Developmental and morphological regulation of clathrin-mediated endocytosis inTrypanosoma brucei

Continuous endosomes form functional subdomains and orchestrate rapid membrane trafficking in trypanosomes

Endocytosis is a common process observed in most eukaryotic cells, although its complexity varies among different organisms. In Trypanosoma brucei, the endocytic machinery is under special selective pressure because rapid membrane recycling is essential for immune evasion. This unicellular parasite effectively removes host antibodies from its cell surface through hydrodynamic drag and fast endocytic internalization. The entire process of membrane recycling occurs exclusively through the flagellar pocket, an extracellular organelle situated at the posterior pole of the spindle-shaped cell. The high-speed dynamics of membrane flux in trypanosomes do not seem compatible with the conventional concept of distinct compartments for early endosomes (EE), late endosomes (LE), and recycling endosomes (RE). To investigate the underlying structural basis for the remarkably fast membrane traffic in trypanosomes, we employed advanced techniques in light and electron microscopy to examine the three-dimensional architecture of the endosomal system. Our findings reveal that the endosomal system in trypanosomes exhibits a remarkably intricate structure. Instead of being compartmentalized, it constitutes a continuous membrane system, with specific functions of the endosome segregated into membrane subdomains enriched with classical markers for EE, LE, and RE. These membrane subdomains can partly overlap or are interspersed with areas that are negative for endosomal markers. This continuous endosome allows fast membrane flux by facilitated diffusion that is not slowed by multiple fission and fusion events.

Fatty acid uptake in Trypanosoma brucei: Host resources and possible mechanisms

Trypanosoma brucei spp. causes African Sleeping Sickness in humans and nagana, a wasting disease, in cattle. As T. brucei goes through its life cycle in its mammalian and insect vector hosts, it is exposed to distinct environments that differ in their nutrient resources. One such nutrient resource is fatty acids, which T. brucei uses to build complex lipids or as a potential carbon source for oxidative metabolism. Of note, fatty acids are the membrane anchoring moiety of the glycosylphosphatidylinositol (GPI)-anchors of the major surface proteins, Variant Surface Glycoprotein (VSG) and the Procyclins, which are implicated in parasite survival in the host. While T. brucei can synthesize fatty acids de novo, it also readily acquires fatty acids from its surroundings. The relative contribution of parasite-derived vs. host-derived fatty acids to T. brucei growth and survival is not known, nor have the molecular mechanisms of fatty acid uptake been defined. To facilitate experimental inquiry into these important aspects of T. brucei biology, we addressed two questions in this review: (1) What is known about the availability of fatty acids in different host tissues where T. brucei can live? (2) What is known about the molecular mechanisms mediating fatty acid uptake in T. brucei? Finally, based on existing biochemical and genomic data, we suggest a model for T. brucei fatty acid uptake that proposes two major routes of fatty acid uptake: diffusion across membranes followed by intracellular trapping, and endocytosis of host lipoproteins.

Combined nanometric and phylogenetic analysis of unique endocytic compartments in Giardia lamblia sheds light on the evolution of endocytosis in Metamonada

Background

Giardia lamblia, a parasitic protist of the Metamonada supergroup, has evolved one of the most diverged endocytic compartment systems investigated so far. Peripheral endocytic compartments, currently known as peripheral vesicles or vacuoles (PVs), perform bulk uptake of fluid phase material which is then digested and sorted either to the cell cytosol or back to the extracellular space.

Results

Here, we present a quantitative morphological characterization of these organelles using volumetric electron microscopy and super-resolution microscopy (SRM). We defined a morphological classification for the heterogenous population of PVs and performed a comparative analysis of PVs and endosome-like organelles in representatives of phylogenetically related taxa, Spironucleus spp. and Tritrichomonas foetus. To investigate the as-yet insufficiently understood connection between PVs and clathrin assemblies in G. lamblia, we further performed an in-depth search for two key elements of the endocytic machinery, clathrin heavy chain (CHC) and clathrin light chain (CLC), across different lineages in Metamonada. Our data point to the loss of a bona fide CLC in the last Fornicata common ancestor (LFCA) with the emergence of a protein analogous to CLC (GlACLC) in the Giardia genus. Finally, the location of clathrin in the various compartments was quantified.

Conclusions

Taken together, this provides the first comprehensive nanometric view of Giardia’s endocytic system architecture and sheds light on the evolution of GlACLC analogues in the Fornicata supergroup and, specific to Giardia, as a possible adaptation to the formation and maintenance of stable clathrin assemblies at PVs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01402-3.

To the Surface and Back: Exo- and Endocytic Pathways in Trypanosoma brucei

Trypanosoma brucei is one of only a few unicellular pathogens that thrives extracellularly in the vertebrate host. Consequently, the cell surface plays a critical role in both immune recognition and immune evasion. The variant surface glycoprotein (VSG) coats the entire surface of the parasite and acts as a flexible shield to protect invariant proteins against immune recognition. Antigenic variation of the VSG coat is the major virulence mechanism of trypanosomes. In addition, incessant motility of the parasite contributes to its immune evasion, as the resulting fluid flow on the cell surface drags immunocomplexes toward the flagellar pocket, where they are internalized. The flagellar pocket is the sole site of endo- and exocytosis in this organism. After internalization, VSG is rapidly recycled back to the surface, whereas host antibodies are thought to be transported to the lysosome for degradation. For this essential step to work, effective machineries for both sorting and recycling of VSGs must have evolved in trypanosomes. Our understanding of the mechanisms behind VSG recycling and VSG secretion, is by far not complete. This review provides an overview of the trypanosome secretory and endosomal pathways. Longstanding questions are pinpointed that, with the advent of novel technologies, might be answered in the near future.

p67: a cryptic lysosomal hydrolase in Trypanosoma brucei?

p67 is a type I transmembrane glycoprotein of the terminal lysosome of African trypanosomes. Its biosynthesis involves transport of an initial gp100 ER precursor to the lysosome, followed by cleavage to N-terminal (gp32) and C-terminal (gp42) subunits that remain non-covalently associated. p67 knockdown is lethal, but the only overt phenotype is an enlarged lysosome (~250 to >1000 nm). Orthologues have been characterized in Dictyostelium and mammals. These have processing pathways similar to p67, and are thought to have phospholipase B-like (PLBL) activity. The mouse PLBD2 crystal structure revealed that the PLBLs represent a subgroup of the larger N-terminal nucleophile (NTN) superfamily, all of which are hydrolases. NTNs activate by internal autocleavage mediated by a nucleophilic residue, i.e. Cys, Ser or Thr, on the upstream peptide bond to form N-terminal α (gp32) and C-terminal β (gp42) subunits that remain non-covalently associated. The N-terminal residue of the β subunit is then catalytic in subsequent hydrolysis reactions. All PLBLs have a conserved Cys/Ser dipeptide at the α/β junction (Cys241/Ser242 in p67), mutation of which renders p67 non-functional in RNAi rescue assays. p67 orthologues are found in many clades of parasitic protozoa, thus p67 is the founding member of a group of hydrolases that likely play a role broadly in the pathogenesis of parasitic infections.

