The trypanosomatids are generally aberrant in their protein N-glycosylation pathways. However, protein N-glycosylation in the African trypanosome Trypanosoma brucei, etiological agent of human African sleeping sickness, is not well understood. Here, we describe the creation of a bloodstream-form T. brucei mutant that is deficient in the endoplasmic reticulum enzyme glucosidase II. Characterization of the variant surface glycoprotein, the main glycoprotein synthesized by the parasite with two N-glycosylation sites, revealed unexpected changes in the N-glycosylation of this molecule. Structural characterization by mass spectrometry, nuclear magnetic resonance spectroscopy, and chemical and enzymatic treatments revealed that one of the two glycosylation sites was occupied by conventional oligomannose structures, whereas the other accumulated unusual structures in the form of Glc␣1-3Man␣1-2Man␣1-2Man␣1-3(Man␣1-6)Man␤1-4GlcNAc␤1-4GlcNAc, Glc␣1-3Man␣1-2Man␣1-2Man␣1-3(GlcNAc␤1-2Man␣1-6)Man␤1-4GlcNAc␤1-4GlcNAc, and Glc␣1-3Man␣1-2Man␣1-2Man␣1-3(Gal␤1-4GlcNAc␤1-2Man␣1-6)Man␤1-4GlcNAc␤1-4GlcNAc. The possibility that these structures might arise from Glc 1 Man 9 GlcNAc 2 by unusually rapid ␣-mannosidase processing was ruled out using a mixture of ␣-mannosidase inhibitors. The results suggest that bloodstream-form T. brucei can transfer both Man 9 GlcNAc 2 and Man 5 GlcNAc 2 to the variant surface glycoprotein in a site-specific manner and that, unlike organisms that transfer exclusively Glc 3 Man 9 GlcNAc 2 , the T. brucei UDP-Glc: glycoprotein glucosyltransferase and glucosidase II enzymes can use Man 5 GlcNAc 2 and Glc 1 Man 5 GlcNAc 2 , respectively, as their substrates. The ability to transfer Man 5 GlcNAc 2 structures to N-glycosylation sites destined to become Man 4 -3 GlcNAc 2 or complex structures may have evolved as a mechanism to conserve dolichol-phosphate-mannose donors for glycosylphosphatidylinositol anchor biosynthesis and points to fundamental differences in the specificities of host and parasite glycosyltransferases that initiate the synthesis of complex N-glycans.The parasitic protozoan Trypanosoma brucei, transmitted by the tsetse fly, is the causative agent of nagana in cattle and African trypanosomiasis or sleeping sickness in humans. During the bloodstream stage of the life cycle, the cells are covered in a densely packed coat of variant surface glycoprotein (VSG) 3 The VSG coat serves as a physical barrier to components of the host complement system and undergoes antigenic variation (1). There are many VSG genes, and each encodes a GPIanchored glycoprotein with one to three N-glycosylation sites (2, 3). The cell line used in this study expresses VSG variant 221 (also known as MiTat1.2). VSG221 carries two occupied N-glycosylation sites, the glycan structures of which have been fully characterized (4). The Asn-428 site, 5 residues from the GPI attachment site, is occupied mostly by oligomannose structures (Man 7-9 GlcNAc 2 ), whereas the Asn-263 site is occupied by small biantennary structures rangin...

Graphical abstractPhenotypic screening of the LOPAC library identified several potent and selective inhibitors of African trypanosomes. The κ-opioid agonist (+)-U50,488 represents a novel lead for drug discovery against sleeping sickness.

Trypanothione reductase (TryR) is a genetically validated drug target in the parasite Trypanosoma brucei, the causative agent of human African trypanosomiasis. Here we report the discovery, synthesis, and development of a novel series of TryR inhibitors based on a 3,4-dihydroquinazoline scaffold. In addition, a high resolution crystal structure of TryR, alone and in complex with substrates and inhibitors from this series, is presented. This represents the first report of a high resolution complex between a noncovalent ligand and this enzyme. Structural studies revealed that upon ligand binding the enzyme undergoes a conformational change to create a new subpocket which is occupied by an aryl group on the ligand. Therefore, the inhibitor, in effect, creates its own small binding pocket within the otherwise large, solvent exposed active site. The TryR–ligand structure was subsequently used to guide the synthesis of inhibitors, including analogues that challenged the induced subpocket. This resulted in the development of inhibitors with improved potency against both TryR and T. brucei parasites in a whole cell assay.

