Glycosylphosphatidylinositol-dependent protein trafficking in bloodstream stage Trypanosoma 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.

Stage-Specific COPII-Mediated Cargo Selectivity in African Trypanosomes

A hallmark of eukaryotic cells is the ability to form a secretory pathway connecting many intracellular compartments. In the early secretory pathway, coated protein complex II (COPII)-coated vesicles mediate the anterograde transport of newly synthesized secretory cargo from the endoplasmic reticulum to the Golgi apparatus. The COPII coat complex is comprised of an inner layer of Sec23/Sec24 heterodimers and an outer layer of Sec13/Sec31 heterotetramers. In African trypanosomes, there are two paralogues each of Sec23 and Sec24, that form obligate heterodimers (TbSec23.2/TbSec24.1, TbSec23.1/TbSec24.2). It is not known if these form distinct homotypic classes of vesicles or one heterotypic class, but it is known that TbSec23.2/TbSec24.1 specifically mediate forward trafficking of GPI-anchored proteins (GPI-APs) in bloodstream-form trypanosomes (BSF). Here, we showed that this selectivity was lost in insect procyclic stage parasites (PCF). All isoforms of TbSec23 and TbSec24 are essential in PCF parasites as judged by RNAi knockdowns. RNAi silencing of each subunit had equivalent effects on the trafficking of GPI-APs and p67, a transmembrane lysosomal protein. However, silencing of the TbSec23.2/TbSec24.1 had heterodimer had a significant impact on COPII mediated trafficking of soluble TbCatL from the ER to the lysosome. This finding suggests a model in which selectivity of COPII transport was altered between the BSF and PCF trypanosomes, possibly as an adaptation to a digenetic life cycle.

IMPORTANCE African trypanosomes synthesize dense surface coats composed of stage-specific glycosylphosphatidylinositol lipid anchored proteins. We previously defined specific machinery in bloodstream stage parasites that mediate the exit of these proteins from the endoplasmic reticulum. Here, we performed similar analyses in the procyclic insect stage and found significant differences in this process. These findings contribute to our understanding of secretory processes in this unusual eukaryotic model system.

Turnover of Variant Surface Glycoprotein in Trypanosoma brucei Is Not Altered in Response to Specific Silencing

African trypanosomes evade the immune system of the mammalian host by the antigenic variation of the predominant glycosylphosphatidylinositol (GPI)-anchored surface protein, variant surface glycoprotein (VSG). VSG is a very stable protein that is turned over from the cell surface with a long half-life (~26 h), allowing newly synthesized VSG to populate the surface. We have recently demonstrated that VSG turnover under normal growth is mediated by a combination of GPI hydrolysis and direct shedding with intact GPI anchors. VSG synthesis is tightly regulated in dividing trypanosomes, and when subjected to RNA interference (RNAi) silencing, cells display rapid cell cycle arrest in order to conserve VSG density on the cell surface (K. Sheader, S. Vaughan, J. Minchin, K. Hughes, et al., Proc Natl Acad Sci U S A 102:8716-8721, 2005, https://doi.org/10.1073/pnas.0501886102). Arrested cells also display an altered morphology of secretory organelles-engorgement of the trans-Golgi cisternae-that may reflect a disruption of post-Golgi secretory transport. We now ask whether trypanosomes under VSG silencing also reduce the rate of VSG turnover to further conserve coat density. Our data indicate that trypanosomes do not regulate VSG turnover according to VSG protein abundance, nor was there any effect on the post-Golgi transport of soluble or GPI-anchored secretory cargo. However, the surface morphology of silenced cells was altered from a typically rugose topology to a smoother profile, consistent with reduced overall membrane trafficking to the cell surface. IMPORTANCE African trypanosomes evade the host immune system by altering the expression of variant surface glycoproteins (VSGs) in a process called antigenic variation. VSG is essential, and when its synthesis is ablated by RNAi silencing, cells enter precytokinesis growth arrest as a means to maintain constant cell surface VSG levels. We have investigated whether arrested cells also alter the rate of natural VSG turnover as a means to conserve the surface coat. This work provides insights into the natural biology of the glycocalyx of this important human and veterinary parasite.

Dynamic, variable oligomerization and the trafficking of variant surface glycoproteins of Trypanosoma brucei

African trypanosomes cause disease in humans and livestock, avoiding host immunity by changing the expression of variant surface glycoproteins (VSGs); the major glycosylphosphatidylinositol (GPI) anchored antigens coating the surface of the bloodstream stage. Proper trafficking of VSGs is therefore critical to pathogen survival. The valence model argues that GPI anchors regulate progression and fate in the secretory pathway and that, specifically, a valence of two (VSGs are dimers) is critical for stable cell surface association. However, recent reports that the MITat1.3 (M1.3) VSG N-terminal domain (NTD) behaves as a monomer in solution and in a crystal structure challenge this model. We now show that the behavior of intact M1.3 VSG in standard in vivo trafficking assays is consistent with an oligomer. Nevertheless, Blue Native Gel electrophoresis and size exclusion chromatography-multiangle light scattering chromatography of purified full length M1.3 VSG indicates a monomer in vitro. However, studies with additional VSGs show that multiple oligomeric states are possible, and that for some VSGs oligomerization is concentration dependent. These data argue that individual VSG monomers possess different propensities to self-oligomerize, but that when constrained at high density to the cell surface, oligomeric species predominate. These results resolve the apparent conflict between the valence hypothesis and the M1.3 NTD VSG crystal structure.

Fluorescent tagging of Plasmodium circumsporozoite protein allows imaging of sporozoite formation but blocks egress from oocysts

The circumsporozoite protein, CSP, is the major surface protein of Plasmodium sporozoites, the form of malaria parasites transmitted by mosquitoes. CSP is involved in sporozoite formation within and egress from oocysts, entry into mosquito salivary glands and mammalian liver as well as migration in the skin. Yet, how CSP facilitates sporozoite formation, oocyst egress and hepatocyte specific invasion is still not fully understood. Here, we aimed at generating a series of parasites expressing full-length versions of CSP with internally inserted green fluorescent protein between known domains at the endogenous csp locus. This enabled the investigation of sporozoite formation in living oocysts. GFP insertion after the signal peptide leads to cleavage of GFP before the fusion protein reached the plasma membrane while insertion of GFP before or after the TSR domain prevented sporozoite egress and liver invasion. These data suggest different strategies for obtaining mature salivary gland sporozoites that express GFP-CSP fusions.

