Null Mutations in Drosophila N-Acetylglucosaminyltransferase I Produce Defects in Locomotion and a Reduced Life Span

Reliable N-Glycan Analysis-Removal of Frequently Occurring Oligosaccharide Impurities by Enzymatic Degradation

Glycosylation, especially N-glycosylation, is one of the most common protein modifications, with immense importance at the molecular, cellular, and organismal level. Thus, accurate and reliable N-glycan analysis is essential in many areas of pharmaceutical and food industry, medicine, and science. However, due to the complexity of the cellular glycosylation process, in-depth glycoanalysis is still a highly challenging endeavor. Contamination of samples with oligosaccharide impurities (OSIs), typically linear glucose homo-oligomers, can cause further complications. Due to their physicochemical similarity to N-glycans, OSIs produce potentially overlapping signals, which can remain unnoticed. If recognized, suspected OSI signals are usually excluded in data evaluation. However, in both cases, interpretation of results can be impaired. Alternatively, sample preparation can be repeated to include an OSI removal step from samples. However, this significantly increases sample amount, time, and effort necessary. To overcome these issues, we investigated the option to enzymatically degrade and thereby remove interfering OSIs as a final sample preparation step. Therefore, we screened ten commercially available enzymes concerning their potential to efficiently degrade maltodextrins and dextrans as most frequently found OSIs. Of these enzymes, only dextranase from Chaetomium erraticum and glucoamylase P from Hormoconis resinae enabled a degradation of OSIs within only 30 min that is free of side reactions with N-glycans. Finally, we applied the straightforward enzymatic degradation of OSIs to N-glycan samples derived from different standard glycoproteins and various stem cell lysates.

The N-glycosylation-related genes as potential targets for RNAi-mediated pest control of the Colorado potato beetle (Leptinotarsa decemlineata)

BACKGROUND

N-glycosylation is one of the most common and important post-translational modifications in the eukaryotic cell. The study of protein N-glycosylation in several model insects confirmed the importance of this process in insect development, immunity, survival and fertility. The Colorado potato beetle (Leptinotarsa decemlineata) (CPB) is a common pest of Solanaceae crops. With the infamous title of champion of insecticide resistance, novel pest control strategies for this insect are needed. Luckily this pest insect is reported as very sensitive for the post-genomic technology of RNA interference (RNAi).

RESULTS

In this project, we investigated the importance of N-glycosylation in the survival and development of CPB using RNAi-mediated gene silencing of N-glycosylation-related genes (NGRGs) during the different transition steps from the larva, through the pupa to the adult stage. High mortality was observed in the larval stage with the silencing of early NGRGs, as STT3a, DAD1 and GCS1. With dsRNA against middle NGRGs, abnormal phenotypes at the ecdysis process and adult formation were observed, while the silencing of late NGRGs did not cause mortality.

CONCLUSION

The lethal phenotypes observed on silencing of the genes involved in the early processing steps of the N-glycosylation pathway suggest these genes are good candidates for RNAi-mediated control of CPB. Next to the gene-specific mechanism of RNAi for biosafety and possible implementation in integrated pest management, we believe these early NGRGs provide a possible alternative to the well-known target genes Snf7 and vacuolar ATPases that are now used in the first commercial RNAi-based products and thus they may be useful in the context of proactive resistance management. © 2021 Society of Chemical Industry.

Temporal analysis of N-acetylglucosamine extension of N-glycans in the middle silk gland of silkworm Bombyx mori

N-glycosylation of proteins is an important post-translational modification in eukaryotic cells. One of the key modifications in protein N-glycosylation is N-acetylglucosamine (GlcNAc) extension mediated by N-acetylglucosaminyltransferase I (GNTI), which triggers N-glycan maturation from high-mannose-type to hybrid- and complex-type structures in Golgi. However, the temporal contributions of GNTI to GlcNAc extension and the resultant N-glycan structures in insects have not been analyzed. Here, focusing on GlcNAc extension of N-glycan in the silkworm Bombyx mori, we analyzed the temporal N-glycan alterations in the middle silk gland (MSG) and characterized the property of key enzyme for complex-type N-glycan biosynthesis, B. mori GNTI (BmGNTI). N-glycan analysis of N-glycoproteins in the MSG demonstrated that BmGNTI identified and characterized in this study consistently contributed to GlcNAc extension of N-glycans, which led to the accumulation of GlcNAc-extended N-glycans as predominant structures throughout the MSG development. The expression profile of GlcNAc extension-related genes revealed that the enzymes contributing to the hydrolysis of GlcNAc showed stage-specific expressions, thereby resulting in accumulations of the end product N-glycans of the enzyme. These results lead to the speculation that not BmGNTI but rather glycosylhydrolases critically influenced the structural formations and the changes in the ratio of N-glycans with GlcNAc residue(s) in MSG.