Background: Deoxyhypusine synthase (DHS) catalyzes the spermidine-dependent modification of translation factor eIF5A.Results: Trypanosomatid DHS activity is increased 3000-fold by heterotetramer formation with a catalytically dead paralog, and both gene products are essential for parasite growth.Conclusion: Trypanosomatid DHS is a complex between catalytically impaired and inactive DHS subunits.Significance: This activation mechanism uniquely evolved for two independent enzymes within the trypanosomatid polyamine pathway.

In this paper, we describe the range of N-linked glycan structures produced by wild-type and glucosidase II null mutant bloodstream form Trypanosoma brucei parasites and the creation and characterization of a bloodstream form Trypanosoma brucei UDP-glucose:glycoprotein glucosyltransferase null mutant. These analyses highlight peculiarities of the Trypanosoma brucei UDP-glucose:glycoprotein glucosyltransferase, including an unusually wide substrate specificity, ranging from Man 5 GlcNAc 2 to Man 9 GlcNAc 2 glycans, and an unusually high efficiency in vivo, quantitatively glucosylating the Asn263 N-glycan of variant surface glycoprotein (VSG) 221 and 75% of all non-VSG N glycosylation sites. We also show that although Trypanosoma brucei UDP-glucose:glycoprotein glucosyltransferase is not essential for parasite growth at 37°C, it is essential for parasite growth and survival at 40°C. The null mutant was also shown to be hypersensitive to the effects of the N glycosylation inhibitor tunicamycin. Further analysis of bloodstream form Trypanosoma brucei under normal conditions and stress conditions suggests that it does not have a classical unfolded protein response triggered by sensing unfolded proteins in the endoplasmic reticulum. Rather, judging by its uniform Grp78/BiP levels, it appears to have an unregulated and constitutively active endoplasmic reticulum protein folding system. We suggest that the latter may be particularly appropriate for this organism, which has an extremely high flux of glycoproteins through its secretory pathway.

There is an urgent need for new drugs for the treatment of tropical parasitic diseases such as human African trypanosomiasis, which is caused by Trypanosoma brucei. The enzyme trypanothione reductase (TryR) is a potential drug target within these organisms. Herein we report the screening of a 62000 compound library against T. brucei TryR. Further work was undertaken to optimise potency and selectivity of two novel-compound series arising from the enzymatic and whole parasite screens and mammalian cell counterscreens. Both of these series, containing either a quinoline or pyrimidinopyrazine scaffold, yielded low micromolar inhibitors of the enzyme and growth of the parasite. The challenges of inhibiting TryR with druglike molecules is discussed.

SCYX-7158, an oxaborole, is currently in Phase I clinical trials for the treatment of human African trypanosomiasis. Here we investigate possible modes of action against Trypanosoma brucei using orthogonal chemo-proteomic and genomic approaches. SILAC-based proteomic studies using an oxaborole analogue immobilised onto a resin was used either in competition with a soluble oxaborole or an immobilised inactive control to identify thirteen proteins common to both strategies. Cell-cycle analysis of cells incubated with sub-lethal concentrations of an oxaborole identified a subtle but significant accumulation of G2 and >G2 cells. Given the possibility of compromised DNA fidelity, we investigated long-term exposure of T. brucei to oxaboroles by generating resistant cell lines in vitro. Resistance proved more difficult to generate than for drugs currently used in the field, and in one of our three cell lines was unstable. Whole-genome sequencing of the resistant cell lines revealed single nucleotide polymorphisms in 66 genes and several large-scale genomic aberrations. The absence of a simple consistent mechanism among resistant cell lines and the diverse list of binding partners from the proteomic studies suggest a degree of polypharmacology that should reduce the risk of resistance to this compound class emerging in the field. The combined genetic and chemical biology approaches have provided lists of candidates to be investigated for more detailed information on the mode of action of this promising new drug class.

Trypanothione reductase (TryR) is a key validated enzyme in the trypanothione-based redox metabolism of pathogenic trypanosomes and leishmania parasites. This system is absent in humans, being replaced with glutathione and glutathione reductase, and as such offers a target for selective inhibition. As part of a program to discover antiparasitic drugs, the LOPAC1280 library of 1266 compounds was screened against TryR and the top hits evaluated against glutathione reductase and T. brucei parasites. The top hits included a number of known tricyclic neuroleptic drugs along with other new scaffolds for TryR. Three novel druglike hits were identified and SAR studies on one of these using information from the tricyclic neuroleptic agents led to the discovery of a competitive inhibitor (Ki=330 nm) with an improved potency against T. brucei (EC50=775 nm).