Complicated N-linked glycans in simple organisms

Alpha-mannosidase-2 modulates arbovirus infection in a pathogen- and Wolbachia-specific manner in Aedes aegypti mosquitoes

Multiple Wolbachia strains can block pathogen infection, replication and/or transmission in Aedes aegypti mosquitoes under both laboratory and field conditions. However, Wolbachia effects on pathogens can be highly variable across systems and the factors governing this variability are not well understood. It is increasingly clear that the mosquito host is not a passive player in which Wolbachia governs pathogen transmission phenotypes; rather, the genetics of the host can significantly modulate Wolbachia-mediated pathogen blocking. Specifically, previous work linked variation in Wolbachia pathogen blocking to polymorphisms in the mosquito alpha-mannosidase-2 (αMan2) gene. Here we use CRISPR-Cas9 mutagenesis to functionally test this association. We developed αMan2 knockouts and examined effects on both Wolbachia and virus levels, using dengue virus (DENV; Flaviviridae) and Mayaro virus (MAYV; Togaviridae). Wolbachia titres were significantly elevated in αMan2 knockout (KO) mosquitoes, but there were complex interactions with virus infection and replication. In Wolbachia-uninfected mosquitoes, the αMan2 KO mutation was associated with decreased DENV titres, but in a Wolbachia-infected background, the αMan2 KO mutation significantly increased virus titres. In contrast, the αMan2 KO mutation significantly increased MAYV replication in Wolbachia-uninfected mosquitoes and did not affect Wolbachia-mediated virus blocking. These results demonstrate that αMan2 modulates arbovirus infection in A. aegypti mosquitoes in a pathogen- and Wolbachia-specific manner, and that Wolbachia-mediated pathogen blocking is a complex phenotype dependent on the mosquito host genotype and the pathogen. These results have a significant impact for the design and use of Wolbachia-based strategies to control vector-borne pathogens.

Culicoides-Specific Fitness Increase of Vesicular Stomatitis Virus in Insect-to-Insect Infections

Vesicular stomatitis virus (VSV) is an arthropod-borne virus affecting livestock. In the United States, sporadic outbreaks result in significant economic losses. During epizootics, Culicoides biting midges are biological vectors and key to the geographic expansion of outbreaks. Additionally, Culicoides may play a role in VSV overwintering because females and males are capable of highly efficient venereal transmission, despite their relatively low virus titers. We hypothesized that VSV propagated within a midge has increased fitness for subsequent midge infections. To evaluate the potential host-specific fitness increase, we propagated three viral isolates of VSV in porcine skin fibroblasts and Culicoides cell lines. We then evaluated the viral infection dynamics of the different cell-source groups in Culicoides sonorensis. Our results indicate that both mammalian- and insect-derived VSV replicate well in midges inoculated via intrathoracic injection, thereby bypassing the midgut barriers. However, when the virus was required to infect and escape the midgut barrier to disseminate after oral acquisition, the insect-derived viruses had significantly higher titers, infection, and dissemination rates than mammalian-derived viruses. Our research suggests that VSV replication in Culicoides cells increases viral fitness, facilitating midge-to-midge transmission and subsequent replication, and further highlights the significance of Culicoides midges in VSV maintenance and transmission dynamics.

Considerations for Glycoprotein Production

This chapter is intended to provide insights for researchers aiming to choose an appropriate expression system for the production of recombinant glycoproteins. Producing glycoproteins is complex, as glycosylation patterns are determined by the availability and abundance of specific enzymes rather than a direct genetic blueprint. Furthermore, the cell systems often employed for protein production are evolutionarily distinct, leading to significantly different glycosylation when utilized for glycoprotein production. The selection of an appropriate production system depends on the intended applications and desired characteristics of the protein. Whether the goal is to produce glycoproteins mimicking native conditions or to intentionally alter glycan structures for specific purposes, such as enhancing immunogenicity in vaccines, understanding glycosylation present in the different systems and in different growth conditions is essential. This chapter will cover Escherichia coli, baculovirus/insect cell systems, Pichia pastoris, as well as different mammalian cell culture systems including Chinese hamster ovary (CHO) cells, human endothelial kidney (HEK) cell lines, and baby hamster kidney (BHK) cells.

Crystal structures of human immune protein FIBCD1 suggest an extended binding site compatible with recognition of pathogen-associated carbohydrate motifs

Fibrinogen C domain-containing protein 1 (FIBCD1) is an immune protein proposed to be involved in host recognition of chitin on the surface of pathogens. As FIBCD1 readily binds acetylated molecules, we have determined the high-resolution crystal structures of a recombinant fragment of the FIBCD1 C-terminal domain complexed with small N-acetyl-containing ligands to determine the mode of recognition. All ligands bind at the conserved N-acetyl-binding site (S1) with galactose and glucose-derived ligands rotated 180° relative to each other. One subunit of a native structure derived from protein expressed in mammalian cells binds glycosylation from a neighboring subunit, in an extended binding site. Across the various structures, the primary S1 binding pocket is occupied by N-acetyl-containing ligands or acetate, with N-acetyl, acetate, or sulfate ion in an adjacent pocket S1(2). Inhibition binding studies of N-acetylglucosamine oligomers, (GlcNAc)n, n = 1, 2, 3, 5, 11, via ELISA along with microscale thermophoresis affinity assays indicate a strong preference of FIBCD1 for longer N-acetylchitooligosaccharides. Binding studies of mutant H396A, located beyond the S1(2) site, showed no significant difference from wildtype, but K381L, within the S1(2) pocket, blocked binding to the model ligand acetylated bovine serum albumin, suggesting that S1(2) may have functional importance in ligand binding. The binding studies, alongside structural definition of diverse N-acetyl monosaccharide binding in the primary S1 pocket and of additional, adjacent binding pockets, able to accommodate both carbohydrate and sulfate functional groups, suggest a versatility in FIBCD1 to recognize chitin oligomers and other pathogen-associated carbohydrate motifs across an extended surface.

Evolution and phylogenetic distribution of endo-α-mannosidase

While glycans underlie many biological processes, such as protein folding, cell adhesion, and cell-cell recognition, deep evolution of glycosylation machinery remains an understudied topic. N-linked glycosylation is a conserved process in which mannosidases are key trimming enzymes. One of them is the glycoprotein endo-α-1,2-mannosidase which participates in the initial trimming of mannose moieties from an N-linked glycan inside the cis-Golgi. It is unique as the only endo-acting mannosidase found in this organelle. Relatively little is known about its origins and evolutionary history; so far it was reported to occur only in vertebrates. In this work, a taxon-rich bioinformatic survey to unravel the evolutionary history of this enzyme, including all major eukaryotic clades and a wide representation of animals, is presented. The endomannosidase was found to be more widely distributed in animals and other eukaryotes. The protein motif changes in context of the canonical animal enzyme were tracked. Additionally, the data show the two canonical vertebrate endomannosidase genes, MANEA and MANEAL, arose at the second round of the two vertebrate genome duplications and one more vertebrate paralog, CMANEAL, is uncovered. Finally, a framework where N-glycosylation co-evolved with complex multicellularity is described. A better understanding of the evolution of core glycosylation pathways is pivotal to understanding biology of eukaryotes in general, and the Golgi apparatus in particular. This systematic analysis of the endomannosidase evolution is one step toward this goal.