A genetic screen in Drosophila reveals the role of fucosylation in host susceptibility to Candida infection

Candida infections constitute a blind spot in global public health as very few new anti-fungal drugs are being developed. Genetic surveys of host susceptibilities to such infections using mammalian models have certain disadvantages in that obtaining results is time-consuming, owing to relatively long lifespans, and these results have low statistical resolution because sample sizes are usually small. Here, we report a targeted genetic screening of 5698 RNAi lines encompassing 4135 Drosophila genes with human homologues, several of which we identify as important for host survival after Candida albicans infection. These include genes in a variety of functional classes encompassing gene expression, intracellular signalling, metabolism and enzymatic regulation. Analysis of one of the screen hits, the infection-induced α-(1,3)-fucosylase FucTA, showed that N-glycan fucosylation has several targets among proteins involved in host defence, which provides multiple avenues of investigation for the mechanistic analysis of host survival to systemic C. albicans infection.

RNAi of the N-glycosylation-related genes confirms their importance in insect development and α-1,6-fucosyltransferase plays a role in the ecdysis event for the hemimetabolous pest insect Nilaparvata lugens

Recently N-glycosylation was found to be required for the postembryonic development and metamorphosis of the holometabolous beetle Tribolium castaneum. However, the role of N-glycosylation in the development of hemimetabolous insects is unknown. To further elucidate the role of N-glycosylation in the development of insects, a functional characterization of the N-glycosylation-related genes (NGRGs) was performed in a model insect for hemimetabolous development, namely the brown planthopper Nilaparvata lugens. In this project, we report the effects of RNAi-mediated silencing of 15 NGRGs on the postembryonic development of N. lugens. Two major observations were made. First, interruption of the early steps of N-glycan processing led to a lethal phenotype during the transition from nymph to adult as was observed in T. castaneum. Second, we report here on an essential function for the α-1,6-fucosyl transferase in the ecdysis event of N. lugens between nymphal instars, since gene-silencing by RNAi led to failure of ecdysis and subsequent mortality of the treated insect.

In-depth Site-specific Analysis of N-glycoproteome in Human Cerebrospinal Fluid and Glycosylation Landscape Changes in Alzheimer's Disease

As the body fluid that directly interchanges with the extracellular fluid of the central nervous system (CNS), cerebrospinal fluid (CSF) serves as a rich source for CNS-related disease biomarker discovery. Extensive proteome profiling has been conducted for CSF, but studies aimed at unraveling site-specific CSF N-glycoproteome are lacking. Initial efforts into site-specific N-glycoproteomics study in CSF yield limited coverage, hindering further experimental design of glycosylation-based disease biomarker discovery in CSF. In the present study, we have developed an N-glycoproteomic approach that combines enhanced N-glycopeptide sequential enrichment by hydrophilic interaction chromatography (HILIC) and boronic acid enrichment with electron transfer and higher-energy collision dissociation (EThcD) for large-scale intact N-glycopeptide analysis. The application of the developed approach to the analyses of human CSF samples enabled identifications of a total of 2893 intact N-glycopeptides from 511 N-glycosites and 285 N-glycoproteins. To our knowledge, this is the largest site-specific N-glycoproteome dataset reported for CSF to date. Such dataset provides molecular basis for a better understanding of the structure-function relationships of glycoproteins and their roles in CNS-related physiological and pathological processes. As accumulating evidence suggests that defects in glycosylation are involved in Alzheimer's disease (AD) pathogenesis, in the present study, a comparative in-depth N-glycoproteomic analysis was conducted for CSF samples from healthy control and AD patients, which yielded a comparable N-glycoproteome coverage but a distinct expression pattern for different categories of glycoforms, such as decreased fucosylation in AD CSF samples. Altered glycosylation patterns were detected for a number of N-glycoproteins including alpha-1-antichymotrypsin, ephrin-A3 and carnosinase CN1 etc., which serve as potentially interesting targets for further glycosylation-based AD study and may eventually lead to molecular elucidation of the role of glycosylation in AD progression.

