Single-subunit oligosaccharyltransferases of Trypanosoma brucei display different and predictable peptide acceptor specificities

Molecular tools to regulate gene expression in Trypanosoma cruzi

Developing molecular strategies to manipulate gene expression in trypanosomatids is challenging, particularly with respect to the unique gene expression mechanisms adopted by these unicellular parasites, such as polycistronic mRNA transcription and multi-gene families. In the case of Trypanosoma cruzi (T. cruzi), the causative agent of Chagas Disease, the lack of RNA interference machinery further complicated functional genetic studies important for understanding parasitic biology and developing biomarkers and potential therapeutic targets. Therefore, alternative methods of performing knockout and/or endogenous labelling experiments were developed to identify and understand the function of proteins for survival and interaction with the host. In this review, we present the main tools for the genetic manipulation of T. cruzi, focusing on the Clustered Regularly Interspaced Short Palindromic Repeats Cas9-associated system technique widely used in this organism. Moreover, we highlight the importance of using these tools to elucidate the function of uncharacterized and glycosylated proteins. Further developments of these technologies will allow the identification of new biomarkers, therapeutic targets and potential vaccines against Chagas disease with greater efficiency and speed.

The endoplasmic reticulum of trypanosomatids: An unrevealed road for chemotherapy

The endoplasmic reticulum (ER) of higher eukaryotic cells forms an intricate membranous network that serves as the main processing facility for folding and assembling of secreted and membrane proteins. The ER is a highly dynamic organelle that interacts with other intracellular structures, as well as endosymbiotic pathogenic and non-pathogenic microorganisms. A strict ER quality control (ERQC) must work to ensure that proteins entering the ER are folded and processed correctly. Unfolded or misfolded proteins are usually identified, selected, and addressed to Endoplasmic Reticulum-Associated Degradation (ERAD) complex. Conversely, when there is a large demand for secreted proteins or ER imbalance, the accumulation of unfolded or misfolded proteins activates the Unfold Protein Response (UPR) to restore the ER homeostasis or, in the case of persistent ER stress, induces the cell death. Pathogenic trypanosomatids, such as Trypanosoma cruzi, Trypanosoma brucei and Leishmania spp are the etiological agents of important neglected diseases. These protozoans have a complex life cycle alternating between vertebrate and invertebrate hosts. The ER of trypanosomatids, like those found in higher eukaryotes, is also specialized for secretion, and depends on the ERAD and non-canonical UPR to deal with the ER stress. Here, we reviewed the basic aspects of ER biology, organization, and quality control in trypanosomatids. We also focused on the unusual way by which T. cruzi, T. brucei, and Leishmania spp. respond to ER stress, emphasizing how these parasites' ER-unrevealed roads might be an attractive target for chemotherapy.

Common and unique features of glycosylation and glycosyltransferases in African trypanosomes

Eukaryotic protein glycosylation is mediated by glycosyl- and oligosaccharyl-transferases. Here, we describe how African trypanosomes exhibit both evolutionary conservation and significant divergence compared with other eukaryotes in how they synthesise their glycoproteins. The kinetoplastid parasites have conserved components of the dolichol-cycle and oligosaccharyltransferases (OSTs) of protein N-glycosylation, and of glycosylphosphatidylinositol (GPI) anchor biosynthesis and transfer to protein. However, some components are missing, and they process and decorate their N-glycans and GPI anchors in unique ways. To do so, they appear to have evolved a distinct and functionally flexible glycosyltransferases (GT) family, the GT67 family, from an ancestral eukaryotic β3GT gene. The expansion and/or loss of GT67 genes appears to be dependent on parasite biology. Some appear to correlate with the obligate passage of parasites through an insect vector, suggesting they were acquired through GT67 gene expansion to assist insect vector (tsetse fly) colonisation. Others appear to have been lost in species that subsequently adopted contaminative transmission. We also highlight the recent discovery of a novel and essential GT11 family of kinetoplastid parasite fucosyltransferases that are uniquely localised to the mitochondria of Trypanosoma brucei and Leishmania major. The origins of these kinetoplastid FUT1 genes, and additional putative mitochondrial GT genes, are discussed.