Unusual β1-4-galactosidase activity of an α1-6-mannosidase from Xanthomonas manihotis in the processing of branched hybrid and complex glycans

Mannosidases are a diverse group of glycoside hydrolases that play crucial roles in mannose trimming of oligomannose glycans, glycoconjugates, and glycoproteins involved in numerous cellular processes, such as glycan biosynthesis and metabolism, structure regulation, cellular recognition, and cell–pathogen interactions. Exomannosidases and endomannosidases cleave specific glycosidic bonds of mannoside linkages in glycans and can be used in enzyme-based methods for sequencing of isomeric glycan structures. α1-6-mannosidase from Xanthomonas manihotis is known as a highly specific exoglycosidase that removes unbranched α1-6 linked mannose residues from oligosaccharides. However, we discovered that this α1-6-mannosidase also possesses an unexpected β1-4-galactosidase activity in the processing of branched hybrid and complex glycans through our use of enzymatic reactions, high performance anion-exchange chromatography, and liquid chromatography mass spectrometric sequencing. Our docking simulation of the α1-6-mannosidase with glycan substrates reveals potential interacting residues in a relatively shallow pocket slightly differing from its homologous enzymes in the glycoside hydrolase 125 family, which may be responsible for the observed higher promiscuity in substrate binding and subsequent terminal glycan hydrolysis. This observation of novel β1-4-galactosidase activity of the α1-6-mannosidase provides unique insights into its bifunctional activity on the substrate structure-dependent processing of terminal α1-6-mannose of unbranched glycans and terminal β1-4-galactose of hybrid and complex glycans. The finding thus suggests the dual glycosidase specificity of this α1-6-mannosidase and the need for careful consideration when used for the structural elucidation of glycan isomers.

Carbohydrates in allergy: from disease to novel immunotherapies

Respiratory allergic disorders are a global public health problem that are responsible for substantial morbidity and healthcare expenditure. Despite the availability of allergen immunotherapy (AIT), its efficacy is suboptimal and regimens are lengthy, with a significant risk of potentially severe side effects. Studies on the recognition of allergens by immune cells through carbohydrate-lectin interactions, which play a crucial role in immune modulation and pathogenesis of allergy, have paved the way for improvements in AIT. We highlight innovative approaches for more effective and safer AIT, including the use of allergens conjugated to specific carbohydrates that bind to C-type lectins (CLRs) and sialic acid-binding immunoglobulin-type lectins (Siglecs) on immune cells to induce suppressive responses.

Involvement of N-glycans in binding of Photorhabdus luminescens Tc toxin

Photorhabdus luminescens Tc toxins are large tripartite ABC-type toxin complexes, composed of TcA, TcB and TcC proteins. Tc toxins are widespread and have shown a tropism for a variety of targets including insect, mammalian and human cells. However, their receptors and the specific mechanisms of uptake into target cells remain unknown. Here, we show that the TcA protein TcdA1 interacts with N-glycans, particularly Lewis X/Y antigens. This is confirmed using N-acetylglucosamine transferase I (Mgat1 gene product)-deficient Chinese hamster ovary (CHO) Lec1 cells, which are highly resistant to intoxication by the Tc toxin complex most likely due to the absence of complex N-glycans. Restoring Mgat1 gene activity, and hence complex N-glycan biosynthesis, recapitulated the sensitivity of these cells to the toxin. Exogenous addition of Lewis X trisaccharide partially inhibits intoxication in wild-type cells. Additionally, sialic acid also largely reduced binding of the Tc toxin. Moreover, proteolytic activation of TcdA1 alters glycan-binding and uptake into target cells. The data suggest that TcdA1-binding is most likely multivalent, and carbohydrates probably work cooperatively to facilitate binding and intoxication.

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.

Characterization of glycan determinants that mediate recognition of the major Wuchereria bancrofti circulating antigen by diagnostic antibodies

The Global Program to Eliminate Lymphatic Filariasis (GPELF) relies heavily on a rapid diagnostic test (RDT) to a Wuchereria bancrofti circulating filarial antigen (Wb-CFA) to identify endemic areas and for determining when mass drug administration can stop. The antigen contains a carbohydrate epitope that is recognized by monoclonal antibody AD12. Og4C3, a monoclonal antibody that is used in a commercial ELISA for Wb-CFA recognizes the same moiety. Despite its diagnostic importance, little is known about the structure and function of this "AD12 epitope". It is also present on other W. bancrofti glycoproteins and on glycoproteins of other filarial worms, but such antigens are not detected in the sera of individuals with most other filarial infections. We report here functional and biochemical analyses that shed light on the interaction between filarial glycoproteins and AD12 and/or Og4C3. Binding of these monoclonal antibodies to a mammalian glycan array suggests the reactive moiety has structural similarity to terminal β-d-glucuronic acid in a 1-3 linkage to other hexoses. However, sera collected from individuals with patent W. bancrofti infection had very low or undetectable serum antibodies to the GlcA-containing array glycans. Unlike other filarial glycoproteins, the Wb-CFA is relatively resistant to protease digestion by pronase and trypsin and completely resistant to the mucinase O-sialoglycoprotein endopeptidase (OSGE). The protease resistance of the Wb-CFA may contribute to its consistent detection in Wb-infected sera.

Binding specificity of ostreolysin A6 towards Sf9 insect cell lipids

Oyster mushrooms (Pleurotus spp.) have recently been shown to produce insecticidal bi-component protein complexes based on the aegerolysin proteins. A role for these proteins is thus indicated for defence and protection of the mushroom, and we propose their use as new environmentally friendly bioinsecticides. These aegerolysin-based protein complexes permeabilise artificial lipid vesicles through aegerolysin binding to an insect-specific sphingolipid, ceramide phosphoethanolamine (CPE), and they are cytotoxic for the Spodoptera frugiferda (Sf9) insect cell line. Tandem mass spectrometry analysis of the Sf9 lipidome uncovered lipids not previously reported in the literature, including in particular C14 sphingosine-based CPE molecular species, which comprised ~4 mol% of the whole lipidome. Further analysis of the lipid binding specificity of an aegerolysin from P. ostreatus, ostreolysin A6 (OlyA6), to lipid vesicles composed of commercial lipids, to lipid vesicles composed of the total lipid extract from Sf9 cells, and to HPLC-separated Sf9 cell lipid fractions containing ceramides, confirmed CPE as the main OlyA6 receptor, but also highlighted the importance of membrane cholesterol for formation of strong and stable interactions of OlyA6 with artificial and natural lipid membranes. Binding assays performed with glycan arrays and surface plasmon resonance, which included invertebrate-specific glycans, excluded these saccharides as potential additional OlyA6 receptors.