Rab11 mediates selective recycling and endocytic trafficking in Trypanosoma brucei

Trypanosoma brucei possesses a streamlined secretory system that guarantees efficient delivery to the cell surface of the critical glycosyl-phosphatidylinositol (GPI)-anchored virulence factors, variant surface glycoprotein (VSG) and transferrin receptor (TfR). Both are thought to be constitutively endocytosed and returned to the flagellar pocket via TbRab11+ recycling endosomes. We use conditional knockdown with established reporters to investigate the role of TbRab11 in specific endomembrane trafficking pathways in bloodstream trypanosomes. TbRab11 is essential. Ablation has a modest negative effect on general endocytosis, but does not affect turnover, steady state levels or surface localization of TfR. Nor are biosynthetic delivery to the cell surface and recycling of VSG affected. TbRab11 depletion also causes increased shedding of VSG into the media by formation of nanotubes and extracellular vesicles. In contrast to GPI-anchored cargo, TbRab11 depletion reduces recycling of the transmembrane invariant surface protein, ISG65, leading to increased lysosomal turnover. Thus, TbRab11 plays a critical role in recycling of transmembrane, but not GPI-anchored surface proteins. We proposed a two-step model for VSG turnover involving release of VSG-containing vesicles followed by GPI hydrolysis. Collectively, our results indicate a critical role of TbRab11 in the homeostatic maintenance of the secretory/endocytic system of bloodstream T. brucei.

Autophagic-related cell death of Trypanosoma brucei induced by bacteriocin AS-48

The parasitic protozoan Trypanosoma brucei is the causative agent of human African trypanosomiasis (sleeping sickness) and nagana. Current drug therapies have limited efficacy, high toxicity and/or are continually hampered by the appearance of resistance. Antimicrobial peptides have recently attracted attention as potential parasiticidal compounds. Here, we explore circular bacteriocin AS-48's ability to kill clinically relevant bloodstream forms of T. brucei gambiense, T. brucei rhodesiense and T. brucei brucei. AS-48 exhibited excellent anti-trypanosomal activity in vitro (EC₅₀ = 1–3 nM) against the three T. brucei subspecies, but it was innocuous to human cells at 10⁴-fold higher concentrations. In contrast to its antibacterial action, AS-48 does not kill the parasite through plasma membrane permeabilization but by targeting intracellular compartments. This was evidenced by the fact that vital dye internalization-prohibiting concentrations of AS-48 could kill the parasite at 37 °C but not at 4 °C. Furthermore, AS-48 interacted with the surface of the parasite, at least in part via VSG, its uptake was temperature-dependent and clathrin-depleted cells were less permissive to the action of AS-48. The bacteriocin also caused the appearance of myelin-like structures and double-membrane autophagic vacuoles. These changes in the parasite's ultrastructure were confirmed by fluorescence microscopy as AS-48 induced the production of EGFP-ATG8.2-labeled autophagosomes. Collectively, these results indicate AS-48 kills the parasite through a mechanism involving clathrin-mediated endocytosis of VSG-bound AS-48 and the induction of autophagic-like cell death. As AS-48 has greater in vitro activity than the drugs currently used to treat T. brucei infection and does not present any signs of toxicity in mammalian cells, it could be an attractive lead compound for the treatment of sleeping sickness and nagana.

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AS-48 kills Trypanosoma brucei efficiently and is innocuous in mammalian cells.

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It has greater in vitro activity than drugs currently in use.

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AS-48 must be internalized by the parasite in order to exert its trypanocidal effect.

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AS-48 uptake involves VSG binding and clathrin-mediated endocytosis.

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AS-48 induces an autophagic-related cell death.

Let's get fISSical: fast in silico synchronization as a new tool for cell division cycle analysis

Cell cycle progression is a question of fundamental biological interest. The coordinated duplication and segregation of all cellular structures and organelles is however an extremely complex process, and one which remains only partially understood even in the most intensively researched model organisms. Trypanosomes are in an unusual position in this respect - they are both outstanding model systems for fundamental questions in eukaryotic cell biology, and pathogens that are the causative agents of three of the neglected tropical diseases. As a failure to successfully complete cell division will be deleterious or lethal, analysis of the cell division cycle is of relevance both to basic biology and drug design efforts. Cell division cycle analysis is however experimentally challenging, as the analysis of phenotypes associated with it remains hypothesis-driven and therefore biased. Current methods of analysis are extremely labour-intensive, and cell synchronization remains difficult and unreliable. Consequently, there exists a need - both in basic and applied trypanosome biology - for a global, unbiased, standardized and high-throughput analysis of cell division cycle progression. In this review, the requirements - both practical and computational - for such a system are considered and compared with existing techniques for cell cycle analysis.

The role of the PI(3,5)P2 kinase TbFab1 in endo/lysosomal trafficking in Trypanosoma brucei

Protein trafficking through endo/lysosomal compartments is critically important to the biology of the protozoan parasite Trypanosoma brucei, but the routes material may take to the lysosome, as well as the molecular factors regulating those routes, remain incompletely understood. Phosphoinositides are signaling phospholipids that regulate many trafficking events by recruiting specific effector proteins to discrete membrane subdomains. In this study, we investigate the role of one phosphoinositide, PI(3,5)P₂ in T. brucei. We find a low steady state level of PI(3,5)P₂ in bloodstream form parasites comparable to that of other organisms. RNAi knockdown of the putative PI(3)P-5 kinase TbFab1 decreases the PI(3,5)P₂ pool leading to rapid cell death. TbFab1 and PI(3,5)P₂ both localize strongly to late endo/lysosomes. While most trafficking functions were intact in TbFab1 deficient cells, including both endocytic and biosynthetic trafficking to the lysosome, lysosomal turnover of an endogenous ubiquitinylated membrane protein, ISG65, was completely blocked suggesting that TbFab1 plays a role in the ESCRT-mediated late endosomal/multivesicular body degradative pathways. Knockdown of a second component of PI(3,5)P₂ metabolism, the PI(3,5)P₂ phosphatase TbFig4, also resulted in delayed turnover of ISG65. Together, these results demonstrate an essential role for PI(3,5)P₂ in the turnover of ubiquitinylated membrane proteins and in trypanosome endomembrane biology.

Lineage-specific proteins essential for endocytosis in trypanosomes

Clathrin-mediated endocytosis (CME) is the most evolutionarily ancient endocytic mechanism known, and in many lineages the sole mechanism for internalisation. Significantly, in mammalian cells CME is responsible for the vast bulk of endocytic flux and has likely undergone multiple adaptations to accommodate specific requirements by individual species. In African trypanosomes, we previously demonstrated that CME is independent of the AP-2 adaptor protein complex, that orthologues to many of the animal and fungal CME protein cohort are absent, and that a novel, trypanosome-restricted protein cohort interacts with clathrin and drives CME. Here, we used a novel cryomilling affinity isolation strategy to preserve transient low-affinity interactions, giving the most comprehensive trypanosome clathrin interactome to date. We identified the trypanosome AP-1 complex, Trypanosoma brucei (Tb)EpsinR, several endosomal SNAREs plus orthologues of SMAP and the AP-2 associated kinase AAK1 as interacting with clathrin. Novel lineage-specific proteins were identified, which we designate TbCAP80 and TbCAP141. Their depletion produced extensive defects in endocytosis and endomembrane system organisation, revealing a novel molecular pathway subtending an early-branching and highly divergent form of CME, which is conserved and likely functionally important across the kinetoplastid parasites.

