These differentiation events require stage-specific changes to basic cell biological processes and reflect responses to environmental stimuli and programmed differentiation events that must occur within a single cell. The organization of cell structure is fundamental to the trypanosome throughout its life cycle. Modulations of the overall cell morphology and positioning of the specialized mitochondrial genome, flagellum and associated basal body provide the classical descriptions of the different life cycle stages of the parasite. The dependency relationships that govern these morphological changes are now beginning to be understood and their molecular basis identified. The overall picture emerging is of a highly organized cell in which the rules established for cell division and morphogenesis in organisms such as yeast and mammalian cells do not necessarily apply. Therefore, understanding the developmental cell biology of the African trypanosome is providing insight into both fundamentally conserved and fundamentally different aspects of the organization of the eukaryotic cell.