Trypanosoma rhodesiense: antigenicity and immunogenicity of flagellar pocket membrane components

Architecture of a Host-Parasite Interface: Complex Targeting Mechanisms Revealed Through Proteomics

Surface membrane organization and composition is key to cellular function, and membrane proteins serve many essential roles in endocytosis, secretion, and cell recognition. The surface of parasitic organisms, however, is a double-edged sword; this is the primary interface between parasites and their hosts, and those crucial cellular processes must be carried out while avoiding elimination by the host immune defenses. For extracellular African trypanosomes, the surface is partitioned such that all endo- and exocytosis is directed through a specific membrane region, the flagellar pocket, in which it is thought the majority of invariant surface proteins reside. However, very few of these proteins have been identified, severely limiting functional studies, and hampering the development of potential treatments. Here we used an integrated biochemical, proteomic and bioinformatic strategy to identify surface components of the human parasite Trypanosoma brucei. This surface proteome contains previously known flagellar pocket proteins as well as multiple novel components, and is significantly enriched in proteins that are essential for parasite survival. Molecules with receptor-like properties are almost exclusively parasite-specific, whereas transporter-like proteins are conserved in model organisms. Validation shows that the majority of surface proteome constituents are bona fide surface-associated proteins and, as expected, most present at the flagellar pocket. Moreover, the largest systematic analysis of trypanosome surface molecules to date provides evidence that the cell surface is compartmentalized into three distinct domains with free diffusion of molecules in each, but selective, asymmetric traffic between. This work provides a paradigm for the compartmentalization of a cell surface and a resource for its analysis.

The adaptative mechanisms of Trypanosoma brucei for sterol homeostasis in its different life-cycle environments

Bloodstream forms of Trypanosoma brucei do not synthesize sterols de novo and therefore cannot survive in medium devoid of lipoproteins. Growth of parasites is essentially supported by receptor-mediated endocytosis of low-density lipoproteins (LDLs), which carry phospholipids and cholesteryl esters. These lipids are released from internalized LDL after apoprotein B-100 is degraded by acidic thiol-proteases in the endolysosomal apparatus and then metabolized, as in mammalian cells. The LDL receptor is recycled and its expression is regulated by the sterol stores. Documented pharmacological and immunological interferences with LDL receptor-mediated lipid supply to the bloodstream forms are summarized, and the potential for new approaches to fight against these parasites is evaluated. In contrast to bloodstream forms, cultured procyclic forms can acquire sterols from both exogenous (lipoprotein endocytosis) and endogenous (biosynthesis of ergosterol) sources. The rate-limiting steps of both endocytosis (surface LDL receptor expression) and biosynthesis (3-hydroxy-3-methylglutaryl coenzyme A reductase activity) are regulated by the cellular content of sterol. These two pathways thus complement each other to yield a balanced sterol supply, which demonstrates adaptative capacities to survive in totally different environments and fine regulatory mechanisms of sterol homeostasis.

An integral membrane glycoprotein associated with an endocytic compartment of Trypanosoma vivax: identification and partial characterization

A 65-kD glycoprotein (gp65) of Trypanosoma (Duttonella) vivax was identified using a murine monoclonal antibody (mAb 4E1) that had been raised against formalin-fixed, in vitro-propagated, uncoated forms. Intracellular localization studies utilizing the mAb in immunofluorescence on fixed, permeabilized T. vivax bloodstream forms and immunoelectron microscopy on thin sections of Lowicryl K4M-embedded cells revealed labeling of vesicles and tubules in the posterior portion of the parasite. Some mAb-labeled vesicles contained endocytosed 10 nm BSA-gold after incubation of the parasites with the marker for 5-30 min at 37 degrees C, and the greatest degree of colocalization was observed after 5 min. Double labeling experiments using the mAb and a polyclonal anti-variant surface glycoprotein (VSG) antibody to simultaneously localize both gp65 and VSG demonstrated that there was little overlap in the distribution of these antigens. Thus, gp65 is associated with tubules and vesicles that are involved in endocytosis but which appear to be distinct from VSG processing pathways within the cell. Using the mAb for immunoblot analyses, gp65 was shown to be enriched in a fraction of solubilized membrane proteins eluted from either immobilized Con A or Ricinus communis agglutinin and was found to possess carbohydrate linkages cleaved by both endoglycosidase H and O-glycosidase, suggesting the presence of N- and O-linked glycans. Protease protection and crosslinking experiments suggest that gp65 is a transmembrane protein with trypsin cleavage and NH2-crosslinking sites on the lumenal face of the vesicles.

Trypanosoma brucei brucei: antigenic stability of its LDL receptor and immunological cross-reactivity with the LDL receptor of the mammalian host

The rapid growth of Trypanosoma brucei brucei in the blood and tissue fluids of vertebrates requires the receptor-mediated endocytosis of LDL from the host (Coppens et al. 1987; Gillett and Owen 1987) and is slowed by a monospecific rabbit antiserum against the purified LDL receptor of the parasite. We have used this antiserum in combination with several well-characterized antigenic variants (originating from stock 427: MITat 1.1a, 1.3a, 1.4a, 1.5a, 1.5d, 1.8b) to examine whether the LDL receptor of T. b. brucei is a stable surface antigen, common to all parasite variants despite antigenic variation of the major surface glycoprotein, and whether it is immunologically distinct from the LDL receptor of the host. At low concentrations, binding at 4 degrees C of rat LDL to several variants of T. b. brucei and to isolated rat hepatocytes was inhibited to a similar extent by the antiserum. In double immunodiffusion, a single precipitation line was observed, showing continuity between the extracts of all variants as well as between that of trypanosomes and of mammalian tissues. In Western blots of trypanosome extracts, the LDL receptor was strongly labeled as a single band of Mr 145,000, whereas with a rat liver extract, a single band of similar electrophoretic mobility was weakly labeled at a high concentration of the antiserum. In conclusion, the LDL receptor occurred in all variants of T. b. brucei, was a stable surface antigen despite variation of the major surface glycoprotein, and displayed biochemical and immunological similarities with the LDL receptor of the rat host.

Structure and transcription of a P-ATPase gene from Trypanosoma brucei

A putative ATPase gene was cloned from Trypanosoma brucei genomic DNA. The length of the gene open reading frame is 3,033 bp, predicting a protein of about 110 kDa. The sequence of this protein shares 10 blocks of homology with other eukaryotic ATPases, including the putative phosphorylation site characteristic of P-ATPases. Its hydropathy profile reveals 8-10 potential membrane-spanning regions. While the amino acid sequence of the T. brucei ATPase shows only 25% overall homology with its counterpart from the related kinetoplastid protozoan Leishmania donovani, 49% sequence conservation is found when compared with the calcium-ATPase from rabbit sarcoplasmic reticulum. This gene is present in only one copy, localized in the large chromosome fraction. It is transcribed at a similar level in procyclic and bloodstream forms, as a 4.3-kb mRNA. Run-on assays suggest continuous transcription of the gene and flanking sequences over at least 10 kb, by a RNA polymerase sensitive to alpha-amanitin. Transcription inhibition by UV irradiation suggests that the ATPase gene is more than 4 kb downstream from its promoter.