Summary: Endocytosis is a vital process in most cells, and here we identify important proteins required for this process in trypanosomes. Significantly, these are unique and not present in animals, fungi or plants.

Antiprotozoal glutathione derivatives with flagellar membrane binding activity against T. brucei rhodesiense

A new series of N-substituted S-(2,4-dinitrophenyl)glutathione dibutyl diesters were synthesized to improve in vitro anti-protozoal activity against the pathogenic parasites Trypanosoma brucei rhodesiense, Trypanosoma cruzi and Leishmania donovani. The results obtained indicate that N-substituents enhance the inhibitory properties of glutathione diesters whilst showing reduced toxicity against KB cells as in the cases of compounds 5, 9, 10, 16, 18 and 19. We suggest that the interaction of N-substituted S-(2,4-dinitrophenyl) glutathione dibutyl diesters with T. b. brucei occurs mainly by weak hydrophobic interactions such as London and van der Waals forces. A QSAR study indicated that the inhibitory activity of the peptide is associated negatively with the average number of C atoms, N_C and positively to S_ZX, the ZX shadow a geometric descriptor related to molecular size and orientation of the compound. HPLC-UV studies in conjunction with optical microscopy indicate that the observed selectivity of inhibition of these compounds against bloodstream form T. b. brucei parasites in comparison to L. donovani under the same conditions is due to intracellular uptake via endocytosis in the flagellar pocket.

Species-Specific Adaptations of Trypanosome Morphology and Motility to the Mammalian Host

African trypanosomes thrive in the bloodstream and tissue spaces of a wide range of mammalian hosts. Infections of cattle cause an enormous socio-economic burden in sub-Saharan Africa. A hallmark of the trypanosome lifestyle is the flagellate's incessant motion. This work details the cell motility behavior of the four livestock-parasites Trypanosoma vivax, T. brucei, T. evansi and T. congolense. The trypanosomes feature distinct swimming patterns, speeds and flagellar wave frequencies, although the basic mechanism of flagellar propulsion is conserved, as is shown by extended single flagellar beat analyses. Three-dimensional analyses of the trypanosomes expose a high degree of dynamic pleomorphism, typified by the 'cellular waveform'. This is a product of the flagellar oscillation, the chirality of the flagellum attachment and the stiffness of the trypanosome cell body. The waveforms are characteristic for each trypanosome species and are influenced by changes of the microenvironment, such as differences in viscosity and the presence of confining obstacles. The distinct cellular waveforms may be reflective of the actual anatomical niches the parasites populate within their mammalian host. T. vivax displays waveforms optimally aligned to the topology of the bloodstream, while the two subspecies T. brucei and T. evansi feature distinct cellular waveforms, both additionally adapted to motion in more confined environments such as tissue spaces. T. congolense reveals a small and stiff waveform, which makes these parasites weak swimmers and destined for cell adherence in low flow areas of the circulation. Thus, our experiments show that the differential dissemination and annidation of trypanosomes in their mammalian hosts may depend on the distinct swimming capabilities of the parasites.

Binding and Endocytosis of Bovine Hololactoferrin by the Parasite Entamoeba histolytica

Entamoeba histolytica is a human parasite that requires iron (Fe) for its metabolic function and virulence. Bovine lactoferrin (B-Lf) and its peptides can be found in the digestive tract after dairy products are ingested. The aim of this study was to compare virulent trophozoites recently isolated from hamster liver abscesses with nonvirulent trophozoites maintained for more than 30 years in cultures in vitro regarding their interaction with iron-charged B-Lf (B-holo-Lf). We performed growth kinetics analyses of trophozoites in B-holo-Lf and throughout several consecutive transfers. The virulent parasites showed higher growth and tolerance to iron than nonvirulent parasites. Both amoeba variants specifically bound B-holo-Lf with a similar K_d. However, averages of 9.45 × 10⁵ and 6.65 × 10⁶ binding sites/cell were found for B-holo-Lf in nonvirulent and virulent amoebae, respectively. Virulent amoebae bound more efficiently to human and bovine holo-Lf, human holo-transferrin, and human and bovine hemoglobin than nonvirulent amoebae. Virulent amoebae showed two types of B-holo-Lf binding proteins. Although both amoebae endocytosed this glycoprotein through clathrin-coated vesicles, the virulent amoebae also endocytosed B-holo-Lf through a cholesterol-dependent mechanism. Both amoeba variants secreted cysteine proteases cleaving B-holo-Lf. These data demonstrate that the B-Lf endocytosis is more efficient in virulent amoebae.

Transferrin: Endocytosis and Cell Signaling in Parasitic Protozoa

Iron is the fourth most abundant element on Earth and the most abundant metal in the human body. This element is crucial for life because almost all organisms need iron for several biological activities. This is the case with pathogenic organisms, which are at the vanguard in the battle with the human host for iron. The latest regulates Fe concentration through several iron-containing proteins, such as transferrin. The transferrin receptor transports iron to each cell that needs it and maintains it away from pathogens. Parasites have developed several strategies to obtain iron as the expression of specific transferrin receptors localized on plasma membrane, internalized through endocytosis. Signal transduction pathways related to the activation of the receptor have functional importance in proliferation. The study of transferrin receptors and other proteins with action in the signaling networks is important because these proteins could be used as therapeutic targets due to their specificity or to differences with the human counterpart. In this work, we describe proteins that participate in signal transduction processes, especially those that involve transferrin endocytosis, and we compare these processes with those found in T. brucei, T. cruzi, Leishmania spp., and E. histolytica parasites.

Iron Homeostasis and Trypanosoma brucei Associated Immunopathogenicity Development: A Battle/Quest for Iron

African trypanosomosis is a chronic debilitating disease affecting the health and economic well-being of developing countries. The immune response during African trypanosome infection consisting of a strong proinflammatory M1-type activation of the myeloid phagocyte system (MYPS) results in iron deprivation for these extracellular parasites. Yet, the persistence of M1-type MYPS activation causes the development of anemia (anemia of chronic disease, ACD) as a most prominent pathological parameter in the mammalian host, due to enhanced erythrophagocytosis and retention of iron within the MYPS thereby depriving iron for erythropoiesis. In this review we give an overview of how parasites acquire iron from the host and how iron modulation of the host MYPS affects trypanosomosis-associated anemia development. Finally, we also discuss different strategies at the level of both the host and the parasite that can/might be used to modulate iron availability during African trypanosome infections.