Expression and purification of recombinant G protein-coupled receptors: A review

Given their extensive role in cell signalling, GPCRs are significant drug targets; despite this, many of these receptors have limited or no available prophylaxis. Novel drug design and discovery significantly rely on structure determination, of which GPCRs are typically elusive. Progress has been made thus far to produce sufficient quantity and quality of protein for downstream analysis. As such, this review highlights the systems available for recombinant GPCR expression, with consideration of their advantages and disadvantages, as well as examples of receptors successfully expressed in these systems. Additionally, an overview is given on the use of detergents and the styrene maleic acid (SMA) co-polymer for membrane solubilisation, as well as purification techniques.

Glycan diversity in the course of vertebrate evolution

Vertebrates are estimated to have arisen over 500 million years ago in the Cambrian Period. Species that survived the Big Five extinction events at a global scale underwent repeated adaptive radiations along with habitat expansions from the sea to the land and sky. The development of the endoskeleton and neural tube enabled more complex body shapes. At the same time, vertebrates became suitable for the invasion and proliferation of foreign organisms. Adaptive immune systems were acquired for responses to a wide variety of pathogens, and more sophisticated systems developed during the evolution of mammals and birds. Vertebrate glycans consist of common core structures and various elongated structures, such as Neu5Gc, Galα1-3Gal, Galα1-4Gal, and Galβ1-4Gal epitopes, depending on the species. During species diversification, complex glycan structures were generated, maintained or lost. Whole-genome sequencing has revealed that vertebrates harbor numerous and even redundant glycosyltransferase genes. The production of various glycan structures is controlled at the genetic level in a species-specific manner. Because cell surface glycans are often targets of bacterial and viral infections, glycan structural diversity is presumed to be protective against infections. However, the maintenance of apparently redundant glycosyltransferase genes and investment in species-specific glycan structures, even in higher vertebrates with highly developed immune systems, are not well explained. This fact suggests that glycans play important roles in unknown biological processes.

Structural basis for the specific cleavage of core-fucosylated N-glycans by endo-β-N-acetylglucosaminidase from the fungus Cordyceps militaris

N-Linked glycans play important roles in various cellular and immunological events. Endo-β-N-acetylglucosaminidase (ENGase) can release or transglycosylate N-glycans and is a promising tool for the chemoenzymatic synthesis of glycoproteins with homogeneously modified glycans. The ability of ENGases to act on core-fucosylated glycans is a key factor determining their therapeutic utility because mammalian N-glycans are frequently α-1,6-fucosylated. Although the biochemistries and structures of various ENGases have been studied extensively, the structural basis for the recognition of the core fucose and the asparagine-linked GlcNAc is unclear. Herein, we determined the crystal structures of a core fucose-specific ENGase from the caterpillar fungus Cordyceps militaris (Endo-CoM), which belongs to glycoside hydrolase family 18. Structures complexed with fucose-containing ligands were determined at 1.75-2.35 Å resolutions. The fucose moiety linked to GlcNAc is extensively recognized by protein residues in a round-shaped pocket, whereas the asparagine moiety linked to the GlcNAc is exposed to the solvent. The N-glycan-binding cleft of Endo-CoM is Y-shaped, and several lysine and arginine residues are present at its terminal regions. These structural features were consistent with the activity of Endo-CoM on fucose-containing glycans on rituximab (IgG) and its preference for a sialobiantennary substrate. Comparisons with other ENGases provided structural insights into their core fucose tolerance and specificity. In particular, Endo-F3, a known core fucose-specific ENGase, has a similar fucose-binding pocket, but the surrounding residues are not shared with Endo-CoM. Our study provides a foothold for protein engineering to develop enzymatic tools for the preparation of more effective therapeutic antibodies.

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.

Comparisons of N-glycans across invertebrate phyla

Many invertebrates are either parasites themselves or vectors involved in parasite transmission; thereby, the interactions of parasites with final or intermediate hosts are often mediated by glycans. Therefore, it is of interest to compare the glycan structures or motifs present across invertebrate species. While a typical vertebrate modification such as sialic acid is rare in lower animals, antennal and core modifications of N-glycans are highly varied and range from core fucose, galactosylated fucose, fucosylated galactose, methyl groups, glucuronic acid and sulphate through to addition of zwitterionic moieties (phosphorylcholine, phosphoethanolamine and aminoethylphosphonate). Only in some cases are the enzymatic bases and the biological function of these modifications known. We are indeed still in the phase of discovering invertebrate glycomes primarily using mass spectrometry, but molecular biology and microarraying techniques are complementary to the determination of novel glycan structures and their functions.

Cryo-EM structures of the pore-forming A subunit from the Yersinia entomophaga ABC toxin

ABC toxins are pore-forming virulence factors produced by pathogenic bacteria. YenTcA is the pore-forming and membrane binding A subunit of the ABC toxin YenTc, produced by the insect pathogen Yersinia entomophaga. Here we present cryo-EM structures of YenTcA, purified from the native source. The soluble pre-pore structure, determined at an average resolution of 4.4 Å, reveals a pentameric assembly that in contrast to other characterised ABC toxins is formed by two TcA-like proteins (YenA1 and YenA2) and decorated by two endochitinases (Chi1 and Chi2). We also identify conformational changes that accompany membrane pore formation by visualising YenTcA inserted into liposomes. A clear outward rotation of the Chi1 subunits allows for access of the protruding translocation pore to the membrane. Our results highlight structural and functional diversity within the ABC toxin subfamily, explaining how different ABC toxins are capable of recognising diverse hosts.

YenTcA is the pore-forming and membrane binding subunit of the ABC toxin YenTc, which is produced by the insect pathogen Yersinia entomophaga. Here authors present cryo-EM structures of YenTcA purified from the native source which implicate associated endochitinases in host cell recognition.

The N-glycome of the hemipteran pest insect Nilaparvata lugens reveals unexpected sex differences

The brown planthopper, Nilaparvata lugens, is a model species for hemimetabolous development and the most important pest insect in rice, which is the major staple crop for about half of the world population. Despite its importance, little is known of the N-glycosylation process in this insect. Here we report on the N-glycome for the post-embryonic stages of N. lugens, revealing unique features that are different from the holometabolous insect models, as the fruit fly Drosophila melanogaster and the beetle Tribolium castaneum. Analysis of the N-glycan fingerprint for male and female adults showed sex-specific N-glycosylation in insects. Specifically, the female adults progress towards a unique glycan profile with a striking increase in high mannose N-glycans. The N-glycome of N. lugens contributes to study pathways differentiating between sexes, and the results shed light on the evolution and differences in development between primitive hemimetabolous insects and more advanced holometabolous insects. The data are discussed in relation to potential function(s) in development and sex specificity.