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.

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.

Evolutionarily conserved and species-specific glycoproteins in the N-glycoproteomes of diverse insect species

N-glycosylation is one of the most abundant and conserved protein modifications in eukaryotes. The attachment of N-glycans to proteins can modulate their properties and influences numerous important biological processes, such as protein folding and cellular attachment. Recently, it has been shown that protein N-glycosylation plays a vital role in insect development and survival, which makes the glycans an interesting target for pest control. Despite the importance of protein N-glycosylation in insects, knowledge about insect N-glycoproteomes is scarce. To fill this gap, the N-glycosites were identified in proteins from three major pest insects, spanning different insect orders and diverging in post-embryonic development, feeding mechanism and evolutionary ancestry: Drosophila melanogaster (Diptera), Tribolium castaneum (Coleoptera) and Acyrthosiphon pisum (Hemiptera). The N-glyco-FASP method for isolation of N-glycopeptides was optimized to study the insect N-glycosites and allowed the identification of 889 N-glycosylation sites in T. castaneum, 941 in D. melanogaster and 1338 in A. pisum. Although a large set of the corresponding glycoproteins is shared among the three insects, species- and order-specific glycoproteins were also identified. The functionality of the insect glycoproteins together with the conservation of the N-glycosites throughout evolution is discussed. This information can help in the elaboration of novel pest insect control strategies based on interference in insect glycosylation.

Study on feasibility of determination of glucosamine content of fermentation process using a micro NIR spectrometer

N-acetyl-d-glucosamine (GlcNAc) is a microbial fermentation product, and NIR spectroscopy is an effective process analytical technology (PAT) tool in detecting the key quality attribute: the GlcNAc content. Meanwhile, the design of NIR spectrometers is under the trend of miniaturization, portability and low-cost nowadays. The aim of this study was to explore a portable micro NIR spectrometer with the fermentation process. First, FT-NIR spectrometer and Micro-NIR 1700 spectrometer were compared with simulated fermentation process solutions. The R_c², R_p², RMSECV and RMSEP of the optimal FT-NIR and Micro-NIR 1700 models were 0.999, 0.999, 3.226 g/L, 1.388 g/L and 0.999, 0.999, 1.821 g/L, 0.967 g/L. Passing-Bablok regression method and paired t-test results showed there were no significant differences between the two instruments. Then the Micro-NIR 1700 was selected for the practical fermentation process, 135 samples from 10 batches were collected. Spectral pretreatment methods and variables selection methods (BiPLS, FiPLS, MWPLS and CARS-PLS) for PLS modeling were discussed. The R_c², R_p², RMSECV and RMSEP of the optimal GlcNAc content PLS model of the practical fermentation process were 0.994, 0.995, 2.792 g/L and 1.946 g/L. The results have a positive reference for application of the Micro-NIR spectrometer. To some extent, it could provide theoretical supports in guiding the microbial fermentation or the further assessment of bioprocess.

MGAT1 is a novel transcriptional target of Wnt/β-catenin signaling pathway

BACKGROUND

The Wnt/β-catenin signaling pathway is an evolutionary conserved pathway, which has important functions in vertebrate early development, axis formation, cellular proliferation and morphogenesis. Additionally, Wnt/β-catenin signaling pathway is one of the most important intracellular pathways that controls cancer progression. To date most of the identified targets of this pathway are shown to harbor tumorigenic properties. We previously showed that Mannosyl glycoprotein acetylglucosaminyl-transferase (MGAT1) enzyme is among the Wnt/β-catenin signaling putative target genes in hepatocellular carcinoma cell lines (Huh7).

METHODS

MGAT1 protein levels were determined by Western Blotting from Huh7 cell lines in which Wnt/β-catenin pathway was activated by means of different approaches such as LiCl treatment and mutant β-catenin overexpression. Luciferase reporter assay was used to analyze the promoter activity of MGAT1. The mRNA levels of MGAT1 were determined by quantitative real-time PCR from Huh7 cells that were treated with either Wnt agonist or GSK-3β inhibitor. Wound healing and XTT cell proliferation assays were performed in order to determine the proliferation and migration capacities of MGAT1 overexpressing stable Huh7 cells. Finally, xenograft experiments were carried out to measure the tumor formation capacities in vivo.