ENTH and ANTH domain proteins participate in AP2-independent clathrin-mediated endocytosis

Clathrin-mediated endocytosis (CME) is a major route of entry into eukaryotic cells. A core of evolutionarily ancient genes encodes many components of this system but much of our mechanistic understanding of CME is derived from a phylogenetically narrow sampling of a few model organisms. In the parasite Trypanosoma brucei, which is distantly related to the better characterised animals and fungi, exceptionally fast endocytic turnover aids its evasion of the host immune system. Although clathrin is absolutely essential for this process, the adaptor protein complex 2 (AP2) has been secondarily lost, suggesting mechanistic divergence. Here, we characterise two phosphoinositide-binding monomeric clathrin adaptors, T. brucei (Tb)EpsinR and TbCALM, which in trypanosomes are represented by single genes, unlike the expansions present in animals and fungi. Depletion of these gene products reveals essential, but partially redundant, activities in CME. Ultrastructural analysis of TbCALM and TbEpsinR double-knockdown cells demonstrated severe defects to clathrin-coated pit formation and morphology associated with a dramatic inhibition of endocytosis. Depletion of TbCALM alone, however, produced a distinct lysosomal segregation phenotype, indicating an additional non-redundant role for this protein. Therefore, TbEpsinR and TbCALM represent ancient phosphoinositide-binding proteins with distinct and vital roles in AP2-independent endocytosis.

Clathrin expression in Trypanosoma cruzi

Background

Clathrin-mediated vesicular trafficking, the mechanism by which proteins and lipids are transported between membrane-bound organelles, accounts for a large proportion of import from the plasma membrane (endocytosis) and transport from the trans-Golgi network towards the endosomal system. Clathrin-mediated events are still poorly understood in the protozoan Trypanosoma cruzi, the causative agent of Chagas disease in Latin America. In this study, clathrin heavy (TcCHC) and light (TcCLC) chain gene expression and protein localization were investigated in different developmental forms of T. cruzi (epimastigotes, trypomastigotes and amastigotes), using both polyclonal and monoclonal antibodies raised against T. cruzi recombinant proteins.

Results

Analysis by confocal microscopy revealed an accumulation of TcCHC and TcCLC at the cell anterior, where the flagellar pocket and Golgi complex are located. TcCLC partially colocalized with the Golgi marker TcRAB7-GFP and with ingested albumin, but did not colocalize with transferrin, a protein mostly ingested via uncoated vesicles at the cytostome/cytopharynx complex.

Conclusion

Clathrin heavy and light chains are expressed in T. cruzi. Both proteins typically localize anterior to the kinetoplast, at the flagellar pocket and Golgi complex region. Our data also indicate that in T. cruzi epimastigotes clathrin-mediated endocytosis of albumin occurs at the flagellar pocket, while clathrin-independent endocytosis of transferrin occurs at the cytostome/cytopharynx complex.

The ancient small GTPase Rab21 functions in intermediate endocytic steps in trypanosomes

Endocytosis is an essential process in nearly all eukaryotic cells, including the African trypanosome Trypanosoma brucei. Endocytosis in these organisms is exclusively clathrin mediated, although several lineage-specific features indicate that precise mechanisms are distinct from those of higher eukaryotes. T. brucei Rab21 is a member of an ancient, pan-eukaryotic, endocytic Rab clade that is retained by trypanosomes. We show that T. brucei Rab21 (TbRab21) localizes to endosomes, partially colocalizing with TbRab5A, TbRab28, and TbVps23, the latter two being present at late endosomes. TbRab21 expression is essential for cellular proliferation, and its suppression results in a partial block in traffic to the lysosome. RNA interference (RNAi)-mediated knockdown of TbRab21 had no effect on TbRab5A expression or location but did result in decreased in trans expression of ESCRT (trypanosome endosomal sorting complex required for transport) components and TbRab28, while knockdown of ESCRT subunit TbVps23 resulted in decreased TbRab21 expression. These data suggest that TbRab21 acts downstream of TbRab5A and functions in intimate connection with the trypanosome ESCRT system.

A comparative analysis of trypanosomatid SNARE proteins

The Kinetoplastida are flagellated protozoa evolutionary distant and divergent from yeast and humans. Kinetoplastida include trypanosomatids, and a number of important pathogens. Trypanosoma brucei, Trypanosoma cruzi and Leishmania spp. inflict significant morbidity and mortality on humans and livestock as the etiological agents of human African trypanosomiasis, Chagas' disease and leishmaniasis respectively. For all of these organisms, intracellular trafficking is vital for maintenance of the host–pathogen interface, modulation/evasion of host immune system responses and nutrient uptake. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are critical components of the intracellular trafficking machinery in eukaryotes, mediating membrane fusion and contributing to organelle specificity. We asked how the SNARE complement evolved across the trypanosomatids. An in silico search of the predicted proteomes of T. b. brucei and T. cruzi was used to identify candidate SNARE sequences. Phylogenetic analysis, including comparisons with yeast and human SNAREs, allowed assignment of trypanosomatid SNAREs to the Q or R subclass, as well as identification of several SNAREs orthologous with those of opisthokonts. Only limited variation in number and identity of SNAREs was found, with Leishmania major having 27 and T. brucei 26, suggesting a stable SNARE complement post-speciation. Expression analysis of T. brucei SNAREs revealed significant differential expression between mammalian and insect infective forms, especially within R and Qb-SNARE subclasses, suggesting possible roles in adaptation to different environments. For trypanosome SNAREs with clear orthologs in opisthokonts, the subcellular localization of TbVAMP7C is endosomal while both TbSyn5 and TbSyn16B are at the Golgi complex, which suggests conservation of localization and possibly also function. Despite highly distinct life styles, the complement of trypanosomatid SNAREs is quite stable between the three pathogenic lineages, suggesting establishment in the last common ancestor of trypanosomes and Leishmania. Developmental changes to SNARE mRNA levels between blood steam and procyclic life stages suggest that trypanosomes modulate SNARE functions via expression. Finally, the locations of some conserved SNAREs have been retained across the eukaryotic lineage.

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SNARE proteins are essential components of intracellular transport.

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These proteins exhibit considerable conservation across pathogenic trypanosomes.

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Some trypanosome SNARE families are expanded or lost.

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Developmental changes in trypanosome SNARE expression are apparent.

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Orthologous SNAREs demonstrate conserved locations and hence function.

Characterization of the late endosomal ESCRT machinery in Trypanosoma brucei

The multivesicular body (MVB) is a specialized Rab7+ late endosome (LE) containing multiple intralumenal vesicles that function in targeting ubiquitinylated cell surface proteins to the lysosome for degradation. African trypanosomes lack a morphologically well-defined MVB, but contain orthologs of the ESCRT (Endosomal Sorting Complex Required for Transport) machinery that mediates MVB formation. We investigate the role of TbVps23, an early ESCRT component, and TbVps4, the terminal ESCRT ATPase, in lysosomal trafficking in bloodstream form trypanosomes. Both localize to the TbRab7+ LE and RNAi silencing of each rapidly blocks growth. TbVps4 silencing results in approximately threefold accumulation of TbVps23 at the LE, consistent with blocking terminal ESCRT disassembly. Trafficking of endocytic and biosynthetic cargo, but not default lysosomal reporters, is also negatively affected. Others reported that TbVps23 mediates ubiquitin-dependent lysosomal degradation of invariant surface glycoproteins (ISG65) (Leung et al., Traffic 2008;9:1698-1716). In contrast, we find that TbVps23 ablation does not affect ISG65 turnover, while TbVps4 silencing markedly enhances lysosomal degradation. We propose several models to accommodate these results, including that the ESCRT machinery actually retrieves ISG65 from the LE to earlier endocytic compartments, and in its absence ISG65 traffics more efficiently to the lysosome. Overall, these results confirm that the ESCRT machinery is essential in Trypanosoma brucei and plays important and novel role(s) in LE function in trypanosomes.