Surface molecules of extracellular vesicles secreted by the helminth pathogen Fasciola hepatica direct their internalisation by host cells

Helminth parasites secrete extracellular vesicles (EVs) that can be internalised by host immune cells resulting in modulation of host immunity. While the molecular cargo of EVs have been characterised in many parasites, little is known about the surface-exposed molecules that participate in ligand-receptor interactions with the host cell surface to initiate vesicle docking and subsequent internalisation. Using a membrane-impermeable biotin reagent to capture proteins displayed on the outer membrane surface of two EV sub-populations (termed 15k and 120k EVs) released by adult F. hepatica, we describe 380 surface proteins including an array of virulence factors, membrane transport proteins and molecules involved in EV biogenesis/trafficking. Proteomics and immunohistochemical analysis show that the 120k EVs have an endosomal origin and may be released from the parasite via the protonephridial (excretory) system whilst the larger 15k EVs are released from the gastrodermal epithelial cells that line the fluke gut. A parallel lectin microarray strategy was used to profile the topology of major surface oligosaccharides of intact fluorogenically-labelled EVs as they would be displayed to the host. Lectin profiles corresponding to glycoconjugates exposed on the surface of the 15 K and 120K EV sub-populations are practically identical but are distinct from those of the parasite surface tegument, although all are predominated by high mannose sugars. We found that while the F. hepatica EVs were resistant to exo- and endo-glycosidases, the glyco-amidase PNGase F drastically remodelled the surface oligosaccharides and blocked the uptake of EVs by host macrophages. In contrast, pre-treatment with antibodies obtained from infected hosts, or purified antibodies raised against the extracellular domains of specific EV surface proteins (DM9-containing protein, CD63 receptor and myoferlin), significantly enhanced their cellular internalisation. This work highlights the diversity of EV biogenesis and trafficking pathways used by F. hepatica and sheds light on the molecular interaction between parasite EVs and host cells.

Over the last decade, it has become recognised that extracellular vesicles (EVs) are important mediators of communication by transferring molecular signals (including proteins, lipids, complex carbohydrates, mRNA, microRNA and other non-coding RNA species), between cells. Variously described as exosomes or microvesicles depending on their cellular origin and mode of biogenesis, EVs perform a variety of roles in the maintenance of normal physiology but also participate in pathological settings. EVs also play an important role in host-pathogen interactions, with recent work suggesting that they contribute to helminth immunomodulatory strategies. Here we have identified the proteins and sugars displayed on the outer surface of two sub-types of EVs released by the helminth pathogen Fasciola hepatica. We show that the proteins are antigenic and direct EV internalisation by host macrophages. Our study provides a better understanding of how parasite-derived EVs interact with host cells which is important for future development of therapeutics/vaccines that target this interface.

Highly modified and immunoactive N-glycans of the canine heartworm

The canine heartworm (Dirofilaria immitis) is a mosquito-borne parasitic nematode whose range is extending due to climate change. In a four-dimensional analysis involving HPLC, MALDI-TOF–MS and MS/MS in combination with chemical and enzymatic digestions, we here reveal an N-glycome of unprecedented complexity. We detect N-glycans of up to 7000 Da, which contain long fucosylated HexNAc-based repeats, as well as glucuronylated structures. While some modifications including LacdiNAc, chitobiose, α1,3-fucose and phosphorylcholine are familiar, anionic N-glycans have previously not been reported in nematodes. Glycan array data show that the neutral glycans are preferentially recognised by IgM in dog sera or by mannose binding lectin when antennal fucose and phosphorylcholine residues are removed; this pattern of reactivity is reversed for mammalian C-reactive protein, which can in turn be bound by the complement component C1q. Thereby, the N-glycans of D. immitis contain features which may either mediate immunomodulation of the host or confer the ability to avoid immune surveillance.

The glycome of parasites can have immunomodulatory properties or help to avoid immune surveillance, but details are unknown. Here, Martini et al. characterize the N-glycome of the canine heartworm, reveal an unprecedented complexity, particularly in anionic N-glycans, and determine recognition by components of the immune system.

Protein-Specific Analysis of Invertebrate Glycoproteins

N-Glycans are posttranslational modifications of proteins attached to the amide side chains of asparagine residues, with possible heterogeneity due to different structures being possible at the same glycosylation site. In contrast to the mammalian systems, invertebrate N-glycosylation presents a challenge in analysis as there exist unfamiliar epitopes and a high degree of structural and isomeric variation between different species. A simple analytical approach to analyze N-glycans on specific glycoproteins is presented, which involves a combination of tryptic peptide mass spectrometry and "off-line" RP-HPLC MALDI-TOF MS/MS complemented by blotting to recognize specific epitopes. An additional N-glycan enrichment and labeling step can facilitate the analysis of single structures and even provide isomeric separation of N-glycans from specific proteins.

CRISPR/Cas9 and glycomics tools for Toxoplasma glycobiology

Infection with the protozoan parasite Toxoplasma gondii is a major health risk owing to birth defects, its chronic nature, ability to reactivate to cause blindness and encephalitis, and high prevalence in human populations. Unlike most eukaryotes, Toxoplasma propagates in intracellular parasitophorous vacuoles, but like nearly all other eukaryotes, Toxoplasma glycosylates many cellular proteins and lipids and assembles polysaccharides. Toxoplasma glycans resemble those of other eukaryotes, but species-specific variations have prohibited deeper investigations into their roles in parasite biology and virulence. The Toxoplasma genome encodes a suite of likely glycogenes expected to assemble N-glycans, O-glycans, a C-glycan, GPI-anchors, and polysaccharides, along with their precursors and membrane transporters. To investigate the roles of specific glycans in Toxoplasma, here we coupled genetic and glycomics approaches to map the connections between 67 glycogenes, their enzyme products, the glycans to which they contribute, and cellular functions. We applied a double-CRISPR/Cas9 strategy, in which two guide RNAs promote replacement of a candidate gene with a resistance gene; adapted MS-based glycomics workflows to test for effects on glycan formation; and infected fibroblast monolayers to assess cellular effects. By editing 17 glycogenes, we discovered novel Glc_0-2-Man₆-GlcNAc₂-type N-glycans, a novel HexNAc-GalNAc-mucin-type O-glycan, and Tn-antigen; identified the glycosyltransferases for assembling novel nuclear O-Fuc-type and cell surface Glc-Fuc-type O-glycans; and showed that they are important for in vitro growth. The guide sequences, editing constructs, and mutant strains are freely available to researchers to investigate the roles of glycans in their favorite biological processes.