RESULTS

In this study we showed that the activation of Wnt/β-catenin pathway culminates in the upregulation of MGAT1 enzyme both at transcriptional and post-transcriptional levels. We also showed that overexpression of the β-catenin gene (CTNNB1) increased the promoter activity of MGAT1. We applied a set of complementary approaches to elucidate the functional importance of MGAT1 as a vital target of Wnt/β-catenin signaling in Huh7 cells. Our analyses related to cell proliferation and migration assays showed that in comparison to the control cells, MGAT1 expressing Huh7 cells have greater proliferative and invasive capabilities. Furthermore, the stable overexpression of MGAT1 gene in Huh7 cell lines lead to a significant increase in tumor growth rate in Severe Combined Immunodeficient (SCID) mice.

CONCLUSIONS

Taken together, we showed for the first time that MGAT is a novel Wnt/β-catenin pathway target that has important implications for tumorigenesis.

Distribution of Glycan Motifs at the Surface of Midgut Cells in the Cotton Leafworm (Spodoptera littoralis) Demonstrated by Lectin Binding

Glycans are involved in many biological phenomena, including signal transduction, cell adhesion, immune response or differentiation. Although a few papers have reported on the role of glycans in the development and proper functioning of the insect midgut, no data are available regarding the localization of the glycan structures on the surface of the cells in the gut of insects. In this paper, we analyzed the spatial distribution of glycans present on the surface of the midgut cells in larvae of the cotton leafworm Spodoptera littoralis, an important agricultural pest insect worldwide. For this purpose, we established primary midgut cell cultures, probed these individual cells that are freely suspended in liquid medium with a selection of seven fluorescently labeled lectins covering a range of different carbohydrate binding specificities [mannose oligomers (GNA and HHA), GalNAc/Gal (RSA and SSA), GlcNAc (WGA and Nictaba) and Neu5Ac(α-2,6)Gal/GalNAc (SNA-I)], and visualized the interaction of these lectins with the different zones of the midgut cells using confocal microscopy. Our analysis focused on the typical differentiated columnar cells with a microvillar brush border at their apical side, which are dominantly present in the Lepidopteran midgut and function in food digestion and absorption, and as well as on the undifferentiated stem cells that are important for midgut development and repair. Confocal microscopy analyses showed that the GalNAc/Gal-binding lectins SSA and RSA and the terminal GlcNAc-recognizing WGA bound preferentially to the apical microvillar zone of the differentiated columnar cells as compared to the basolateral pole. The reverse result was observed for the mannose-binding lectins GNA and HHA, as well as Nictaba that binds preferentially to GlcNAc oligomers. Furthermore, differences in lectin binding to the basal and lateral zones of the cell membranes of the columnar cells were apparent. In the midgut stem cells, GNA and Nictaba bound more strongly to the membrane of these undifferentiated cells compared to the microvillar pole of the columnar cells, while SSA, HHA, WGA, and SNA-I showed stronger binding to the microvilli. Our results indicated that polarization of the midgut cells is also reflected by a specific distribution of glycans, especially between the basal and microvillar pole. The data are discussed in relation to the functioning and development of the insect midgut.

Heterologous expression of the N-acetylglucosaminyltransferase I dictates a reinvestigation of the N-glycosylation pathway in Chlamydomonas reinhardtii

Eukaryotic N-glycosylation pathways are dependent of N-acetylglucosaminyltransferase I (GnTI), a key glycosyltransferase opening the door to the formation of complex-type N-glycans by transferring a N-acetylglucosamine residue onto the Man₅GlcNAc₂ intermediate. In contrast, glycans N-linked to Chlamydomonas reinhardtii proteins arise from a GnTI-independent Golgi processing of oligomannosides giving rise to Man₅GlcNAc₂ substituted eventually with one or two xylose(s). Here, complementation of C. reinhardtii with heterologous GnTI was investigated by expression of GnTI cDNAs originated from Arabidopsis and the diatom Phaeodactylum tricornutum. No modification of the N-glycans was observed in the GnTI transformed cells. Consequently, the structure of the Man₅GlcNAc₂ synthesized by C. reinhardtii was reinvestigated. Mass spectrometry analyses combined with enzyme sequencing showed that C. reinhardtii proteins carry linear Man₅GlcNAc₂ instead of the branched structure usually found in eukaryotes. Moreover, characterization of the lipid-linked oligosaccharide precursor demonstrated that C. reinhardtii exhibit a Glc₃Man₅GlcNAc₂ dolichol pyrophosphate precursor. We propose that this precursor is then trimmed into a linear Man₅GlcNAc₂ that is not substrate for GnTI. Furthermore, cells expressing GnTI exhibited an altered phenotype with large vacuoles, increase of ROS production and accumulation of starch granules, suggesting the activation of stress responses likely due to the perturbation of the Golgi apparatus.