Identification and Functional Characterisation of CRK12:CYC9, a Novel Cyclin-Dependent Kinase (CDK)-Cyclin Complex in Trypanosoma brucei

The protozoan parasite, Trypanosoma brucei, is spread by the tsetse fly and causes trypanosomiasis in humans and animals. Both the life cycle and cell cycle of the parasite are complex. Trypanosomes have eleven cdc2-related kinases (CRKs) and ten cyclins, an unusually large number for a single celled organism. To date, relatively little is known about the function of many of the CRKs and cyclins, and only CRK3 has previously been shown to be cyclin-dependent in vivo. Here we report the identification of a previously uncharacterised CRK:cyclin complex between CRK12 and the putative transcriptional cyclin, CYC9. CRK12:CYC9 interact to form an active protein kinase complex in procyclic and bloodstream T. brucei. Both CRK12 and CYC9 are essential for the proliferation of bloodstream trypanosomes in vitro, and we show that CRK12 is also essential for survival of T. brucei in a mouse model, providing genetic validation of CRK12:CYC9 as a novel drug target for trypanosomiasis. Further, functional characterisation of CRK12 and CYC9 using RNA interference reveals roles for these proteins in endocytosis and cytokinesis, respectively.

Proteomic analysis of clathrin interactions in trypanosomes reveals dynamic evolution of endocytosis

Endocytosis is a vital cellular process maintaining the cell surface, modulating signal transduction and facilitating nutrient acquisition. In metazoa, multiple endocytic modes are recognized, but for many unicellular organisms the process is likely dominated by the ancient clathrin-mediated pathway. The endocytic system of the highly divergent trypanosomatid Trypanosoma brucei exhibits many unusual features, including a restricted site of internalization, dominance of the plasma membrane by GPI-anchored proteins, absence of the AP2 complex and an exceptionally high rate. Here we asked if the proteins subtending clathrin trafficking in trypanosomes are exclusively related to those of higher eukaryotes or if novel, potentially taxon-specific proteins operate. Co-immunoprecipitation identified twelve T. brucei clathrin-associating proteins (TbCAPs), which partially colocalized with clathrin. Critically, eight TbCAPs are restricted to trypanosomatid genomes and all of these are required for robust cell proliferation. A subset, TbCAP100, TbCAP116, TbCAP161 and TbCAP334, were implicated in distinct endocytic steps by detailed analysis of knockdown cells. Coupled with the absence of orthologs for many metazoan and fungal endocytic factors, these data suggest that clathrin interactions in trypanosomes are highly lineage-specific, and indicate substantial evolutionary diversity within clathrin-mediated endocytosis mechanisms across the eukaryotes.

Proteomics on the rims: insights into the biology of the nuclear envelope and flagellar pocket of trypanosomes

Trypanosomatids represent the causative agents of major diseases in humans, livestock and plants, with inevitable suffering and economic hardship as a result. They are also evolutionarily highly divergent organisms, and the many unique aspects of trypanosome biology provide opportunities in terms of identification of drug targets, the challenge of exploiting these putative targets and, at the same time, significant scope for exploration of novel and divergent cell biology. We can estimate from genome sequences that the degree of divergence of trypanosomes from animals and fungi is extreme, with perhaps one third to one half of predicted trypanosome proteins having no known function based on homology or recognizable protein domains/architecture. Two highly important aspects of trypanosome biology are the flagellar pocket and the nuclear envelope, where in silico analysis clearly suggests great potential divergence in the proteome. The flagellar pocket is the sole site of endo- and exocytosis in trypanosomes and plays important roles in immune evasion via variant surface glycoprotein (VSG) trafficking and providing a location for sequestration of various invariant receptors. The trypanosome nuclear envelope has been largely unexplored but, by analogy with higher eukaryotes, roles in the regulation of chromatin and most significantly, in controlling VSG gene expression are expected. Here we discuss recent successful proteomics-based approaches towards characterization of the nuclear envelope and the endocytic apparatus, the identification of conserved and novel trypanosomatid-specific features, and the implications of these findings.

Form and function in the trypanosomal secretory pathway

Recent advances in secretory biology of African trypanosomes reveal both similarities and striking differences with other model eukaryotic organisms. Secretion is streamlined for rapid and selective transport of the major cargo, VSG. Selectivity in the early and post-Golgi compartments is dependent on glycosylphosphatidyl inositol anchors. Streamlining includes reduced organellar abundance, and close association of ER exit sites with Golgi and with unique flagellar cytoskeletal elements that govern organellar replication and segregation. These elements include a novel centrin containing bilobe structure. Innate signals for post-Golgi sorting of biosynthetic lysosomal cargo trafficking have been defined, as have pathways for both biosynthetic and endocytic trafficking to the lysosome. Less well-defined secretory organelles such as the multivesicular body and acidocalcisomes are receiving closer scrutiny.

Endocytosis genes facilitate protein and membrane transport in C. elegans sensory cilia

BACKGROUND

Multiple intracellular transport pathways drive the formation, maintenance, and function of cilia, a compartmentalized organelle associated with motility, chemo-/mechano-/photosensation, and developmental signaling. These pathways include cilium-based intraflagellar transport (IFT) and poorly understood membrane trafficking events. Defects in ciliary transport contribute to the etiology of human ciliary disease such as Bardet-Biedl syndrome (BBS). In this study, we employ the genetically tractable nematode Caenorhabditis elegans to investigate whether endocytosis genes function in cilium formation and/or the transport of ciliary membrane or ciliary proteins.

RESULTS

Here we show that localization of the clathrin light chain, AP-2 clathrin adaptor, dynamin, and RAB-5 endocytic proteins overlaps with a morphologically discrete periciliary membrane compartment associated with sensory cilia. In addition, ciliary transmembrane proteins such as G protein-coupled receptors concentrate at periciliary membranes. Disruption of endocytic gene function causes expansion of ciliary and/or periciliary membranes as well as defects in the ciliary targeting and/or transport dynamics of ciliary transmembrane and IFT proteins. Finally, genetic analyses reveal that the ciliary membrane expansions in dynamin and AP-2 mutants require bbs-8 and rab-8 function and that sensory signaling and endocytic genes may function in a common pathway to regulate ciliary membrane volume.

CONCLUSIONS

These data implicate C. elegans endocytosis proteins localized at the ciliary base in regulating ciliary and periciliary membrane volume and suggest that membrane retrieval from these compartments is counterbalanced by BBS-8 and RAB-8-mediated membrane delivery.