The Secretome and N-Glycosylation Profiles of the Charophycean Green Alga, Penium margaritaceum, Resemble Those of Embryophytes

The secretome can be defined as the population of proteins that are secreted into the extracellular environment. Many proteins that are secreted by eukaryotes are N-glycosylated. However, there are striking differences in the diversity and conservation of N-glycosylation patterns between taxa. For example, the secretome and N-glycosylation structures differ between land plants and chlorophyte green algae, but it is not clear when this divergence took place during plant evolution. A potentially valuable system to study this issue is provided by the charophycean green algae (CGA), which is the immediate ancestors of land plants. In this study, we used lectin affinity chromatography (LAC) coupled with mass spectrometry to characterize the secretome including secreted N-glycoproteins of Penium margaritaceum, which is a member of the CGA. The identified secreted proteins and N-glycans were compared to those known from the chlorophyte green alga Chlamydomonas reinhardtii and the model land plant, Arabidopsis thaliana, to establish their evolutionary context. Our approach allowed the identification of cell wall proteins and proteins modified with N-glycans that are identical to those of embryophytes, which suggests that the P. margaritaceum secretome is more closely related to those of land plants than to those of chlorophytes. The results of this study support the hypothesis that many of the proteins associated with plant cell wall modification as well as other extracellular processes evolved prior to the colonization of terrestrial habitats.

N-Glycoproteomic Characterization of Mannosidase and Xylosyltransferase Mutant Strains of Chlamydomonasreinhardtii

At present, only little is known about the enzymatic machinery required for N-glycosylation in Chlamydomonas reinhardtii, leading to the formation of N-glycans harboring Xyl and methylated Man. This machinery possesses new enzymatic features, as C. reinhardtii N-glycans are independent of β1,2-N-acetylglucosaminyltransferase I. Here we have performed comparative N-glycoproteomic analyses of insertional mutants of mannosidase 1A (IM Man1A ) and xylosyltransferase 1A (IM XylT1A ). The disruption of man1A affected methylation of Man and the addition of terminal Xyl. The absence of XylT1A led to shorter N-glycans compared to the wild type. The use of a IM Man1A xIM XylT1A double mutant revealed that the absence of Man1A suppressed the IM XylT1A phenotype, indicating that the increased N-glycan trimming is regulated by core β1,2-Xyl and is dependent on Man1A activity. These data point toward an enzymatic cascade in the N-glycosylation pathway of C. reinhardtii with interlinked roles of Man1A and XylT1A. The results described herein represent the first step toward a functional characterization of the enzymatic N-glycosylation machinery in C. reinhardtii.

Identification of multiple isomeric core chitobiose-modified high-mannose and paucimannose N-glycans in the planarian Schmidtea mediterranea

Cell surface–associated glycans mediate many cellular processes, including adhesion, migration, signaling, and extracellular matrix organization. The galactosylation of core fucose (GalFuc epitope) in paucimannose and complex-type N-glycans is characteristic of protostome organisms, including flatworms (planarians). Although uninvestigated, the structures of these glycans may play a role in planarian regeneration. Whole-organism MALDI-MS analysis of N-linked oligosaccharides from the planarian Schmidtea mediterranea revealed the presence of multiple isomeric high-mannose and paucimannose structures with unusual mono-, di-, and polygalactosylated (n = 3–5) core fucose structures; the latter structures have not been reported in other systems. Di- and trigalactosylated core fucoses were the most dominant glycomers. N-Glycans showed extensive, yet selective, methylation patterns, ranging from non-methylated to polymethylated glycoforms. Although the majority of glycoforms were polymethylated, a small fraction also consisted of non-methylated glycans. Remarkably, monogalactosylated core fucose remained unmethylated, whereas its polygalactosylated forms were methylated, indicating structurally selective methylation. Using database searches, we identified two potential homologs of the Galβ1–4Fuc–synthesizing enzyme from nematodes (GALT-1) that were expressed in the prepharyngeal, pharyngeal, and mesenchymal regions in S. mediterranea. The presence of two GALT-1 homologs suggests different requirements for mono- and polygalactosylation of core fucose for the formation of multiple isomers. Furthermore, we observed variations in core fucose glycosylation patterns in different planarian strains, suggesting evolutionary adaptation in fucose glycosylation. The various core chitobiose modifications and methylations create >60 different glycoforms in S. mediterranea. These results contribute greatly to our understanding of N-glycan biosynthesis and suggest the presence of a GlcNAc-independent biosynthetic pathway in S. mediterranea.

Cryptosporidium parvum vaccine candidates are incompletely modified with O-linked-N-acetylgalactosamine or contain N-terminal N-myristate and S-palmitate

Cryptosporidium parvum (studied here) and Cryptosporidium hominis are important causes of diarrhea in infants and immunosuppressed persons. C. parvum vaccine candidates, which are on the surface of sporozoites, include glycoproteins with Ser- and Thr-rich domains (Gp15, Gp40, and Gp900) and a low complexity, acidic protein (Cp23). Here we used mass spectrometry to determine that O-linked GalNAc is present in dense arrays on a glycopeptide with consecutive Ser derived from Gp40 and on glycopeptides with consecutive Thr derived from Gp20, a novel C. parvum glycoprotein with a formula weight of ~20 kDa. In contrast, the occupied Ser or Thr residues in glycopeptides from Gp15 and Gp900 are isolated from one another. Gly at the N-terminus of Cp23 is N-myristoylated, while Cys, the second amino acid, is S-palmitoylated. In summary, C. parvum O-GalNAc transferases, which are homologs of host enzymes, densely modify arrays of Ser or Thr, as well as isolated Ser and Thr residues on C. parvum vaccine candidates. The N-terminus of an immunodominant antigen has lipid modifications similar to those of host cells and other apicomplexan parasites. Mass spectrometric demonstration here of glycopeptides with O-glycans complements previous identification C. parvum O-GalNAc transferases, lectin binding to vaccine candidates, and human and mouse antibodies binding to glycopeptides. The significance of these post-translational modifications is discussed with regards to the function of these proteins and the design of serological tests and vaccines.