Trypanosoma brucei transferrin receptor: Functional replacement of the GPI anchor with a transmembrane domain

The transferrin receptor of Trypanosoma brucei (TbTfR) is a heterodimer of a glycosylphosphatidylinositol (GPI)-anchored ESAG6 subunit and an ESAG7 subunit. To investigate whether the GPI-anchor is essential for the function of the TbTfR, an ESAG6 with a transmembrane domain instead of a GPI-anchor (ESAG6tmd) was inducibly expressed in bloodstream form trypanosomes. It is shown that the ESAG6tmd is able to dimerise with ESAG7 to form a TbTfR that can bind transferrin. Fractionation experiments clearly demonstrated that the transmembrane-anchored TbTfR is exclusively associated with the membrane fraction. No difference in the uptake of transferrin was observed between trypanosomes inducibly expressing a transmembrane-anchored TbTfR and trypanosomes inducibly expressing a GPI-anchored TbTfR. Differences in glycosylation pattern of ESAG6tmd and native ESAG6 may indicate different intracellular trafficking of transmembrane- and GPI-anchored TbTfRs. The findings suggest that the GPI-anchor is not essential for the function of the TbTfR in bloodstream forms of T. brucei.

VSG mRNA levels are regulated by the production of functional VSG protein

The bloodstream form of Trypanosoma brucei persists in mammalian hosts through a population survival strategy depending on antigenic variation of a cell surface coat composed of the variant surface glycoprotein (VSG). The integrity of the VSG coat is essential and blocking its synthesis results in a cell division cycle arrest just prior to cytokinesis. This observation indicates that VSG levels are monitored and that the cell has mechanisms to respond to a disruption of synthesis. Here, the regulation of VSG mRNA levels has been investigated by first measuring VSG mRNA copy number, and second using ectopic expression of VSG transgenes containing premature termination codons. The findings are that (i) VSG mRNA copy number varies with the identity of the VSG and (ii) a pathway detects synthesis of non-functional VSG protein and results in an increase in VSG mRNA levels.

p97 Inhibitor CB-5083 Blocks ERAD in Trypanosoma brucei

Misfolded proteins trapped in the endoplasmic reticulum (ER) are specifically recognized and retrotranslocated to the cytosol by the ER-Associated Degradation (ERAD) system and delivered to the proteasome for destruction. This process was recently described in Trypanosoma brucei (T. brucei) using the misfolded epitope tagged Transferrin Receptor subunits ESAG7:Ty and HA:ESAG6 (HA:E6). Critical to this work was the proteasomal inhibitor MG132. However, MG132 has off-target inhibitory effects on lysosomal Cathepsin L that could cause misinterpretation of turnover results. Here, we evaluate an orally bioavailable p97 inhibitor, CB-5083, for use in T. brucei. p97 is a ubiquitous protein involved in many cellular events including the membrane extraction step of ERAD. CB-5083 strongly inhibits turnover of HA:E6, with comparable protein recovery to MG132 treatment. Interestingly, little deglycosylated cytoplasmic species accumulates, though it normally emerges with MG132 treatment. This suggests that CB-5083 blocks ERAD upstream of the proteasome, as expected for inhibition of the trypanosomal p97 orthologue TbVCP. Under CB-5083 treatment, HA:E6 is also strongly membrane-associated, suggesting ER localization. Finally, we provide an experimental example where CB-5083 treatment offers clarity to the off-target effects of MG132 treatment.

Evolution of Antigenic Variation in African Trypanosomes: Variant Surface Glycoprotein Expression, Structure, and Function

The process of antigenic variation in parasitic African trypanosomes is a remarkable mechanism for outwitting the immune system of the mammalian host, but it requires a delicate balancing act for the monoallelic expression, folding and transport of a single variant surface glycoprotein (VSG). Only one of hundreds of VSG genes is expressed at time, and this from just one of ≈15 dedicated expression sites. By switching expression of VSGs the parasite presents a continuously shifting antigenic facade leading to prolonged chronic infections lasting months to years. The basics of VSG structure and switching have been known for several decades, but recent studies have brought higher resolution to many aspects this process. New VSG structures, in silico modeling of infections, studies of VSG codon usage, and experimental ablation of VSG expression provide insights that inform how this remarkable system may have evolved.

Endoplasmic reticulum-associated degradation and disposal of misfolded GPI-anchored proteins in Trypanosoma brucei

Misfolded secretory proteins are retained by endoplasmic reticulum quality control (ERQC) and degraded in the proteasome by ER-associated degradation (ERAD). However, in yeast and mammals, misfolded glycosylphosphatidylinositol (GPI)-anchored proteins are preferentially degraded in the vacuole/lysosome. We investigate this process in the divergent eukaryotic pathogen Trypanosoma brucei using a misfolded GPI-anchored subunit (HA:E6) of the trypanosome transferrin receptor. HA:E6 is N-glycosylated and GPI-anchored and accumulates in the ER as aggregates. Treatment with MG132, a proteasome inhibitor, generates a smaller protected polypeptide (HA:E6*), consistent with turnover in the proteasome. HA:E6* partitions between membrane and cytosol fractions, and both pools are proteinase K-sensitive, indicating cytosolic disposition of membrane-associated HA:E6*. HA:E6* is de-N-glycosylated and has a full GPI-glycan structure from which dimyristoylglycerol has been removed, indicating that complete GPI removal is not a prerequisite for proteasomal degradation. However, HA:E6* is apparently not ubiquitin-modified. The trypanosome GPI anchor is a forward trafficking signal; thus the dynamic tension between ERQC and ER exit favors degradation by ERAD. These results differ markedly from the standard eukaryotic model systems and may indicate an evolutionary advantage related to pathogenesis.

Genome-wide analysis of Excretory/Secretory proteins in Trypanosoma brucei brucei: Insights into functional characteristics and identification of potential targets by immunoinformatics approach

Trypanosoma brucei brucei (T.b.brucei) is an extra-cellular parasite that causes Animal African Trypanosomiasis (AAT) disease in animals. Till day, this disease is more difficult to treat and control due to lack of efficient vaccines and early diagnosis of the parasite infection. T.b.brucei Excretory/Secretory (ES) proteins were involved in pathogenesis and key for understanding the host-parasite interactions. Functions of T.b.brucei's ES proteins were poorly investigated and experimental identification is expensive and time-consuming. Bioinformatics approaches are cost-effective by facilitating the experimental analysis of potential drug targets for parasitic diseases. Here we applied several bioinformatics tools to predict and functionalize the annotation of 1104 ES proteins and immunoinformatics approaches carried out to predict and evaluate the epitopes in T.b.brucei. Secretory information, functional annotations and potential epitopes of each ES proteins were available at http://tbb.insilico.in. This study provides functional information of T.b.brucei for experimental studies to identify potential targets for diagnosis and therapeutics development.