CdTe and CdSe quantum dots cytotoxicity: a comparative study on microorganisms

Quantum dots (QDs) are colloidal semiconductor nanocrystals of a few nanometers in diameter, being their size and shape controlled during the synthesis. They are synthesized from atoms of group II–VI or III–V of the periodic table, such as cadmium telluride (CdTe) or cadmium selenium (CdSe) forming nanoparticles with fluorescent characteristics superior to current fluorophores. The excellent optical characteristics of quantum dots make them applied widely in the field of life sciences. Cellular uptake of QDs, location and translocation as well as any biological consequence, such as cytotoxicity, stimulated a lot of scientific research in this area. Several studies pointed to the cytotoxic effect against micoorganisms. In this mini-review, we overviewed the synthesis and optical properties of QDs, and its advantages and bioapplications in the studies about microorganisms such as protozoa, bacteria, fungi and virus.

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.

Antimicrobial peptide killing of African trypanosomes

The diseases caused by trypanosomes are medically and economically devastating to the population of Sub-Saharan Africa. Parasites of the genus Trypanosoma infect both humans, causing African sleeping sickness, and livestock, causing Nagana. The development of effective treatment strategies has suffered from severe side effects of approved drugs, resistance and major difficulties in delivering drugs. Antimicrobial peptides (AMPs) are ubiquitous components of immune defence and are being rigorously pursued as novel sources of new therapeutics for a variety of pathogens. Here, we review the role of AMPs in the innate immune response of the tsetse fly to African trypanosomes, catalogue trypanocidal AMPs from diverse organisms and highlight the susceptibility of bloodstream form African trypanosomes to killing by unconventional toxic peptides.

Adaptor protein-3 (AP-3) complex mediates the biogenesis of acidocalcisomes and is essential for growth and virulence of Trypanosoma brucei

Acidocalcisomes are acidic calcium and polyphosphate storage organelles found in a diverse range of organisms. Here we present evidence that the biogenesis of acidocalcisomes in Trypanosoma brucei is linked to the expression of adaptor protein-3 (AP-3) complex. Localization studies in cell lines expressing β3 and δ subunits of AP-3 fused to epitope tags revealed their partial co-localization with the vacuolar proton pyrophosphatase, a marker of acidocalcisomes, with the Golgi marker Golgi reassembly and stacking protein, and with antibodies against the small GTPase Rab11. Ablation of the β3 subunit by RNA interference (RNAi) resulted in disappearance of acidocalcisomes from both procyclic and bloodstream form trypanosomes, as revealed by immmunofluorescence and electron microscopy assays, with no alterations in trafficking of different markers to lysosomes. Knockdown of the β3 subunit resulted in lower acidic calcium, pyrophosphate, and polyphosphate content as well as defects in growth in culture, resistance to osmotic stress, and virulence in mice. Similar results were obtained by knocking down the expression of the δ subunit of AP-3. These results indicate that AP-3 is essential for the biogenesis of acidocalcisomes and for growth and virulence of T. brucei.

Late endosomal Rab7 regulates lysosomal trafficking of endocytic but not biosynthetic cargo in Trypanosoma brucei

We present the first functional analysis of the small GTPase, TbRab7, in Trypanosoma brucei. TbRab7 defines discrete late endosomes closely juxtaposed to the terminal p67(+) lysosome. RNAi indicates that TbRab7 is essential in bloodstream trypanosomes. Initial rates of endocytosis were unaffected, but lysosomal delivery of cargo, including tomato lectin (TL) and trypanolytic factor (TLF) were blocked. These accumulate in a dispersed internal compartment of elevated pH, likely derived from the late endosome. Surface binding of TL but not TLF was reduced, suggesting that cellular distribution of flagellar pocket receptors is differentially regulated by TbRab7. TLF activity was reduced approximately threefold confirming that lysosomal delivery is critical for trypanotoxicity. Unexpectedly, delivery of endogenous proteins, p67 and TbCatL, were unaffected indicating that TbRab7 does not regulate biosynthetic lysosomal trafficking. Thus, unlike mammalian cells and yeast, lysosomal trafficking of endocytosed and endogenous proteins occur via different routes and/or are regulated differentially. TbRab7 silencing had no effect on a cryptic default pathway to the lysosome, suggesting that the default lysosomal reporters p67ΔTM, p67ΔCD and VSGΔGPI do not utilize the endocytic pathway as previously proposed. Surprisingly, conditional knockout indicates that TbRab7 may be non-essential in procyclic insect form trypanosomes.

Neospora caninum: comparative gene expression profiling of Neospora caninum wild type and a temperature sensitive clone

To understand the genetic basis of virulence, gene expression profiles of a temperature-sensitive clone (NCts-8, relatively avirulent) and its wild type (NC-1) of Neospora caninum were characterized and compared using a high-density microarray with approximately 63,000 distinct oligonucleotides. This microarray consists of 5692 unique N. caninum sequences, including 1980 Tentative Consensus sequences and 3712 singleton ESTs from the TIGR N. caninum Gene Index (NCGI, release 5.0). Each sequence was represented by 11 distinct 60mer oligonucleotides synthesized in situ on the microarray. The results showed that 111 genes were significantly repressed and no up-regulated genes were identified in the NCts-8 clone. The level of 10 randomly selected genes from the repressed genes was confirmed using real-time RT-PCR. Of the 111 repressed genes, 58 were hypothetical protein products and 53 were annotated genes. Over 70% of the repressed genes identified in this study are clustered on five chromosomes (I, VII, VIII, X and XII). These results suggest that the down-regulated genes may be in part responsible for the reduced pathogenesis of NCts-8; further characterization of the regulated genes may aid in understanding of molecular basis of virulence and development of countermeasures against neosporosis.

Specializations in a successful parasite: what makes the bloodstream-form African trypanosome so deadly?

Most trypanosomatid parasites have both arthropod and mammalian or plant hosts, and the ability to survive and complete a developmental program in each of these very different environments is essential for life cycle progression and hence being a successful pathogen. For African trypanosomes, where the mammalian stage is exclusively extracellular, this presents specific challenges and requires evasion of both the acquired and innate immune systems, together with adaptation to a specific nutritional environment and resistance to mechanical and biochemical stresses. Here we consider the basis for these adaptations, the specific features of the mammalian infective trypanosome that are required to meet these challenges, and how these processes both inform on basic parasite biology and present potential therapeutic targets.

The distinct binding properties between avian/human influenza A virus NS1 and Postsynaptic density protein-95 (PSD-95), and inhibition of nitric oxide production

BACKGROUND

The NS1 protein of influenza A virus is able to bind with many proteins that affect cellular signal transduction and protein synthesis in infected cells. The NS1 protein consists of approximately 230 amino acids and the last 4 amino acids of the NS1 C-terminal form a PDZ binding motif. Postsynaptic Density Protein-95 (PSD-95), which is mainly expressed in neurons, has 3 PDZ domains. We hypothesise that NS1 binds to PSD-95, and this binding is able to affect neuronal function.