Structures and developmental alterations of N-glycans of zebrafish embryos

Zebrafish is a model organism suitable for studying vertebrate development. We analyzed the N-glycan structures of zebrafish embryos and their alterations during zebrafish embryogenesis to obtain basic data for studying the roles of N-glycosylation. Multiple modes of high-performance liquid chromatography and multistage mass spectrometry were used for structural analysis of N-glycans. The N-glycans from deyolked embryos at 36 hours postfertilization, a mid-pharyngula stage, contained relatively higher amounts of complex- and hybrid-type glycans with LacNAc (Galβ1-4GlcNAc) and/or sialyl LacNAc without additional β1,4-Gal, which are commonly found in mammalian tissues, as well as abundant oligomannose-type glycans. Some of the complex- and hybrid-type glycans possessed various extended LacNAc structures, such as Galβ1-4LacNAc, LacNAc-repeat or unique (+/- dHex)-GalNAcα1-GlcNAcβ1-LacNAc. In contrast, the yolk of the embryo contains predominant oligomannose-type glycans and complex-type glycans with Galβ1-4(Siaα2-3)Galβ1-4(Fucα1-3)GlcNAc antennae. N-Glycan profiles obtained from deyolked embryos at different stages showed stage-dependent variation of complex- and hybrid-type glycans. At gastrula and early segmentation stages, complex- and hybrid-type glycans were minor components, and their antenna structures were mainly sialyl LacdiNAc (Siaα2-6GalNAcβ1-4GlcNAc). From the mid-segmentation to pharyngula stages, those with LacNAc and/or α2,6-sialyl LacNAc antenna structures increased remarkably, and those with α2,3-sialyl LacNAc antenna, core α1,6-Fuc and bisecting GlcNAc modifications increased gradually. These results suggest the presence of mechanisms for regulating the antenna structures of complex/hybrid N-glycan biosynthesis in the phylotypic stage of vertebrate development.

N-glycan structures of β-HlH subunit of Helix lucorum hemocyanin

The carbohydrate structures of molluscan hemocyanins have recently received particular interest due to their specific monosaccharide composition, as well as their immunostimulatory properties and application in clinical studies. For the first time, we investigated N-glycans of the structural subunit β-HlH of hemocyanin isolated from Helix lucorum. In total, 32 different glycans were enzymatically liberated and characterized by tandem mass spectrometry using a Q-Trap mass spectrometer. Our study revealed a highly heterogeneous mixture of glycans with composition Hex_3-7HexNAc_2-5MeHex_0-4Pent_0-1Fuc_0-1. The oligosaccharide chains are mostly modified at the inner core by β1-2-linked xylose to β-mannose, by α1-6-fucosylation of the innermost GlcNAc residue (the Asn-bound GlcNAc), and by methylation. The glycans of β-HlH mainly contain a terminal MeHex residue; in some cases even two, three or four of these residues occur. Several carbohydrate chains in β-HlH are core-fucosylated without Xyl and also possess a high degree of methylation. This study shows the presence of mono- and bi-antennary N-glycans as well as hybrid type structures with or without core-fucosylation.

Asparagine-Linked Glycans of Cryptosporidium parvum Contain a Single Long Arm, Are Barely Processed in the Endoplasmic Reticulum (ER) or Golgi, and Show a Strong Bias for Sites with Threonine

Cryptosporidium parvum causes severe diarrhea in infants in developing countries and in immunosuppressed persons, including those with AIDS. We are interested in the Asn-linked glycans (N-glycans) of C. parvum, because (1) the N-glycan precursor is predicted to contain five mannose and two glucose residues on a single long arm versus nine mannose and three glucose residues on the three-armed structure common in host N-glycans, (2) C. parvum is a rare eukaryote that lacks the machinery for N-glycan-dependent quality control of protein folding in the lumen of the Endoplasmic Reticulum (ER), and (3) ER and Golgi mannosidases, as well as glycosyltransferases that build complex N-glycans, are absent from the predicted proteome. The C. parvum N-glycans reported here, which were determined using a combination of collision-induced dissociation and electronic excitation dissociation, contain a single, unprocessed mannose arm ± terminal glucose on the trimannosyl chitobiose core. Upon nanoUPLC-MS/MS separation and analysis of the C. parvum tryptic peptides, the total ion and extracted oxonium ion chromatograms delineated 32 peptides with occupied N-glycan sites; these were derived from 16 glycoproteins. Although the number of potential N-glycan sites with Thr (NxT) is only about twice that with Ser (NxS), almost 90% of the occupied N-glycan sites contain NxT. The two most abundant C. parvum proteins modified with N-glycans were an immunodominant antigen on the surface of sporozoites (gp900) and the possible oocyst wall protein 1 (POWP1). Seven other glycoproteins with N-glycans were unique to C. parvum; five shared common ancestry with other apicomplexans; two glycoproteins shared common ancestry with many organisms. In summary, C. parvum N-glycans are remarkable for the absence of ER and Golgi modification and for the strong bias toward occupancy of N-glycan motifs containing Thr.

Analysis of Invertebrate and Protist N-Glycans

N-glycans from invertebrates and protists have often unusual structures which present analytical challenges. Both core and antennal modifications can be quite different from the more familiar vertebrate glycan motifs; thereby, contrary to the concept that "simple" organisms have "simple" N-glycans, rather complex oligosaccharides structures, including zwitterionic and anionic ones, have been found in a range of species. Thus, to facilitate the optimized elucidation of the maximal possible range of structures, the analytical workflow for glycomics of these organisms should include sequential release and fractionation steps. Peptide:N-glycosidase F is sufficient to isolate N-glycans from fungi and some protists, but in most invertebrates core α1,3-fucose is present, so release of the glycans from glycopeptides with peptide:N-glycosidases A is required. Subsequent solid-phase extraction with graphitized carbon and reversed phase resins enables different classes of N-glycans to be separated prior to high-pressure liquid chromatography (HPLC) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Depending on the types and numbers of glycans present, either reversed- or normal-phase HPLC (or both in series) enable even single isomeric or isobaric structures to be separated prior to MALDI-TOF MS and MS/MS. The use of enzymatic or chemical treatments allows further insights to be gained, although some glycan modifications (especially methylation) are resistant. Using a battery of methods, sometimes up to 100 structures from a single organism can be assigned, a complexity which raises evolutionary questions regarding the function of these glycans.

Helminth glycomics - glycan repertoires and host-parasite interactions

Glycoproteins and glycolipids of parasitic helminths play important roles in biology and host-parasite interaction. This review discusses recent helminth glycomics studies that have been expanding our insights into the glycan repertoire of helminths. Structural data are integrated with biological and immunological observations to highlight how glycomics advances our understanding of the critical roles that glycans and glycan motifs play in helminth infection biology. Prospects and challenges in helminth glycomics and glycobiology are discussed.

The multiple evolutionary origins of the eukaryotic N-glycosylation pathway

BACKGROUND

The N-glycosylation is an essential protein modification taking place in the membranes of the endoplasmic reticulum (ER) in eukaryotes and the plasma membranes in archaea. It shares mechanistic similarities based on the use of polyisoprenol lipid carriers with other glycosylation pathways involved in the synthesis of bacterial cell wall components (e.g. peptidoglycan and teichoic acids). Here, a phylogenomic analysis was carried out to examine the validity of rival hypotheses suggesting alternative archaeal or bacterial origins to the eukaryotic N-glycosylation pathway.