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.

Life Stage-Specific Cargo Receptors Facilitate Glycosylphosphatidylinositol-Anchored Surface Coat Protein Transport in Trypanosoma brucei

The critical virulence factor of bloodstream-form Trypanosoma brucei is the glycosylphosphatidylinositol (GPI)-anchored variant surface glycoprotein (VSG). Endoplasmic reticulum (ER) exit of VSG is GPI dependent and relies on a discrete subset of COPII machinery (TbSec23.2/TbSec24.1). In other systems, p24 transmembrane adaptor proteins selectively recruit GPI-anchored cargo into nascent COPII vesicles. Trypanosomes have eight putative p24s (TbERP1 to TbERP8) that are constitutively expressed at the mRNA level. However, only four TbERP proteins (TbERP1, -2, -3, and -8) are detectable in bloodstream-form parasites. All four colocalize to ER exit sites, are required for efficient GPI-dependent ER exit, and are interdependent for steady-state stability. These results suggest shared function as an oligomeric ER GPI-cargo receptor. This cohort also mediates rapid forward trafficking of the soluble lysosomal hydrolase TbCatL. Procyclic insect-stage trypanosomes have a distinct surface protein, procyclin, bearing a different GPI anchor structure. A separate cohort of TbERP proteins (TbERP1, -2, -4, and -8) are expressed in procyclic parasites and also function in GPI-dependent ER exit. Collectively, these results suggest developmentally regulated TbERP cohorts, likely in obligate assemblies, that may recognize stage-specific GPI anchors to facilitate GPI-cargo trafficking throughout the parasite life cycle. IMPORTANCE African trypanosomes are protozoan parasites that cause African sleeping sickness. Critical to the success of the parasite is the variant surface glycoprotein (VSG), which covers the parasite cell surface and which is essential for evasion of the host immune system. VSG is membrane bound by a glycolipid (GPI) anchor that is attached in the earliest compartment of the secretory pathway, the endoplasmic reticulum (ER). We have previously shown that the anchor acts as a positive forward trafficking signal for ER exit, implying a cognate receptor mechanism for GPI recognition and loading in coated cargo vesicles leaving the ER. Here, we characterize a family of small transmembrane proteins that act at adaptors for this process. This work adds to our understanding of general GPI function in eukaryotic cells and specifically in the synthesis and transport of the critical virulence factor of pathogenic African trypanosomes.

Controlling transferrin receptor trafficking with GPI-valence in bloodstream stage African trypanosomes

Bloodstream-form African trypanosomes encode two structurally related glycosylphosphatidylinositol (GPI)-anchored proteins that are critical virulence factors, variant surface glycoprotein (VSG) for antigenic variation and transferrin receptor (TfR) for iron acquisition. Both are transcribed from the active telomeric expression site. VSG is a GPI² homodimer; TfR is a GPI¹ heterodimer of GPI-anchored ESAG6 and ESAG7. GPI-valence correlates with secretory progression and fate in bloodstream trypanosomes: VSG (GPI²) is a surface protein; truncated VSG (GPI⁰) is degraded in the lysosome; and native TfR (GPI¹) localizes in the flagellar pocket. Tf:Fe starvation results in up-regulation and redistribution of TfR to the plasma membrane suggesting a saturable mechanism for flagellar pocket retention. However, because such surface TfR is non-functional for ligand binding we proposed that it represents GPI² ESAG6 homodimers that are unable to bind transferrin—thereby mimicking native VSG. We now exploit a novel RNAi system for simultaneous lethal silencing of all native TfR subunits and exclusive in-situ expression of RNAi-resistant TfR variants with valences of GPI^0–2. Our results conform to the valence model: GPI⁰ ESAG7 homodimers traffick to the lysosome and GPI² ESAG6 homodimers to the cell surface. However, when expressed alone ESAG6 is up-regulated ~7-fold, leaving the issue of saturable retention in the flagellar pocket in question. Therefore, we created an RNAi-resistant GPI² TfR heterodimer by fusing the C-terminal domain of ESAG6 to ESAG7. Co-expression with ESAG6 generates a functional heterodimeric GPI² TfR that restores Tf uptake and cell viability, and localizes to the cell surface, without overexpression. These results resolve the longstanding issue of TfR trafficking under over-expression and confirm GPI valence as a critical determinant of intracellular sorting in trypanosomes.

African trypanosomes, protozoan parasites that cause African Sleeping Sickness, have two structurally related secretory proteins that are critical for their success as pathogens: variant surface glycoprotein (VSG), which is responsible for evasion of host immune responses, and transferrin receptor (TfR), which is responsible for acquisition of essential iron for nutritional purposes. Both are dimers and both are attached to cell membranes by glycolipid anchors. VSG has two anchors and is found on the outer plasma membrane; TfR has just one anchor and is found in the flagellar pocket, a small restricted invagination of the plasma membrane that is the portal for transport in and out of the cell. These locations are critical to the function of each protein. To test the hypothesis that number of anchors, or valence, controls the localization of these proteins we have genetically engineered a trypanosome cell line that allows controlled expression of TfR with 0, 1, or 2 glycolipid anchors. Detailed studies of the localization and intracellular trafficking of these reporters confirm that glycolipid valence controls ultimate localization, and thus is critical to the essential functions of both VSG and TfR.