Recent Chemical and Chemoenzymatic Strategies to Complex-Type N-Glycans

Glycosylation is one of the major forms of protein post-translational modification. N-glycans attached to proteins by covalent bonds play an indispensable role in intercellular interaction and immune function. In human bodies, most of the cell surface glycoproteins and secreted glycopeptides are modified with complex-type N-glycans. Thus, for analytical or medicinal purposes, efficient and universal methods to provide homogeneous complex-type N-glycans have been an urgent need. Despite the extremely complicated structures, tremendous progress in the synthesis of N-glycans has been achieved. On one hand, chemical strategies are shown to be effective to prepare core oligosaccharides of N-glycans by focusing on stereoselective glycosylations such as β-mannosylation and α-sialylation, as well as the methodology of the N-glycan assembly. On the other hand, chemoenzymatic strategies have also become increasingly powerful in recent years. This review attempts to highlight the very recent advancements in chemical and chemoenzymatic strategies for eukaryotic complex-type N-glycans.

Visualisation of experimentally determined and predicted protein N-glycosylation and predicted glycosylphosphatidylinositol anchor addition in Trypanosoma brucei

Background: Trypanosoma brucei is a protozoan parasite and the etiological agent of human and animal African trypanosomiasis. The organism cycles between its mammalian host and tsetse vector. The host-dwelling bloodstream form of the parasite is covered with a monolayer of variant surface glycoprotein (VSG) that enables it to escape both the innate and adaptive immune systems. Within this coat reside lower-abundance surface glycoproteins that function as receptors and/or nutrient transporters. The glycosylation of the Trypanosoma brucei surface proteome is essential to evade the immune response and is mediated by three oligosaccharyltransferase genes; two of which, TbSTT3A and TbSTT3B, are expressed in the bloodstream form of the parasite.

Methods: We processed a recent dataset of our laboratory to visualise putative glycosylation sites of the Trypanosoma brucei proteome. We provided a visualisation for the predictions of glycosylation carried by TbSTT3A and TbSTT3B, and we augmented the visualisation with predictions for Glycosylphosphatidylinositol anchoring sites, domains and topology of the Trypanosoma brucei proteome.

Conclusions: We created a web service to explore the glycosylation sites of the Trypanosoma brucei oligosaccharyltransferases substrates, using data described in a recent publication of our laboratory. We also made a machine learning algorithm available as a web service, described in our recent publication, to distinguish between TbSTT3A and TbSTT3B substrates.

A Trypanosoma brucei β3 glycosyltransferase superfamily gene encodes a β1-6 GlcNAc-transferase mediating N-glycan and GPI anchor modification

The parasite Trypanosoma brucei exists in both a bloodstream form (BSF) and a procyclic form (PCF), which exhibit large carbohydrate extensions on the N-linked glycans and glycosylphosphatidylinositol (GPI) anchors, respectively. The parasite's glycoconjugate repertoire suggests at least 38 glycosyltransferase (GT) activities, 16 of which are currently uncharacterized. Here, we probe the function(s) of the uncharacterized GT67 glycosyltransferase family and a β3 glycosyltransferase (β3GT) superfamily gene, TbGT10. A BSF-null mutant, created by applying the diCre/loxP method in T. brucei for the first time, showed a fitness cost but was viable in vitro and in vivo and could differentiate into the PCF, demonstrating nonessentiality of TbGT10. The absence of TbGT10 impaired the elaboration of N-glycans and GPI anchor side chains in BSF and PCF parasites, respectively. Glycosylation defects included reduced BSF glycoprotein binding to the lectin ricin and monoclonal antibodies mAb139 and mAbCB1. The latter bind a carbohydrate epitope present on lysosomal glycoprotein p67 that we show here consists of (-6Galβ1-4GlcNAcβ1-)≥4 poly-N-acetyllactosamine repeats. Methylation linkage analysis of Pronase-digested glycopeptides isolated from BSF wild-type and TbGT10 null parasites showed a reduction in 6-O-substituted- and 3,6-di-O-substituted-Gal residues. These data define TbGT10 as a UDP-GlcNAc:βGal β1-6 GlcNAc-transferase. The dual role of TbGT10 in BSF N-glycan and PCF GPI-glycan elaboration is notable, and the β1-6 specificity of a β3GT superfamily gene product is unprecedented. The similar activities of trypanosome TbGT10 and higher-eukaryote I-branching enzyme (EC 2.4.1.150), which belong to glycosyltransferase families GT67 and GT14, respectively, in elaborating N-linked glycans, are a novel example of convergent evolution.