RESULTS

The comparison of several polyisoprenol-based glycosylation pathways from the three domains of life shows that most of the implicated proteins belong to a limited number of superfamilies. The N-glycosylation pathway enzymes are ancestral to the eukaryotes, but their origins are mixed: Alg7, Dpm and maybe also one gene of the glycosyltransferase 1 (GT1) superfamily and Stt3 have proteoarchaeal (TACK superphylum) origins; alg2/alg11 may have resulted from the duplication of the original GT1 gene; the lumen glycosyltransferases were probably co-opted and multiplied through several gene duplications during eukaryogenesis; Alg13/Alg14 are more similar to their bacterial homologues; and Alg1, Alg5 and a putative flippase have unknown origins.

CONCLUSIONS

The origin of the eukaryotic N-glycosylation pathway is not unique and less straightforward than previously thought: some basic components likely have proteoarchaeal origins, but the pathway was extensively developed before the eukaryotic diversification through multiple gene duplications, protein co-options, neofunctionalizations and even possible horizontal gene transfers from bacteria. These results may have important implications for our understanding of the ER evolution and eukaryogenesis.

REVIEWERS

This article was reviewed by Pr. Patrick Forterre and Dr. Sergei Mekhedov (nominated by Editorial Board member Michael Galperin).

Identification of a dTDP-rhamnose biosynthetic pathway that oscillates with the molting cycle in Caenorhabditis elegans

The rhamnose biosynthetic pathway, which is highly conserved across nematode species, was characterized in the nematode Caenorhabditis elegans. The pathway is up-regulated before each larval molt, suggesting that rhamnose biosynthesis plays a role in cuticle or surface coat synthesis.

L-Rhamnose is a common component of cell-wall polysaccharides, glycoproteins and some natural products in bacteria and plants, but is rare in fungi and animals. In the present study, we identify and characterize a biosynthetic pathway for dTDP-rhamnose in Caenorhabditis elegans that is highly conserved across nematode species. We show that RML-1 activates glucose 1-phosphate (Glc-1-P) in the presence of either dTTP or UTP to yield dTDP-glucose or UDP-glucose, respectively. RML-2 is a dTDP-glucose 4,6-dehydratase, converting dTDP-glucose into dTDP-4-keto-6-deoxyglucose. Using mass spectrometry and NMR spectroscopy, we demonstrate that coincubation of dTDP-4-keto-6-deoxyglucose with RML-3 (3,5-epimerase) and RML-4 (4-keto-reductase) produces dTDP-rhamnose. RML-4 could only be expressed and purified in an active form through co-expression with a co-regulated protein, RML-5, which forms a complex with RML-4. Analysis of the sugar nucleotide pool in C. elegans established the presence of dTDP-rhamnose in vivo. Targeting the expression of the rhamnose biosynthetic genes by RNAi resulted in significant reductions in dTDP-rhamnose, but had no effect on the biosynthesis of a closely related sugar, ascarylose, found in the ascaroside pheromones. Therefore, the rhamnose and ascarylose biosynthetic pathways are distinct. We also show that transcriptional reporters for the rhamnose biosynthetic genes are expressed highly in the embryo, in the hypodermis during molting cycles and in the hypodermal seam cells specifically before the molt to the stress-resistant dauer larval stage. These expression patterns suggest that rhamnose biosynthesis may play an important role in hypodermal development or the production of the cuticle or surface coat during molting.

Comparison of RP-HPLC modes to analyse the N-glycome of the free-living nematode Pristionchus pacificus

Pristionchus pacificus is a free-living nematode increasingly used as an organism for comparison to the more familiar model Caenorhabditis elegans. In this study, we examined the N-glycans of this organism isolated after serial release with peptide:N-glycosidases F and A; after fluorescent labelling with 2-aminopyridine, chromatographic fractionation by three types of RP-HPLC (with either classical C18, fused core C18 or alkylamide-bonded phases) followed by mass spectrometric analyses revealed key features of its N-glycome. In addition to paucimannosidic and oligomannosidic glycans typical of invertebrates, N-glycans with two core fucose residues were detected. Furthermore, a range of glycans carrying up to three phosphorylcholine residues was observed whereas, unlike C. elegans, no tetrafucosylated N-glycans were detected. Structures with three fucose residues, unusual methylation of core α1,3-fucose or with galactosylated fucose motifs were found in low amounts; these features may correlate with a different ensemble or expression of glycosyltransferase genes as compared to C. elegans. From an analytical perspective, both the alkylamide RP-amide and fused core C18 columns, as compared to a classical C18 material, offer advantages in terms of resolution and of elution properties, as some minor pyridylamino-labelled glycans (e.g. those carrying phosphorylcholine) appear in earlier fractions and so potential losses of such structures due to insufficient gradient length can be avoided.

The fucomic potential of mosquitoes: Fucosylated N-glycan epitopes and their cognate fucosyltransferases

Fucoconjugates are key mediators of protein-glycan interactions in prokaryotes and eukaryotes. As examples, N-glycans modified with the non-mammalian core α1,3-linked fucose have been detected in various organisms ranging from plants to insects and are immunogenic in mammals. The rabbit polyclonal antibody raised against plant horseradish peroxidase (anti-HRP) is able to recognize the α1,3-linked fucose epitope and is also known to specifically stain neural tissues in the fruit fly Drosophila melanogaster. In this study, we have detected and localized the anti-HRP cross-reactivity in another insect species, the malaria mosquito vector Anopheles gambiae. We were able to identify and structurally elucidate fucosylated N-glycans including core mono- and difucosylated structures (responsible for anti-HRP cross reactivity) as well as a Lewis-type antennal modification on mosquito anionic N-glycans by applying enzymatic and chemical treatments. The three mosquito fucosyltransferase open reading frames (FucT6, FucTA and FucTC) required for the in vivo biosynthesis of the fucosylated N-glycan epitopes were identified in the Anopheles gambiae genome, cloned and recombinantly expressed in Pichia pastoris. Using a robust MALDI-TOF MS approach, we characterised the activity of the three recombinant fucosyltransferases in vitro and demonstrate that they share similar enzymatic properties as compared to their homologues from D. melanogaster and Apis mellifera. Thus, not only do we confirm the neural reactivity of anti-HRP in a mosquito species, but also demonstrate enzymatic activity for all its α1,3- and α1,6-fucosyltransferase homologues, whose specificity matches the results of glycomic analyses.