RESULT

We conducted a yeast two-hybrid analysis, GST-pull down assays and co-immunoprecipitations to detect the interaction between NS1 and PSD-95. The results showed that NS1 of avian influenza virus H5N1 (A/chicken/Guangdong/1/2005) is able to bind to PSD-95, whereas NS1 of human influenza virus H1N1 (A/Shantou/169/2006) is unable to do so. The results also revealed that NS1 of H5N1 significantly reduces the production of nitric oxide (NO) in rat hippocampal neurons.

CONCLUSION

In summary, our study indicates that NS1 of influenza A virus can bind with neuronal PSD-95, and the avian H5N1 and human H1N1 influenza A viruses possess distinct binding properties.

Acetaldehyde/alcohol dehydrogenase-2 (EhADH2) and clathrin are involved in internalization of human transferrin by Entamoeba histolytica

Transferrin (Tf) is a host glycoprotein capable of binding two ferric-iron ions to become holotransferrin (holoTf), which transports iron in to all cells. Entamoeba histolytica is a parasitic protozoan able to use holoTf as a sole iron source in vitro. The mechanism by which this parasite scavenges iron from holoTf is unknown. An E. histolytica holoTf-binding protein (EhTfbp) was purified by using an anti-human transferrin receptor (TfR) monoclonal antibody. EhTfbp was identified by MS/MS analysis and database searches as E. histolytica acetaldehyde/alcohol dehydrogenase-2 (EhADH2), an iron-dependent enzyme. Both EhTfbp and EhADH2 bound holoTf and were recognized by the anti-human TfR antibody, indicating that they correspond to the same protein. It was found that the amoebae internalized holoTf through clathrin-coated pits, suggesting that holoTf endocytosis could be important for the parasite during colonization and invasion of the intestinal mucosa and liver.

The plasma membrane of bloodstream-form African trypanosomes confers susceptibility and specificity to killing by hydrophobic peptides

Trypanosoma brucei is the causative agent of both a veterinary wasting disease and human African trypanosomiasis, or sleeping sickness. The cell membrane of the developmental stage found within the mammalian host, the bloodstream form (BSF), is highly dynamic, exhibiting rapid rates of endocytosis and lateral flow of glycosylphosphatidylinositol-anchored proteins. Here, we show that the cell membrane of these organisms is a target for killing by small hydrophobic peptides that increase the rigidity of lipid bilayers. Specifically, we have derived trypanocidal peptides that are based upon the hydrophobic N-terminal signal sequences of human apolipoproteins. These peptides selectively partitioned into the plasma membrane of BSF trypanosomes, resulting in an increase in the rigidity of the bilayer, dramatic changes in cell motility, and subsequent cell death. No killing of the developmental stage found within the insect midgut, the procyclic form, was observed. Additionally, the peptides exhibited no toxicity toward mammalian cell lines and did not induce hemolysis. Studies with model liposomes indicated that bilayer fluidity dictates the susceptibility of membranes to manipulation by hydrophobic peptides. We suggest that the composition of the BSF trypanosome cell membrane confers a high degree of fluidity and unique susceptibility to killing by hydrophobic peptides and is therefore a target for the development of trypanocidal drugs.

Evidence that low endocytic activity is not directly responsible for human serum resistance in the insect form of African trypanosomes

BACKGROUND

In Trypanosoma brucei, the African trypanosome, endocytosis is developmentally regulated and substantially more active in all known mammalian infective stages. In both mammalian and insect stages endocytic activity is likely required for nutrient acquisition, but in bloodstream forms increased endocytosis is involved in recycling the variant surface glycoprotein and removing host immune factors from the surface. However, a rationale for low endocytic activity in insect stages has not been explored. Here we asked if endocytic down-regulation in the procyclic form was associated with resistance to innate trypanolytic immune factors in the blood meal or tsetse fly midgut.

FINDINGS

Using a well-characterized procyclic parasite with augmented endocytic flux mediated via TbRab5A overexpression, we found that insect stage parasites were able to grow both in the presence of trypanosome lytic factor (TLF) provided in human serum, and also in tsetse flies. Additionally, by placing blood stage parasites in restricted glucose medium, we observed that enlargement of the flagellar pocket, a key morphology associated with defective endocytosis, manifests in parallel with loss of cellular ATP levels.

CONCLUSIONS

These observations suggest that a high rate of endocytosis per se is insufficient to render insect form parasites sensitive to TLF or tsetse-derived trypanocidal factors. However, the data do suggest that endocytosis is energetically burdensome, as endocytic activity is rapidly compromised on energy depletion in bloodstream stages. Hence an important aspect of endocytic modulation in the nutrient-poor tsetse midgut is likely energetic conservation.

Iron metabolism in trypanosomatids, and its crucial role in infection

Iron is almost ubiquitous in living organisms due to the utility of its redox chemistry. It is also dangerous as it can catalyse the formation of reactive free radicals - a classical double-edged sword. In this review, we examine the uptake and usage of iron by trypanosomatids and discuss how modulation of host iron metabolism plays an important role in the protective response. Trypanosomatids require iron for crucial processes including DNA replication, antioxidant defence, mitochondrial respiration, synthesis of the modified base J and, in African trypanosomes, the alternative oxidase. The source of iron varies between species. Bloodstream-form African trypanosomes acquire iron from their host by uptake of transferrin, and Leishmania amazonensis expresses a ZIP family cation transporter in the plasma membrane. In other trypanosomatids, iron uptake has been poorly characterized. Iron-withholding responses by the host can be a major determinant of disease outcome. Their role in trypanosomatid infections is becoming apparent. For example, the cytosolic sequestration properties of NRAMP1, confer resistance against leishmaniasis. Conversely, cytoplasmic sequestration of iron may be favourable rather than detrimental to Trypanosoma cruzi. The central role of iron in both parasite metabolism and the host response is attracting interest as a possible point of therapeutic intervention.

Bloodstream form trypanosome plasma membrane proteins: antigenic variation and invariant antigens

Trypanosoma bruceiis exposed to the adaptive immune system and complement in the blood of its mammalian hosts. The aim of this review is to analyse the role and regulation of the proteins present on the external face of the plasma membrane in the long-term persistence of an infection and transmission. In particular, the following are addressed: (1) antigenic variation of the variant surface glycoprotein (VSG), (2) the formation of an effective VSG barrier shielding invariant surface proteins, and (3) the rapid uptake of VSG antibody complexes combined with degradation of the immunoglobulin and recycling of the VSG.

The trypanosome Rab-related proteins RabX1 and RabX2 play no role in intracellular trafficking but may be involved in fly infectivity

Background

Rab GTPases constitute the largest subgroup of the Ras superfamily and are primarily involved in vesicle targeting. The full extent of Rab family function is unexplored. Several divergent Rab-like proteins are known but few have been characterized. In Trypanosoma brucei there are sixteen Rab genes, but RabX1, RabX2 and RabX3 are divergent within canonical sequence regions. Where known, trypanosome Rab functions are broadly conserved when orthologous relationships may be robustly established, but specific functions for RabX1, X2 and X3 have yet to be determined. RabX1 and RabX2 originated via tandem duplication and subcellular localization places RabX1 at the endoplasmic reticulum, while RabX2 is at the Golgi complex, suggesting distinct functions. We wished to determine whether RabX1 and RabX2 are involved in vesicle transport or other cellular processes.