N-Glycosylation in Piroplasmids: Diversity within Simplicity

N-glycosylation has remained mostly unexplored in Piroplasmida, an order of tick-transmitted pathogens of veterinary and medical relevance. Analysis of 11 piroplasmid genomes revealed three distinct scenarios regarding N-glycosylation: Babesia sensu stricto (s.s.) species add one or two N-acetylglucosamine (NAcGlc) molecules to proteins; Theileria equi and Cytauxzoon felis add (NAcGlc)2-mannose, while B. microti and Theileria s.s. synthesize dolichol-P-P-NAcGlc and dolichol-P-P-(NAcGlc)2 without subsequent transfer to proteins. All piroplasmids possess the gene complement needed for the synthesis of the N-glycosylation substrates, dolichol-P and sugar nucleotides. The oligosaccharyl transferase of Babesia species, T. equi and C. felis, is predicted to be composed of only two subunits, STT3 and Ost1. Occurrence of short N-glycans in B. bovis merozoites was experimentally demonstrated by fluorescence microscopy using a NAcGlc-specific lectin. In vitro growth of B. bovis was significantly impaired by tunicamycin, an inhibitor of N-glycosylation, indicating a relevant role for N-glycosylation in this pathogen. Finally, genes coding for N-glycosylation enzymes and substrate biosynthesis are transcribed in B. bovis blood and tick stages, suggesting that this pathway is biologically relevant throughout the parasite life cycle. Elucidation of the role/s exerted by N-glycans will increase our understanding of these successful parasites, for which improved control measures are needed.

Glycomics, Glycoproteomics, and Glycogenomics: An Inter-Taxa Evolutionary Perspective

Glycosylation is a highly diverse set of co- and posttranslational modifications of proteins. For mammalian glycoproteins, glycosylation is often site-, tissue-, and species-specific and diversified by microheterogeneity. Multitudinous biochemical, cellular, physiological, and organismic effects of their glycans have been revealed, either intrinsic to the carrier proteins or mediated by endogenous reader proteins with carbohydrate recognition domains. Furthermore, glycans frequently form the first line of access by or defense from foreign invaders, and new roles for nucleocytoplasmic glycosylation are blossoming. We now know enough to conclude that the same general principles apply in invertebrate animals and unicellular eukaryotes-different branches of which spawned the plants or fungi and animals. The two major driving forces for exploring the glycomes of invertebrates and protists are (i) to understand the biochemical basis of glycan-driven biology in these organisms, especially of pathogens, and (ii) to uncover the evolutionary relationships between glycans, their biosynthetic enzyme genes, and biological functions for new glycobiological insights. With an emphasis on emerging areas of protist glycobiology, here we offer an overview of glycan diversity and evolution, to promote future access to this treasure trove of glycobiological processes.

Protein glycosylation inLeishmaniaspp

Protein glycosylation is a co- and post-translational modification that, inLeishmaniaparasites, plays key roles in vector–parasite–vertebrate host interaction.