Blocking Synthesis of the Variant Surface Glycoprotein Coat in Trypanosoma brucei Leads to an Increase in Macrophage Phagocytosis Due to Reduced Clearance of Surface Coat Antibodies

The extracellular bloodstream form parasite Trypanosoma brucei is supremely adapted to escape the host innate and adaptive immune system. Evasion is mediated through an antigenically variable Variant Surface Glycoprotein (VSG) coat, which is recycled at extraordinarily high rates. Blocking VSG synthesis triggers a precytokinesis arrest where stalled cells persist for days in vitro with superficially intact VSG coats, but are rapidly cleared within hours in mice. We therefore investigated the role of VSG synthesis in trypanosome phagocytosis by activated mouse macrophages. T. brucei normally effectively evades macrophages, and induction of VSG RNAi resulted in little change in phagocytosis of the arrested cells. Halting VSG synthesis resulted in stalled cells which swam directionally rather than tumbling, with a significant increase in swim velocity. This is possibly a consequence of increased rigidity of the cells due to a restricted surface coat in the absence of VSG synthesis. However if VSG RNAi was induced in the presence of anti-VSG221 antibodies, phagocytosis increased significantly. Blocking VSG synthesis resulted in reduced clearance of anti-VSG antibodies from the trypanosome surface, possibly as a consequence of the changed motility. This was particularly marked in cells in the G2/ M cell cycle stage, where the half-life of anti-VSG antibody increased from 39.3 ± 4.2 seconds to 99.2 ± 15.9 seconds after induction of VSG RNAi. The rates of internalisation of bulk surface VSG, or endocytic markers like transferrin, tomato lectin or dextran were not significantly affected by the VSG synthesis block. Efficient elimination of anti-VSG-antibody complexes from the trypanosome cell surface is therefore essential for trypanosome evasion of macrophages. These experiments highlight the essentiality of high rates of VSG recycling for the rapid removal of host opsonins from the parasite surface, and identify this process as a key parasite virulence factor during a chronic infection.

Surface proteins, ERAD and antigenic variation in Trypanosoma brucei

Variant surface glycoprotein (VSG) is central to antigenic variation in African trypanosomes. Although much prior work documents that VSG is efficiently synthesized and exported to the cell surface, it was recently claimed that 2-3 fold more is synthesized than required, the excess being eliminated by ER-Associated Degradation (ERAD) (Field et al., ). We now reinvestigate VSG turnover and find no evidence for rapid degradation, consistent with a model whereby VSG synthesis is precisely regulated to match requirements for a functional surface coat on each daughter cell. However, using a mutated version of the ESAG7 subunit of the transferrin receptor (E7:Ty) we confirm functional ERAD in trypanosomes. E7:Ty fails to assemble into transferrin receptors and accumulates in the ER, consistent with retention of misfolded protein, and its turnover is selectively rescued by the proteasomal inhibitor MG132. We also show that ER accumulation of E7:Ty does not induce an unfolded protein response. These data, along with the presence of ERAD orthologues in the Trypanosoma brucei genome, confirm ERAD in trypanosomes. We discuss scenarios in which ERAD could be critical to bloodstream parasites, and how these may have contributed to the evolution of antigenic variation in trypanosomes.

Acylation in trypanosomatids: an essential process and potential drug target

Fatty acylation--the addition of fatty acid moieties such as myristate and palmitate to proteins--is essential for the survival, growth, and infectivity of the trypanosomatids: Trypanosoma brucei, Trypanosoma cruzi, and Leishmania. Myristoylation and palmitoylation are critical for parasite growth, targeting and localization, and the intrinsic function of some proteins. The trypanosomatids possess a single N-myristoyltransferase (NMT) and multiple palmitoyl acyltransferases, and these enzymes and their protein targets are only now being characterized. Global inhibition of either process leads to cell death in trypanosomatids, and genetic ablation of NMT compromises virulence. Moreover, NMT inhibitors effectively cure T. brucei infection in rodents. Thus, protein acylation represents an attractive target for the development of new trypanocidal drugs.

Life and times: synthesis, trafficking, and evolution of VSG

Evasion of the acquired immune response in African trypanosomes is principally mediated by antigenic variation, the sequential expression of distinct variant surface glycoproteins (VSGs) at extremely high density on the cell surface. Sequence diversity between VSGs facilitates escape of a subpopulation of trypanosomes from antibody-mediated killing. Significant advances have increased understanding of the mechanisms underpinning synthesis and maintenance of the VSG coat. In this review, we discuss the biosynthesis, trafficking, and turnover of VSG, emphasising those unusual mechanisms that act to maintain coat integrity and to protect against immunological attack. We also highlight new findings that suggest the presence of unique or highly divergent proteins that may offer therapeutic opportunities, as well as considering aspects of VSG biology that remain to be fully explored.

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.

Intracellular trafficking and glycobiology of TbPDI2, a stage-specific protein disulfide isomerase in Trypanosoma brucei

Trypanosoma brucei protein disulfide isomerase 2 (TbPDI2) is a bloodstream stage-specific lumenal endoplasmic reticulum (ER) glycoprotein. ER localization is dependent on the TbPDI2 C-terminal tetrapeptide (KQDL) and is mediated by TbERD2, an orthologue of the yeast ER retrieval receptor. Consistent with this function, TbERD2 localizes prominently to ER exit sites, and RNA interference (RNAi) knockdown results in specific secretion of a surrogate ER retention reporter, BiPN:KQDL. TbPDI2 is highly N-glycosylated and is reactive with tomato lectin, suggesting the presence of poly-N-acetyllactosamine modifications, which are common on lyso/endosomal proteins in trypanosomes but are inconsistent with ER localization. However, TbPDI2 is reactive with tomato lectin immediately following biosynthesis-far too rapidly for transport to the Golgi compartment, the site of poly-N-acetyllactosamine addition. TbPDI2 also fails to react with Erythrina cristagalli lectin, confirming the absence of terminal N-acetyllactosamine units. We propose that tomato lectin binds the Manβ1-4GlcNAcβ1-4GlcNAc trisaccharide core of paucimannose glycans on both newly synthesized and mature TbPDI2. Consistent with this proposal, α-mannosidase treatment renders oligomannose N-glycans on the T. brucei cathepsin L orthologue TbCatL reactive with tomato lectin. These findings resolve contradictory evidence on the location and glycobiology of TbPDI2 and provide a cautionary note on the use of tomato lectin as a poly-N-acetyllactosamine-specific reagent.

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.

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.