Methodology/Principal Findings

Using comparative genomics we find that RabX1 and RabX2 are restricted to trypanosomatids. Gene knockout indicates that RabX1 and RabX2 are non-essential. Simultaneous RNAi knockdown of both RabX1 and RabX2, while partial, was also non-lethal and may suggest non-redundant function, consistent with the distinct locations of the proteins. Analysis of the knockout cell lines unexpectedly failed to uncover a defect in exocytosis, endocytosis or in the morphology or location of multiple markers for the endomembrane system, suggesting that neither RabX1 nor RabX2 has a major role in intracellular transport. However, it was apparent that RabX1 and RabX2 knockout cells displayed somewhat enhanced survival within flies.

Conclusions/Significance

RabX1 and RabX2, two members of the trypanosome Rab subfamily, were shown to have no major detectable role in intracellular transport, despite the localization of each gene product to highly specific endomembrane compartments. These data extend the functional scope of Rab proteins in trypanosomes to include non-canonical roles in differentiation-associated processes in protozoa.

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.

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.

The single ENTH-domain protein of trypanosomes; endocytic functions and evolutionary relationship with epsin

Epsin N-terminal homology (ENTH) domains occur in proteins of either the epsin or epsin-related (epsinR) form. They principally function in clathrin-mediated trafficking and membrane deformation. Both epsin and epsinR possess clathrin-binding motifs, but only epsin incorporates a ubiquitin-interaction motif (UIM). To better understand the origins of ENTH-domain proteins and their functions, we performed detailed comparative genomics and phylogenetics on the epsin family. The epsin ENTH-UIM configuration is an architecture restricted to yeast and animals. Further, we undertook functional analysis in Trypanosoma brucei (T. brucei), a divergent organism possessing a single ENTH-domain protein (TbEpsinR). TbEpsinR has a cellular location similar to both epsin and epsinR at plasma membrane clathrin budding sites and endosomal compartments, and associates with clathrin, as demonstrated by coimmunoprecipitation. Knockdown of TbEpsinR leads to a significant decrease in the intracellular pools of multiple surface antigens, without affecting bulk membrane internalization. Therefore, despite lacking the UIM, TbEpsinR maintains a similar role to metazoan epsin in endocytosis and participates as a clathrin-associated adaptor. We suggest that recruitment of a UIM to the ENTH-domain proteins was not essential for participation in endocytosis of ubiquitylated molecules, and is presumably a specific innovation restricted to higher eukaryotes.

Flagellum elongation is required for correct structure, orientation and function of the flagellar pocket in Trypanosoma brucei

In trypanosomes, the flagellum is rooted in the flagellar pocket, a surface micro-domain that is the sole site for endocytosis and exocytosis. By analysis of anterograde or retrograde intraflagellar transport in IFT88RNAi or IFT140RNAi mutant cells, we show that elongation of the new flagellum is not required for flagellar pocket formation but is essential for its organisation, orientation and function. Transmission electron microscopy revealed that the flagellar pocket exhibited a modified shape (smaller, distorted and/or deeper) in cells with abnormally short or no flagella. Scanning electron microscopy analysis of intact and detergent-extracted cells demonstrated that the orientation of the flagellar pocket collar was more variable in trypanosomes with short flagella. The structural protein BILBO1 was present but its localisation and abundance was altered. The membrane flagellar pocket protein CRAM leaked out of the pocket and reached the short flagella. CRAM also accumulated in intracellular compartments, indicating defects in routing of resident flagellar pocket proteins. Perturbations of vesicular trafficking were obvious; vesicles were observed in the lumen of the flagellar pocket or in the short flagella, and fluid-phase endocytosis was drastically diminished in non-flagellated cells. We propose a model to explain the role of flagellum elongation in correct flagellar pocket organisation and function.

Evolution of specificity in the eukaryotic endomembrane system

Two hundred years after Darwin's birth, our understanding of genetic mechanisms and cell biology has advanced to a level unimaginable in the 19th century. We now know that eukaryotic cells contain a huge variety of internal compartments, each with their own function, identity and history. For the compartments that together form the membrane-trafficking system, one of the central questions is how that identity is encoded and how it evolved. Here we review the key components involved in membrane-trafficking events, including SNAREs, Rabs, vesicle coats, and tethers and what is known about their evolutionary history. Our current understanding suggests a possible common mechanism by which the membrane-trafficking organelles might have evolved. This model of increased organellar complexity by gene duplication and co-evolution of multiple, interacting, specificity-encoding proteins could well be applicable to other non-endosymbiotic organelles as well. The application of basic evolutionary principles well beyond their original scope has been exceedingly powerful not only in reconstructing the history of cellular compartments, but for medical and applied research as well, and underlines the contributions of Darwin's ideas in modern biology.

The trypanosome transcriptome is remodelled during differentiation but displays limited responsiveness within life stages

BACKGROUND

Trypanosomatids utilise polycistronic transcription for production of the vast majority of protein-coding mRNAs, which operates in the absence of gene-specific promoters. Resolution of nascent transcripts by polyadenylation and trans-splicing, together with specific rates of mRNA turnover, serve to generate steady state transcript levels that can differ in abundance across several orders of magnitude and can be developmentally regulated. We used a targeted oligonucleotide microarray, representing the strongly developmentally-regulated T. brucei membrane trafficking system and approximately 10% of the Trypanosoma brucei genome, to investigate both between-stage, or differentiation-dependent, transcriptome changes and within-stage flexibility in response to various challenges.

RESULTS

6% of the gene cohort are developmentally regulated, including several small GTPases, SNAREs, vesicle coat factors and protein kinases both consistent with and extending previous data. Therefore substantial differentiation-dependent remodeling of the trypanosome transcriptome is associated with membrane transport. Both the microarray and qRT-PCR were then used to analyse transcriptome changes resulting from specific gene over-expression, knockdown, altered culture conditions and chemical stress. Firstly, manipulation of Rab5 expression results in co-ordinate changes to clathrin protein expression levels and endocytotic activity, but no detectable changes to steady-state mRNA levels, which indicates that the effect is mediated post-transcriptionally. Secondly, knockdown of clathrin or the variant surface glycoprotein failed to perturb transcription. Thirdly, exposure to dithiothreitol or tunicamycin revealed no evidence for a classical unfolded protein response, mediated in higher eukaryotes by transcriptional changes. Finally, altered serum levels invoked little transcriptome alteration beyond changes to expression of ESAG6/7, the transferrin receptor.

CONCLUSION

While trypanosomes regulate mRNA abundance to effect the major changes accompanying differentiation, a given differentiated state appears transcriptionally inflexible. The implications of the absence of a transcriptome response in trypanosomes for both virulence and models of life cycle progression are discussed.