Human protein paucimannosylation: cues from the eukaryotic kingdoms

Paucimannosidic proteins (PMPs) are bioactive glycoproteins carrying truncated α- or β-mannosyl-terminating asparagine (N)-linked glycans widely reported across the eukaryotic domain. Our understanding of human PMPs remains limited, despite findings documenting their existence and association with human disease glycobiology. This review comprehensively surveys the structures, biosynthetic routes and functions of PMPs across the eukaryotic kingdoms with the aim of synthesising an improved understanding on the role of protein paucimannosylation in human health and diseases. Convincing biochemical, glycoanalytical and biological data detail a vast structural heterogeneity and fascinating tissue- and subcellular-specific expression of PMPs within invertebrates and plants, often comprising multi-α1,3/6-fucosylation and β1,2-xylosylation amongst other glycan modifications and non-glycan substitutions e.g. O-methylation. Vertebrates and protists express less-heterogeneous PMPs typically only comprising variable core fucosylation of bi- and trimannosylchitobiose core glycans. In particular, the Manα1,6Manβ1,4GlcNAc(α1,6Fuc)β1,4GlcNAcβAsn glycan (M2F) decorates various human neutrophil proteins reportedly displaying bioactivity and structural integrity demonstrating that they are not degradation products. Less-truncated paucimannosidic glycans (e.g. M3F) are characteristic glycosylation features of proteins expressed by human cancer and stem cells. Concertedly, these observations suggest the involvement of human PMPs in processes related to innate immunity, tumorigenesis and cellular differentiation. The absence of human PMPs in diverse bodily fluids studied under many (patho)physiological conditions suggests extravascular residence and points to localised functions of PMPs in peripheral tissues. Absence of PMPs in Fungi indicates that paucimannosylation is common, but not universally conserved, in eukaryotes. Relative to human PMPs, the expression of PMPs in plants, invertebrates and protists is more tissue-wide and constitutive yet, similar to their human counterparts, PMP expression remains regulated by the physiology of the producing organism and PMPs evidently serve essential functions in development, cell-cell communication and host-pathogen/symbiont interactions. In most PMP-producing organisms, including humans, the N-acetyl-β-hexosaminidase isoenzymes and linkage-specific α-mannosidases are glycoside hydrolases critical for generating PMPs via N-acetylglucosaminyltransferase I (GnT-I)-dependent and GnT-I-independent truncation pathways. However, the identity and structure of many species-specific PMPs in eukaryotes, their biosynthetic routes, strong tissue- and development-specific expression, and diverse functions are still elusive. Deep exploration of these PMP features involving, for example, the characterisation of endogenous PMP-recognising lectins across a variety of healthy and N-acetyl-β-hexosaminidase-deficient human tissue types and identification of microbial adhesins reactive to human PMPs, are amongst the many tasks required for enhanced insight into the glycobiology of human PMPs. In conclusion, the literature supports the notion that PMPs are significant, yet still heavily under-studied biomolecules in human glycobiology that serve essential functions and create structural heterogeneity not dissimilar to other human N-glycoprotein types. Human PMPs should therefore be recognised as bioactive glycoproteins that are distinctly different from the canonical N-glycoprotein classes and which warrant a more dedicated focus in glycobiological research.

Analysis of substrate specificity of Trypanosoma brucei oligosaccharyltransferases (OSTs) by functional expression of domain-swapped chimeras in yeast

N-Linked protein glycosylation is an essential and highly conserved post-translational modification in eukaryotes. The transfer of a glycan from a lipid-linked oligosaccharide (LLO) donor to the asparagine residue of a nascent polypeptide chain is catalyzed by an oligosaccharyltransferase (OST) in the lumen of the endoplasmic reticulum (ER). Trypanosoma brucei encodes three paralogue single-protein OSTs called TbSTT3A, TbSTT3B, and TbSTT3C that can functionally complement the Saccharomyces cerevisiae OST, making it an ideal experimental system to study the fundamental properties of OST activity. We characterized the LLO and polypeptide specificity of all three TbOST isoforms and their chimeric forms in the heterologous expression host S. cerevisiae where we were able to apply yeast genetic tools and newly developed glycoproteomics methods. We demonstrated that TbSTT3A accepted LLO substrates ranging from Man₅GlcNAc₂ to Man₇GlcNAc₂ In contrast, TbSTT3B required more complex precursors ranging from Man₆GlcNAc₂ to Glc₃Man₉GlcNAc₂ structures, and TbSTT3C did not display any LLO preference. Sequence differences between the isoforms cluster in three distinct regions. We have swapped the individual regions between different OST proteins and identified region 2 to influence the specificity toward the LLO and region 1 to influence polypeptide substrate specificity. These results provide a basis to further investigate the molecular mechanisms and contribution of single amino acids in OST interaction with its substrates.