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.

Diversity and functions of protein glycosylation in insects

The majority of proteins is modified with carbohydrate structures. This modification, called glycosylation, was shown to be crucial for protein folding, stability and subcellular location, as well as protein-protein interactions, recognition and signaling. Protein glycosylation is involved in multiple physiological processes, including embryonic development, growth, circadian rhythms, cell attachment as well as maintenance of organ structure, immunity and fertility. Although the general principles of glycosylation are similar among eukaryotic organisms, insects synthesize a distinct repertoire of glycan structures compared to plants and vertebrates. Consequently, a number of unique insect glycans mediate functions specific to this class of invertebrates. For instance, the core α1,3-fucosylation of N-glycans is absent in vertebrates, while in insects this modification is crucial for the development of wings and the nervous system. At present, most of the data on insect glycobiology comes from research in Drosophila. Yet, progressively more information on the glycan structures and the importance of glycosylation in other insects like beetles, caterpillars, aphids and bees is becoming available. This review gives a summary of the current knowledge and recent progress related to glycan diversity and function(s) of protein glycosylation in insects. We focus on N- and O-glycosylation, their synthesis, physiological role(s), as well as the molecular and biochemical basis of these processes.

Biological roles of glycans

Simple and complex carbohydrates (glycans) have long been known to play major metabolic, structural and physical roles in biological systems. Targeted microbial binding to host glycans has also been studied for decades. But such biological roles can only explain some of the remarkable complexity and organismal diversity of glycans in nature. Reviewing the subject about two decades ago, one could find very few clear-cut instances of glycan-recognition-specific biological roles of glycans that were of intrinsic value to the organism expressing them. In striking contrast there is now a profusion of examples, such that this updated review cannot be comprehensive. Instead, a historical overview is presented, broad principles outlined and a few examples cited, representing diverse types of roles, mediated by various glycan classes, in different evolutionary lineages. What remains unchanged is the fact that while all theories regarding biological roles of glycans are supported by compelling evidence, exceptions to each can be found. In retrospect, this is not surprising. Complex and diverse glycans appear to be ubiquitous to all cells in nature, and essential to all life forms. Thus, >3 billion years of evolution consistently generated organisms that use these molecules for many key biological roles, even while sometimes coopting them for minor functions. In this respect, glycans are no different from other major macromolecular building blocks of life (nucleic acids, proteins and lipids), simply more rapidly evolving and complex. It is time for the diverse functional roles of glycans to be fully incorporated into the mainstream of biological sciences.

Synaptic roles for phosphomannomutase type 2 in a new Drosophila congenital disorder of glycosylation disease model

Congenital disorders of glycosylation (CDGs) constitute a rapidly growing family of human diseases resulting from heritable mutations in genes driving the production and modification of glycoproteins. The resulting symptomatic hypoglycosylation causes multisystemic defects that include severe neurological impairments, revealing a particularly critical requirement for tightly regulated glycosylation in the nervous system. The most common CDG, CDG-Ia (PMM2-CDG), arises from phosphomannomutase type 2 (PMM2) mutations. Here, we report the generation and characterization of the first Drosophila CDG-Ia model. CRISPR-generated pmm2-null Drosophila mutants display severely disrupted glycosylation and early lethality, whereas RNAi-targeted knockdown of neuronal PMM2 results in a strong shift in the abundance of pauci-mannose glycan, progressive incoordination and later lethality, closely paralleling human CDG-Ia symptoms of shortened lifespan, movement impairments and defective neural development. Analyses of the well-characterized Drosophila neuromuscular junction (NMJ) reveal synaptic glycosylation loss accompanied by defects in both structural architecture and functional neurotransmission. NMJ synaptogenesis is driven by intercellular signals that traverse an extracellular synaptomatrix and are co-regulated by glycosylation and matrix metalloproteinases (MMPs). Specifically, trans-synaptic signaling by the Wnt protein Wingless (Wg) depends on the heparan sulfate proteoglycan (HSPG) co-receptor Dally-like protein (Dlp), which is regulated by synaptic MMP activity. Loss of synaptic MMP2, Wg ligand, Dlp co-receptor and downstream trans-synaptic signaling occurs with PMM2 knockdown. Taken together, this Drosophila CDG disease model provides a new avenue for the dissection of cellular and molecular mechanisms underlying neurological impairments and is a means by which to discover and test novel therapeutic treatment strategies.