Endosomal localization of the serum resistance-associated protein in African trypanosomes confers human infectivity

Trypanosoma brucei rhodesiense is the causative agent of human African sleeping sickness. While the closely related subspecies T. brucei brucei is highly susceptible to lysis by a subclass of human high-density lipoproteins (HDL) called trypanosome lytic factor (TLF), T. brucei rhodesiense is resistant and therefore able to establish acute and fatal infections in humans. This resistance is due to expression of the serum resistance-associated (SRA) gene, a member of the variant surface glycoprotein (VSG) gene family. Although much has been done to establish the role of SRA in human serum resistance, the specific molecular mechanism of SRA-mediated resistance remains a mystery. Thus, we report the trafficking and steady-state localization of SRA in order to provide more insight into the mechanism of SRA-mediated resistance. We show that SRA traffics to the flagellar pocket of bloodstream-form T. brucei organisms, where it localizes transiently before being endocytosed to its steady-state localization in endosomes, and we demonstrate that the critical point of colocalization between SRA and TLF occurs intracellularly.

Immunobiology of African trypanosomes: need of alternative interventions

Trypanosomiasis is one of the major parasitic diseases for which control is still far from reality. The vaccination approaches by using dominant surface proteins have not been successful, mainly due to antigenic variation of the parasite surface coat. On the other hand, the chemotherapeutic drugs in current use for the treatment of this disease are toxic and problems of resistance are increasing (see Kennedy (2004) and Legros et al. (2002)). Therefore, alternative approaches in both treatment and vaccination against trypanosomiasis are needed at this time. To be able to design and develop such alternatives, the biology of this parasite and the host response against the pathogen need to be studied. These two aspects of this disease with few examples of alternative approaches are discussed here.

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.

Streamlined architecture and glycosylphosphatidylinositol-dependent trafficking in the early secretory pathway of African trypanosomes

The variant surface glycoprotein (VSG) of bloodstream form Trypanosoma brucei (Tb) is a critical virulence factor. The VSG glycosylphosphatidylinositol (GPI)-anchor strongly influences passage through the early secretory pathway. Using a dominant-negative mutation of TbSar1, we show that endoplasmic reticulum (ER) exit of secretory cargo in trypanosomes is dependent on the coat protein complex II (COPII) machinery. Trypanosomes have two orthologues each of the Sec23 and Sec24 COPII subunits, which form specific heterodimeric pairs: TbSec23.1/TbSec24.2 and TbSec23.2/TbSec24.1. RNA interference silencing of each subunit is lethal but has minimal effects on trafficking of soluble and transmembrane proteins. However, silencing of the TbSec23.2/TbSec24.1 pair selectively impairs ER exit of GPI-anchored cargo. All four subunits colocalize to one or two ER exit sites (ERES), in close alignment with the postnuclear flagellar adherence zone (FAZ), and closely juxtaposed to corresponding Golgi clusters. These ERES are nucleated on the FAZ-associated ER. The Golgi matrix protein Tb Golgi reassembly stacking protein defines a region between the ERES and Golgi, suggesting a possible structural role in the ERES:Golgi junction. Our results confirm a selective mechanism for GPI-anchored cargo loading into COPII vesicles and a remarkable degree of streamlining in the early secretory pathway. This unusual architecture probably maximizes efficiency of VSG transport and fidelity in organellar segregation during cytokinesis.

Fate of glycosylphosphatidylinositol (GPI)-less procyclin and characterization of sialylated non-GPI-anchored surface coat molecules of procyclic-form Trypanosoma brucei

A Trypanosoma brucei TbGPI12 null mutant that is unable to express cell surface procyclins and free glycosylphosphatidylinositols (GPI) revealed that these are not the only surface coat molecules of the procyclic life cycle stage. Here, we show that non-GPI-anchored procyclins are N-glycosylated, accumulate in the lysosome, and appear as proteolytic fragments in the medium. We also show, using lectin agglutination and galactose oxidase-NaB(3)H(4) labeling, that the cell surface of the TbGPI12 null parasites contains glycoconjugates that terminate in sialic acid linked to galactose. Following desialylation, a high-apparent-molecular-weight glycoconjugate fraction was purified by ricin affinity chromatography and gel filtration and shown to contain mannose, galactose, N-acetylglucosamine, and fucose. The latter has not been previously reported in T. brucei glycoproteins. A proteomic analysis of this fraction revealed a mixture of polytopic transmembrane proteins, including P-type ATPase and vacuolar proton-translocating pyrophosphatase. Immunolocalization studies showed that both could be labeled on the surfaces of wild-type and TbGPI12 null cells. Neither galactose oxidase-NaB(3)H(4) labeling of the non-GPI-anchored surface glycoconjugates nor immunogold labeling of the P-type ATPase was affected by the presence of procyclins in the wild-type cells, suggesting that the procyclins do not, by themselves, form a macromolecular barrier.

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.

A parasite cysteine protease is key to host protein degradation and iron acquisition

Cysteine proteases of the Clan CA (papain) family are the predominant protease group in primitive invertebrates. Cysteine protease inhibitors arrest infection by the protozoan parasite, Trypanosoma brucei. RNA interference studies implicated a cathepsin B-like protease, tbcatB, as a key inhibitor target. Utilizing parasites in which one of the two alleles of tbcatb has been deleted, the key role of this protease in degradation of endocytosed host proteins is delineated. TbcatB deficiency results in a decreased growth rate and dysmorphism of the flagellar pocket and the subjacent endocytic compartment. Western blot and microscopic analysis indicate that deficiency in tbcatB results in accumulation of both host and parasite proteins, including the lysosomal marker p67. A critical function for parasitism is the degradation of host transferrin, which is necessary for iron acquisition. Substrate specificity analysis of recombinant tbcatB revealed the optimal peptide cleavage sequences for the enzyme and these were confirmed experimentally using FRET-based substrates. Degradation of transferrin was validated by SDS-PAGE and the specific cleavage sites identified by N-terminal sequencing. Because even a modest deficiency in tbcatB is lethal for the parasite, tbcatB is a logical target for the development of new anti-trypanosomal chemotherapy.

RNA interference of Trypanosoma brucei cathepsin B and L affects disease progression in a mouse model

We investigated the roles played by the cysteine proteases cathepsin B and cathepsin L (brucipain) in the pathogenesis of Trypansoma brucei brucei in both an in vivo mouse model and an in vitro model of the blood–brain barrier. Doxycycline induction of RNAi targeting cathepsin B led to parasite clearance from the bloodstream and prevent a lethal infection in the mice. In contrast, all mice infected with T. brucei containing the uninduced Trypanosoma brucei cathepsin B (TbCatB) RNA construct died by day 13. Induction of RNAi against brucipain did not cure mice from infection; however, 50% of these mice survived 60 days longer than uninduced controls. The ability of T. b. brucei to cross an in vitro model of the human blood–brain barrier was also reduced by brucipain RNAi induction. Taken together, the data suggest that while TbCatB is the more likely target for the development of new chemotherapy, a possible role for brucipain is in facilitating parasite entry into the brain.