Comparisons of Caenorhabditis Fucosyltransferase Mutants Reveal a Multiplicity of Isomeric N-Glycan Structures

Recent studies have shown a remarkable degree of plasticity in the N-glycome of the model nematode Caenorhabditis elegans; ablation of glycosylation-relevant genes can result in radically altered N-glycan profiles despite only minor biological phenotypic effects. Up to four fucose residues and five different linkages of fucose are known on the N-glycans of C. elegans. Due to the complexity in the wild type, we established three mutant strains defective in two core fucosyltransferases each (fut-1;fut-6, fut-1;fut-8, and fut-6;fut-8). Enzymatically released N-glycans were subject to HPLC and MALDI-TOF MS/MS, in combination with various treatments, to verify structural details. The N-glycome of the fut-1;fut-6 mutant was the most complex of the three double-mutant strains due to the extension of the core α1,6-fucose as well as the presence of fucose on the bisecting galactose. In contrast, maximally two fucoses were found on N-glycans of the fut-1;fut-8 and fut-6;fut-8 strains. The different locations and capping of fucose meant that up to 13 isomeric structures, many highly galactosylated, were determined for some single masses. These data not only show the high variability of the N-glycomic capacity of a "simple" nematode but also exemplify the need for multiple approaches to reveal individual glycan structures within complex invertebrate glycomes.

Bunyaviruses: from transmission by arthropods to virus entry into the mammalian host first-target cells

The Bunyaviridae constitute a large family of animal RNA viruses distributed worldwide, most members of which are transmitted to vertebrate hosts by arthropods and can cause severe pathologies in humans and livestock. With an increasing number of outbreaks, arthropod-borne bunyaviruses (arbo-bunyaviruses) present a global threat to public health and agricultural productivity. Yet transmission, tropism, receptors and cell entry remain poorly characterized. The focus of this review is on the initial infection of mammalian hosts by arbo-bunyaviruses from cellular and molecular perspectives, with particular attention to the human host. We address current knowledge and advances regarding the identity of the first-target cells and the subsequent processes of entry and penetration into the cytosol. Aspects of the vector-to-host switch that influence the early steps of cell infection in mammalian skin, where incoming particles are introduced by infected arthropods, are also highlighted and discussed.

Evolutionary diversity of social amoebae N-glycomes may support interspecific autonomy

Multiple species of cellular slime mold (CSM) amoebae share overlapping subterranean environments near the soil surface. Despite similar life-styles, individual species form independent starvation-induced fruiting bodies whose spores can renew the life cycle. N-glycans associated with the cell surface glycocalyx have been predicted to contribute to interspecific avoidance, resistance to pathogens, and prey preference. N-glycans from five CSM species that diverged 300-600 million years ago and whose genomes have been sequenced were fractionated into neutral and acidic pools and profiled by MALDI-TOF-MS. Glycan structure models were refined using linkage specific antibodies, exoglycosidase digestions, MALDI-MS/MS, and chromatographic studies. Amoebae of the type species Dictyostelium discoideum express modestly trimmed high mannose N-glycans variably modified with core α3-linked Fuc and peripherally decorated with 0-2 residues each of β-GlcNAc, Fuc, methylphosphate and/or sulfate, as reported previously. Comparative analyses of D. purpureum, D. fasciculatum, Polysphondylium pallidum, and Actyostelium subglobosum revealed that each displays a distinctive spectrum of high-mannose species with quantitative variations in the extent of these modifications, and qualitative differences including retention of Glc, mannose methylation, and absence of a peripheral GlcNAc, fucosylation, or sulfation. Starvation-induced development modifies the pattern in all species but, except for universally observed increased mannose-trimming, the N-glycans do not converge to a common profile. Correlations with glycogene repertoires will enable future reverse genetic studies to eliminate N-glycomic differences to test their functions in interspecific relations and pathogen evasion.

Purification of galectin-1 mutants using an immobilized Galactoseβ1-4Fucose affinity adsorbent

Galectins are a family of lectins characterized by their carbohydrate recognition domains containing eight conserved amino acid residues, which allows the binding of galectin to β-galactoside sugars such as Galβ1-4GlcNAc. Since galectin-glycan interactions occur extracellularly, recombinant galectins are often used for the functional analysis of these interactions. Although it is relatively easy to purify galectins via affinity to Galβ1-4GlcNAc using affinity adsorbents such as asialofetuin-Sepharose, it could be difficult to do so with mutated galectins, which may have reduced affinity towards their endogenous ligands. However, this is not the case with Caenorhabditis elegans galectin LEC-6; binding to its endogenous recognition unit Galβ1-4Fuc, a unique disaccharide found only in invertebrates, is not necessarily affected by point mutations of the eight well-conserved amino acids. In this study, we constructed mutants of mouse galectin-1 carrying substitutions of each of the eight conserved amino acid residues (H44F, N46D, R48H, V59A, N61D, W68F, E71Q, and R73H) and examined their affinity for Galβ1-4GlcNAc and Galβ1-4Fuc. These mutants, except W68F, had very low affinity for asialofetuin-Sepharose; however, most of them (with the exception of H44F and R48H) could be purified using Galβ1-4Fuc-Sepharose. The affinity of the purified mutant galectins for glycans containing Galβ1-4Fuc or Galβ1-4GlcNAc moieties was quantitatively examined by frontal affinity chromatography, and the results indicated that the mutants retained the affinity only for Galβ1-4Fuc. Given that other mammalian galectins are known to bind Galβ1-4Fuc, our data suggest that immobilized Galβ1-4Fuc ligands could be generally used for easy one-step affinity purification of mutant galectins.

Human neutrophils secrete bioactive paucimannosidic proteins from azurophilic granules into pathogen-infected sputum

Unlike plants and invertebrates, mammals reportedly lack proteins displaying asparagine (N)-linked paucimannosylation (mannose(1-3)fucose(0-1)N-acetylglucosamine(2)Asn). Enabled by technology advancements in system-wide biomolecular characterization, we document that protein paucimannosylation is a significant host-derived molecular signature of neutrophil-rich sputum from pathogen-infected human lungs and is negligible in pathogen-free sputum. Five types of paucimannosidic N-glycans were carried by compartment-specific and inflammation-associated proteins of the azurophilic granules of human neutrophils including myeloperoxidase (MPO), azurocidin, and neutrophil elastase. The timely expressed human azurophilic granule-resident β-hexosaminidase A displayed the capacity to generate paucimannosidic N-glycans by trimming hybrid/complex type N-glycan intermediates with relative broad substrate specificity. Paucimannosidic N-glycoepitopes showed significant co-localization with β-hexosaminidase A and the azurophilic marker MPO in human neutrophils using immunocytochemistry. Furthermore, promyelocyte stage-specific expression of genes coding for paucimannosidic proteins and biosynthetic enzymes indicated a novel spatio-temporal biosynthetic route in early neutrophil maturation. The absence of bacterial exoglycosidase activities and paucimannosidic N-glycans excluded exogenous origins of paucimannosylation. Paucimannosidic proteins from isolated and sputum neutrophils were preferentially secreted upon inoculation with virulent Pseudomonas aeruginosa. Finally, paucimannosidic proteins displayed affinities to mannose-binding lectin, suggesting immune-related functions of paucimannosylation in activated human neutrophils. In conclusion, we are the first to document that human neutrophils produce, store and, upon activation, selectively secrete bioactive paucimannosidic proteins into sputum of lungs undergoing pathogen-based inflammation.