Drosophila Collection: This work generates a new Drosophila congenital disorder of glycosylation model for the most common disease category, caused by phosphomannomutase-2 mutation, and reveals a synaptic mechanism underlying associated neurological impairments.

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.

The role of protein N-glycosylation in neural transmission

Recent studies have explored the function of N-linked glycosylation in the nervous system, demonstrating essential roles of carbohydrate structures in neural development. The function of N-glycans in neural physiology remains less understood; however, increasing evidence indicates that N-glycans can play specific modulatory roles controlling neural transmission and excitability of neural circuits. These roles are mediated via effects on synaptic proteins involved in neurotransmitter release, transporters that regulate nerotransmitter concentrations, neurotransmitter receptors, as well as via regulation of proteins that control excitability and response to milieu stimuli, such as voltage-gated ion channels and transient receptor potential channels, respectively. Sialylated N-glycan structures are among the most potent modulators of cell excitability, exerting prominent effects on voltage gated Na(+) and K(+) channels. This modulation appears to be underlain by complex molecular mechanisms involving electrostatic effects, as well as interaction modes based on more specific steric effects and interactions with lectins and other molecules. Data also indicate that particular features of N-glycans, such as their location on a protein and structural characteristics, can be specifically associated with the effect of glycosylation. These features and their functional implications can vary between different cell types, which highlight the importance of in vivo analyses of glycan functions. Experimental challenges are associated with the overwhelming complexity of the nervous system and glycosylation pathways in vertebrates, and thus model organisms like Drosophila should help elucidate evolutionarily conserved mechanisms underlying glycan functions. Recent studies supported this notion and shed light on functions of several glycosylation genes involved in the regulation of the nervous system.

N-glycosylation in regulation of the nervous system

Protein N-glycosylation can influence the nervous system in a variety of ways by affecting functions of glycoproteins involved in nervous system development and physiology. The importance of N-glycans for different aspects of neural development has been well documented. For example, some N-linked carbohydrate structures were found to play key roles in neural cell adhesion and axonal targeting during development. At the same time, the involvement of glycosylation in the regulation of neural physiology remains less understood. Recent studies have implicated N-glycosylation in the regulation of neural transmission, revealing novel roles of glycans in synaptic processes and the control of neural excitability. N-Glycans were found to markedly affect the function of several types of synaptic proteins involved in key steps of synaptic transmission, including neurotransmitter release, reception, and uptake. Glycosylation also regulates a number of channel proteins, such as TRP channels that control responses to environmental stimuli and voltage-gated ion channels, the principal determinants of neuronal excitability. Sialylated carbohydrate structures play a particularly prominent part in the modulation of voltage-gated ion channels. Sialic acids appear to affect channel functions via several mechanisms, including charge interactions, as well as other interactions that probably engage steric effects and interactions with other molecules. Experiments also indicated that some structural features of glycans can be particularly important for their function. Since glycan structures can vary significantly between different cell types and depend on the metabolic state of the cell, it is important to analyze glycan functions using in vivo approaches. While the complexity of the nervous system and intricacies of glycosylation pathways can create serious obstacles for in vivo experiments in vertebrates, recent studies have indicated that more simple and experimentally tractable model organisms like Drosophila should provide important advantages for elucidating evolutionarily conserved functions of N-glycosylation in the nervous system.