African trypanosomiasis, or sleeping sickness, is caused by the single-cell parasite Trypanosoma brucei (T. brucei). Two parasite-derived enzyme proteins have been hypothesized to play an important role in the viability of the parasite or its ability to produce disease in the human host. Utilizing RNA interference that blocks the production of these proteins in the parasite, we show that elimination of parasite cathepsin B cures infection in mice. RNAi of the second enzyme protein, brucipain, results in the prolongation of life of half the infected mice, but does not cure. Further experiments carried out in a culture system show that brucipain facilitates the migration of parasites across a model of the blood–brain barrier. This suggests that while brucipain is not necessary for the viability of the organisms, it may play a role in infection by allowing parasites to reach the central nervous system and produce the severe second stage of sleeping sickness.

Role of protein translocation pathways across the endoplasmic reticulum in Trypanosoma brucei

The translocation of secretory and membrane proteins across the endoplasmic reticulum (ER) membrane is mediated by co-translational (via the signal recognition particle (SRP)) and post-translational mechanisms. In this study, we investigated the relative contributions of these two pathways in trypanosomes. A homologue of SEC71, which functions in the post-translocation chaperone pathway in yeast, was identified and silenced by RNA interference. This factor is essential for parasite viability. In SEC71-silenced cells, signal peptide (SP)-containing proteins traversed the ER, but several were mislocalized, whereas polytopic membrane protein biogenesis was unaffected. Surprisingly trypanosomes can interchangeably utilize two of the pathways to translocate SP-containing proteins except for glycosylphosphatidylinositol-anchored proteins, whose level was reduced in SEC71-silenced cells but not in cells depleted for SRP68, an SRP-binding protein. Entry of SP-containing proteins to the ER was significantly blocked only in cells co-silenced for the two translocation pathways (SEC71 and SRP68). SEC63, a factor essential for both translocation pathways in yeast, was identified and silenced by RNA interference. SEC63 silencing affected entry to the ER of both SP-containing proteins and polytopic membrane proteins, suggesting that, as in yeast, this factor is essential for both translocation pathways in vivo. This study suggests that, unlike bacteria or other eukaryotes, trypanosomes are generally promiscuous in their choice of mechanism for translocating SP-containing proteins to the ER, although the SRP-independent pathway is favored for glycosylphosphatidylinositol-anchored proteins, which are the most abundant surface proteins in these parasites.

The LAMP-like protein p67 plays an essential role in the lysosome of African trypanosomes

RNAi knockdown was employed to study the function of p67, a lysosome-associated membrane protein (LAMP)-like type I transmembrane lysosomal glycoprotein in African trypanosomes. Conditional induction of p67 dsRNA resulted in specific approximately 90% reductions in de novo p67 synthesis in both mammalian bloodstream and procyclic insect-stage parasites. Bloodstream cell growth was severely retarded with extensive death after > 24 h of induction. Biosynthetic trafficking of residual p67, and of the soluble lysosomal protease trypanopain, were unimpaired. Endocytosis of tomato lectin, a surrogate receptor-mediated cargo, was only mildly impaired (approximately 20%), but proper lysosomal targeting was unaffected. p67 ablation had dramatic effects on lysosomal morphology with gross enlargement (four- to fivefold) and internal membrane profiles reminiscent of autophagic vacuoles. Ablation of p67 expression rendered bloodstream trypanosomes refractory to lysis by human trypanolytic factor (TLF), a lysosomally activated host innate immune mediator. Similar effects on lysosomal morphology and TLF sensitivity were also obtained by two pharmacological agents that neutralize lysosomal pH--chloroquine and bafilomycin A1. Surprisingly, however, lysosomal pH was not affected in ablated cells suggesting that other physiological alterations must account for increased resistance to TLF. These results indicate p67 plays an essential role in maintenance of normal lysosomal structure and physiology in bloodstream-stage African trypanosomes.

Intracellular Trafficking in the Trypanosomatids

Trypanosomes are members of the kinetoplastida, a group of divergent protozoan parasites responsible for considerable morbidity and mortality worldwide. These organisms have highly complex life cycles requiring modification of their cell surface together with engagement of immune evasion systems to effect survival; both processes intimately involve the membrane trafficking system. The completion of three trypanosomatid and several additional protist genomes in the last few years is providing an exciting opportunity to evaluate, at the molecular level, the evolution and diversity of membrane trafficking across deep evolutionary time as well as to analyse in unprecedented detail the membrane trafficking systems of trypanosomes.

De novo sphingolipid synthesis is essential for viability, but not for transport of glycosylphosphatidylinositol-anchored proteins, in African trypanosomes

De novo sphingolipid synthesis is required for the exit of glycosylphosphatidylinositol (GPI)-anchored membrane proteins from the endoplasmic reticulum in yeast. Using a pharmacological approach, we test the generality of this phenomenon by analyzing the transport of GPI-anchored cargo in widely divergent eukaryotic systems represented by African trypanosomes and HeLa cells. Myriocin, which blocks the first step of sphingolipid synthesis (serine + palmitate --> 3-ketodihydrosphingosine), inhibited the growth of cultured bloodstream parasites, and growth was rescued with exogenous 3-ketodihydrosphingosine. Myriocin also blocked metabolic incorporation of [3H]serine into base-resistant sphingolipids. Biochemical analyses indicate that the radiolabeled lipids are not sphingomyelin or inositol phosphorylceramide, suggesting that bloodstream trypanosomes synthesize novel sphingolipids. Inhibition of de novo sphingolipid synthesis with myriocin had no adverse effect on either general secretory trafficking or GPI-dependent trafficking in trypanosomes, and similar results were obtained with HeLa cells. A mild effect on endocytosis was seen for bloodstream trypanosomes after prolonged incubation with myriocin. These results indicate that de novo synthesis of sphingolipids is not a general requirement for secretory trafficking in eukaryotic cells. However, in contrast to the closely related kinetoplastid Leishmania major, de novo sphingolipid synthesis is essential for the viability of bloodstream-stage African trypanosomes.