Biological significance of complex N-glycans in plants and their impact on plant physiology

Asparagine (N)-linked protein glycosylation is a ubiquitous co- and post-translational modification which can alter the biological function of proteins and consequently affects the development, growth, and physiology of organisms. Despite an increasing knowledge of N-glycan biosynthesis and processing, we still understand very little about the biological function of individual N-glycan structures in plants. In particular, the N-glycan-processing steps mediated by Golgi-resident enzymes create a structurally diverse set of protein-linked carbohydrate structures. Some of these complex N-glycan modifications like the presence of β1,2-xylose, core α1,3-fucose or the Lewis a-epitope are characteristic for plants and are evolutionary highly conserved. In mammals, complex N-glycans are involved in different cellular processes including molecular recognition and signaling events. In contrast, the complex N-glycan function is still largely unknown in plants. Here, in this short review, I focus on important recent developments and discuss their implications for future research in plant glycobiology and plant biotechnology.

N-glycosylation requirements in neuromuscular synaptogenesis

Neural development requires N-glycosylation regulation of intercellular signaling, but the requirements in synaptogenesis have not been well tested. All complex and hybrid N-glycosylation requires MGAT1 (UDP-GlcNAc:α-3-D-mannoside-β1,2-N-acetylglucosaminyl-transferase I) function, and Mgat1 nulls are the most compromised N-glycosylation condition that survive long enough to permit synaptogenesis studies. At the Drosophila neuromuscular junction (NMJ), Mgat1 mutants display selective loss of lectin-defined carbohydrates in the extracellular synaptomatrix, and an accompanying accumulation of the secreted endogenous Mind the gap (MTG) lectin, a key synaptogenesis regulator. Null Mgat1 mutants exhibit strongly overelaborated synaptic structural development, consistent with inhibitory roles for complex/hybrid N-glycans in morphological synaptogenesis, and strengthened functional synapse differentiation, consistent with synaptogenic MTG functions. Synapse molecular composition is surprisingly selectively altered, with decreases in presynaptic active zone Bruchpilot (BRP) and postsynaptic Glutamate receptor subtype B (GLURIIB), but no detectable change in a wide range of other synaptic components. Synaptogenesis is driven by bidirectional trans-synaptic signals that traverse the glycan-rich synaptomatrix, and Mgat1 mutation disrupts both anterograde and retrograde signals, consistent with MTG regulation of trans-synaptic signaling. Downstream of intercellular signaling, pre- and postsynaptic scaffolds are recruited to drive synaptogenesis, and Mgat1 mutants exhibit loss of both classic Discs large 1 (DLG1) and newly defined Lethal (2) giant larvae [L(2)GL] scaffolds. We conclude that MGAT1-dependent N-glycosylation shapes the synaptomatrix carbohydrate environment and endogenous lectin localization within this domain, to modulate retention of trans-synaptic signaling ligands driving synaptic scaffold recruitment during synaptogenesis.

Complex N-glycans: the story of the "yellow brick road"

The synthesis of complex asparagine-linked glycans (N-glycans) involves a multi-step process that starts with a five mannose N-glycan structure: [Manα1-6(Manα1-3)Manα1-6][Manα1-3]-R where R = Manβ1-4GlcNAcβ1-4GlcNAcβ1-Asn-protein. N-acetylglucosaminyltransferase I (GlcNAc-TI) first catalyzes addition of GlcNAc in β1-2 linkage to the Manα1-3-R terminus of the five-mannose structure. Mannosidase II then removes two Man residues exposing the Manα1-6 terminus that serves as a substrate for GlcNAc-T II and addition of a second GlcNAcβ1-2 residue. The resulting structure is the complex N-glycan: GlcNAcβ1-2Manα1-6(GlcNAcβ1-2Manα1-3)-R. This structure is the precursor to a large assortment of branched complex N-glycans involving four more N-acetylglucosaminyltransferases. This short review describes the experiments (done in the early 1970s) that led to the discovery of GlcNAc-TI and II.