Regulation of Protein Trafficking by Glycosylphosphatidylinositol Valence in African Trypanosomes

The structure, biosynthesis, and attachment of glycosylphosphatidylinositol (GPI) anchors were all first determined for the variant surface glycoprotein (VSG) of African trypanosomes, and all of the basic aspects of this work have been shown to be universal in eukaryotic organisms. However, the role of GPI anchors in protein trafficking within trypanosomes has lagged behind the more standard eukaryotic model systems such as yeast and polarized epithelial cells. Trypanosomes are also highly polarized cells in which all endocytosis and exocytosis intersect at a discrete domain of the plasma membrane, the flagellar pocket. Within these convergent pathways trafficking of GPI anchored proteins correlates strongly with valence: homodimeric VSG with two GPIs is stably incorporated into the cell surface coat, heterodimeric transferrin receptor with a single GPI is found in the flagellar pocket and is slowly delivered to the lysosome for degradation, and recombinant GPI minus VSG reporters are rapidly degraded in the lysosome. Here we summarize recent data confirming this correlation using a tool kit of recombinant GPI anchored reporters, including a reporter designed to be conditionally modulated between a GPI valence of one and two.

The trypanolytic factor of human serum

African trypanosomes (the prototype of which is Trypanosoma brucei brucei) are protozoan parasites that infect a wide range of mammals. Human blood, unlike the blood of other mammals, has efficient trypanolytic activity, and this needs to be counteracted by these parasites. Resistance to this activity has arisen in two subspecies of Trypanosoma brucei - Trypanosoma brucei rhodesiense and Trypanosoma brucei gambiense - allowing these parasites to infect humans, and this results in sleeping sickness in East Africa and West Africa, respectively. Study of the mechanism by which T. b. rhodesiense escapes lysis by human serum led to the identification of an ionic-pore-forming apolipoprotein - known as apolipoprotein L1 - that is associated with high-density-lipoprotein particles in human blood. In this Opinion article, we argue that apolipoprotein L1 is the factor that is responsible for the trypanolytic activity of human serum.

Secretory organelles of pathogenic protozoa

Secretory processes play an important role on the biology and life cycles of parasitic protozoa. This review focus on basic aspects, from a cell biology perspective, of the secretion of (a) micronemes, rhoptries and dense granules in members of the Apicomplexa group, where these organelles are involved in the process of protozoan penetration into the host cell, survival within the parasitophorous vacuole and subsequent egress from the host cell, (b) the Maurer's cleft in Plasmodium, a structure involved in the secretion of proteins synthesized by the intravacuolar parasite and transported through vesicles to the erythrocyte surface, (c) the secretion of macromolecules into the flagellar pocket of trypanosomatids, and (d) the secretion of proteins which make the cyst wall of Giardia and Entamoeba, with the formation of encystation vesicles.

Regulation of surface coat exchange by differentiating African trypanosomes

African trypanosomes (Trypanosoma brucei) have a digenetic lifecycle that alternates between the mammalian bloodstream and the tsetse fly vector. In the bloodstream, replicating long slender parasites transform into non-dividing short stumpy forms. Upon transmission into the fly midgut, short stumpy cells differentiate into actively dividing procyclics. A hallmark of this process is the replacement of the bloodstream-stage surface coat composed of variant surface glycoprotein (VSG) with a new coat composed of procyclin. Pre-existing VSG is shed by a zinc metalloprotease activity (MSP-B) and glycosylphosphatidylinositol-specific phospholipase C (GPI-PLC). We now provide a detailed analysis of the coordinate and inverse regulation of these activities during synchronous differentiation. MSP-B mRNA and protein levels are upregulated during differentiation at the same time as proteolysis whereas GPI-PLC levels decrease. When transcription or translation is inhibited, VSG release is incomplete and a substantial amount of protein stays cell-associated. Both modes of release are still evident under these conditions, but GPI hydrolysis plays a quantitatively minor role during normal differentiation. Nevertheless, GPI biosynthesis shifts early in differentiation from a GPI-PLC sensitive structure to a resistant procyclic-type anchor. Translation inhibition also results in a marked increase in the mRNA levels of both MSP-B and GPI-PLC, consistent with negative regulation by labile protein factors. The relegation of short stumpy surface GPI-PLC to a secondary role in differentiation suggests that it may play a more important role as a virulence factor within the mammalian host.

A GPI-anchored co-receptor for tissue factor pathway inhibitor controls its intracellular trafficking and cell surface expression

BACKGROUND

Tissue factor pathway inhibitor (TFPI) lacks a membrane attachment signal but it remains associated with the endothelial surface via its association with an, as yet, unidentified glycosyl phosphatidylinositol (GPI)-anchored co-receptor.

OBJECTIVES/METHODS

Cellular trafficking of TFPI within aerolysin-resistant ECV304 and EA.hy926 cells, which do not express GPI-anchored proteins on their surface, was compared with their wild-type counterparts.

RESULTS AND CONCLUSIONS

Although aerolysin-resistant cells produce normal amounts of TFPI mRNA, TFPI is not expressed on the cell surface and total cellular TFPI is greatly decreased compared with wild-type cells. Additionally, normal, not increased, amounts of TFPI are secreted into conditioned media indicating that TFPI is degraded within the aerolysin-resistant cells. Confocal microscopy and studies using metabolic inhibitors demonstrate that aerolysin-resistant cells produce TFPI and transport it into the Golgi with subsequent degradation in lysosomes. The experimental results provide no evidence that cell surface TFPI originates from secreted TFPI that binds back to a GPI-anchored protein. Instead, the data suggest that TFPI tightly, but reversibly, binds to a GPI anchored co-receptor in the ER/Golgi. The co-receptor then acts as a molecular chaperone for TFPI by trafficking it to the cell surface of wild-type cells or to lysosomes of aerolysin-resistant cells. TFPI that escapes co-receptor binding is secreted through the same pathway in both wild-type and aerolysin-resistant cells. The data provide a framework for understanding how TFPI is expressed on endothelium.

GPI valence and the fate of secretory membrane proteins in African trypanosomes

Progression of GPI-anchored proteins in bloodstream African trypanosomes correlates with GPI-valence: homodimeric VSG (2 GPI) is a surface protein; heterodimeric transferrin receptor (1 GPI) localizes in the flagellar pocket; homodimeric GPI-minus VSG (0 GPI) is rapidly degraded in the lysosome. We test this relationship using three native secretory/endocytic proteins as monomeric GPI-plus and -minus reporters. GPI-minus procyclin trafficks to the lysosome and is degraded. GPI-plus procyclin trafficks to the flagellar pocket/cell surface and is released (∼50%) with an intact anchor, the remainder (∼50%) is degraded in the lysosome. GPI-plus BiPNHP, derived from the ER marker BiP, is released quantitatively (>80%), while GPI-plus p67HP, derived from the lysosomal marker p67, turns over by both release (∼15%) and lysosomal degradation (>50%). Turnover of endogenous transferrin receptor occurs primarily by lysosomal degradation (>90%). Thus shedding of monovalent GPI reporters correlates inversely with lysosomal targeting. We propose that mono-GPI reporters cycle through the flagellar pocket and endosome until they are disposed of by either shedding or lysosomal targeting. Partitioning between these fates may be a function of individual physical properties. Release is likely due to the exclusive use of C-14:0 myristate in the bloodstream stage GPI anchor. Up-regulation of transferrin receptor by culture in dog serum resulted in prominent cell surface localization, but not in elevated release. Surface receptor was non-functional for ligand binding suggesting that it may be bivalent homodimers of the GPI-anchored ESAG6 receptor subunit.

Mutational analysis of the GPI-anchor addition sequence from the circumsporozoite protein of Plasmodium

The plasma membrane of Plasmodium sporozoites is uniformly covered by the glycosylphosphatidylinositol (GPI)-anchored circumsporozoite (CS) protein. Sporozoites form in the mosquito midgut through a budding process that occurs within a multinucleate oocyst underneath the basal lamina of the gut. Earlier genetic studies established that normal sporozoite development requires CS. Mutant parasites lacking CS [CS (-)] do not form sporozoites. Ultrastructural analysis of the oocysts from these parasites revealed that there is an early block in the cytokinesis that occurs within the multinucleate oocysts to generate individual sporozoites. Parasites that are hypomorphic for CS expression gave rise to sporozoites with abnormal morphology. These results proved that CS plays a direct role in the maturation of oocysts and in the normal budding of sporozoites. In this article, we examined if the membrane localization of CS via a GPI-anchor, is crucial for its function during sporozoite formation. We generated three mutants in Plasmodium berghei CS, CS-DeltaGPI, CS-TM1 and CS-TM2. In CS-DeltaGPI, we deleted the signal sequence required for the addition of a GPI-anchor to CS. The resulting protein was found only in the cytoplasm of the oocyst. In CS-TM1 and CS-TM2, the GPI-anchor addition sequence of CS was substituted by the transmembrane domain and truncated (to different degrees) cytoplasmic tail of Plasmodium thrombospondin-related anonymous protein (TRAP). The resulting CS protein was detected on the plasma membrane of the oocysts. The amount of CS in the mutants was similar to that of wild type. The sporozoite budding and development were abrogated in both CS-DeltaGPI and CS-TM mutants. The ultrastructure of the mutant oocysts was indistinguishable from that of the CS (-) parasites. Our results suggest that the GPI-anchor of the CS protein is required for sporogenesis.

The membrane-bound histidine acid phosphatase TbMBAP1 is essential for endocytosis and membrane recycling in Trypanosoma brucei

In the parasitic protozoan Trypanosoma brucei, endocytosis and exocytosis occur exclusively at an invagination of the plasma membrane around the base of the flagellum, called the flagellar pocket, which actively communicates by vesicular membrane flow with cisternal/tubulovesicular endosomes. The division of the cell surface into three morphologically distinct sub-domains and the rapid plasma membrane turnover establishes T. brucei as an interesting model for investigations on the sorting and recycling of membrane proteins. In this study we show that the type I membrane protein TbMBAP1, an L-(+)-tartrate-sensitive acid phosphatase, is present in all endosomal membranes but is virtually absent from the lysosome membrane (where this type of protein is mainly found in other organisms) and is not detectable at the cell surface. The endosomal localization of TbMBAP1 is a function of protein abundance. Moderate overexpression (three- to fourfold) leads to an increased appearance within the flagellar pocket membrane. At higher levels the protein is found in the flagellum, and routing to the pellicular plasma membrane is observed at levels 10- to 25-fold above that of wild type. In other organisms L-(+)-tartrate-sensitive acid phosphatases appear to be dispensable but TbMBAP1 is essential, as shown by RNA interference, which causes growth arrest followed by cell death. Comparison of the phenotype of TbMBAP1-depleted cells with that of cells in which endocytosis or exocytosis has been specifically inhibited by RNAi against clathrin of RAB11, reveals that TbMBAP1 is essential for both incoming and recycling membrane traffic. During differentiation of the organism from bloodstream to insect stage, TbMBAP1 is down-regulated and differentially modified in parallel with a 10-fold decrease in the rate of endocytosis.

TbRAB1 and TbRAB2 mediate trafficking through the early secretory pathway of Trypanosoma brucei

The African trypanosome possesses a total of 16 small GTPases of the Rab family, which are involved in control of various membrane transport events. Recently the roles of these proteins in the endocytosis and recycling of the major surface antigen of the bloodstream form, the variant surface glycoprotein (VSG), have been described but little has been reported on the roles of Rab proteins in exocytic pathways in trypanosomatids. Whilst phylogenetic analysis based on sequence similarity indicates a comparatively well conserved core set of Rab proteins, the evolutionary distance of the trypanosome lineage from crown eukaryote model systems requires direct experimental evidence to support these sequence data. By database searching we identified two further Rab genes, TbRAB1 and TbRAB2, which are the trypanosome sequence orthologues of mammalian Rab1 and Rab2, important mediators of ER to Golgi and intra-Golgi transport processes. A remarkably high level of sequence conservation is retained between the trypanosome and higher eukaryote orthologues. By immunolocalisation we find that both TbRAB1 and TbRAB2 reside on membranes in intimate association with the Golgi complex. By heterologous expression in mammalian cells we also demonstrate conservation of targeting information in the TbRAB1 and TbRAB2 proteins, whilst TbRAB1, but not TbRAB2, can complement a Ypt1(ts) conditional mutant in Saccharomyces cerevisiae. The roles of TbRAB1 and TbRAB2 in exocytosis were examined using RNAi. Suppression of TbRAB1 or TbRAB2 was strongly inhibitory to growth and most importantly both TbRAB1 and TbRAB2 were required for normal progression of VSG through the early secretory pathway. These data indicate conservation of function for these proteins between trypanosomes and crown eukaryotes.