Deficiency of α-glucosidase I alters glycoprotein glycosylation and lifespan in Caenorhabditis elegans

Endoplasmic reticulum (ER) α-glucosidase I is an enzyme that trims the distal α1,2-linked glucose (Glc) residue from the Glc3Man9GlcNAc2 oligosaccharide following its addition to nascent glycoproteins in the initial step of processing. This reaction is critical to the subsequent processing of N-glycans and thus defects in α-glucosidase I gene in human cause congenital disorder of glycosylation (CDG) type IIb. We identified the Caenorhabditis elegans α-glucosidase I gene (F13H10.4, designated agl-1) that encodes a polypeptide with 36% identity to human α-glucosidase I. The agl-1 cDNA restored the expression of complex-type N-glycans on the cell surface of α-glucosidase I-defective Chinese hamster ovary Lec23 cells. RNAi knockdown of agl-1 [agl-1(RNAi)] produced worms that were visibly similar to wild-type, but lifespan was reduced to about half of the control. Analyses of N-glycosylation in agl-1(RNAi) animals by western blotting and mass spectrometry showed reduction of paucimannose and complex-type glycans and dramatic increase of glucosylated oligomannose glycans. In addition, a significant amount of unusual terminally fucosylated N-glycans were found in agl-1(RNAi) animals. ER stress response was also provoked, leading to the accumulation of large amounts of triglucosylated free oligosaccharides (FOSs) (Glc3Man4-5GlcNAc1-2) in agl-1(RNAi) animals. Acceleration of ER-associated degradation in response to the accumulation of unfolded glycoproteins and insufficient interaction with calnexin/calreticulin in the ER lumen likely accounts for the increase of FOSs. Taken together, these studies in C. elegans demonstrate that decreased ER α-glucosidase I affects the entire N-glycan profile and induces chronic ER stress, which may contribute to the pathophysiology of CDG-IIb in humans.

N-glycan maturation is crucial for cytokinin-mediated development and cellulose synthesis in Oryza sativa

To explore the physiological significance of N-glycan maturation in the plant Golgi apparatus, gnt1, a mutant with loss of N-acetylglucosaminyltransferase I (GnTI) function, was isolated in Oryza sativa. gnt1 exhibited complete inhibition of N-glycan maturation and accumulated high-mannose N-glycans. Phenotypic analyses revealed that gnt1 shows defective post-seedling development and incomplete cell wall biosynthesis, leading to symptoms such as failure in tiller formation, brittle leaves, reduced cell wall thickness, and decreased cellulose content. The developmental defects of gnt1 ultimately resulted in early lethality without transition to the reproductive stage. However, callus induced from gnt1 seeds could be maintained for periods, although it exhibited a low proliferation rate, small size, and hypersensitivity to salt stress. Shoot regeneration and dark-induced leaf senescence assays indicated that the loss of GnTI function results in reduced sensitivity to cytokinin in rice. Reduced expression of A-type O. sativa response regulators that are rapidly induced by cytokinins in gnt1 confirmed that cytokinin signaling is impaired in the mutant. These results strongly support the proposed involvement of N-glycan maturation in transport as well as in the function of membrane proteins that are synthesized via the endomembrane system.

Large-scale identification of target proteins of a glycosyltransferase isozyme by Lectin-IGOT-LC/MS, an LC/MS-based glycoproteomic approach

Model organisms containing deletion or mutation in a glycosyltransferase-gene exhibit various physiological abnormalities, suggesting that specific glycan motifs on certain proteins play important roles in vivo. Identification of the target proteins of glycosyltransferase isozymes is the key to understand the roles of glycans. Here, we demonstrated the proteome-scale identification of the target proteins specific for a glycosyltransferase isozyme, β1,4-galactosyltransferase-I (β4GalT-I). Although β4GalT-I is the most characterized glycosyltransferase, its distinctive contribution to β1,4-galactosylation has been hardly described so far. We identified a large number of candidates for the target proteins specific to β4GalT-I by comparative analysis of β4GalT-I-deleted and wild-type mice using the LC/MS-based technique with the isotope-coded glycosylation site-specific tagging (IGOT) of lectin-captured N-glycopeptides. Our approach to identify the target proteins in a proteome-scale offers common features and trends in the target proteins, which facilitate understanding of the mechanism that controls assembly of a particular glycan motif on specific proteins.

The N's and O's of Drosophila glycoprotein glycobiology

The past 25 years have seen significant advances in understanding the diversity and functions of glycoprotein glycans in Drosophila melanogaster. Genetic screens have captured mutations that reveal important biological activities modulated by glycans, including protein folding and trafficking, as well as cell signaling, tissue morphogenesis, fertility, and viability. Many of these glycan functions have parallels in vertebrate development and disease, providing increasing opportunities to dissect pathologic mechanisms using Drosophila genetics. Advances in the sensitivity of structural analytic techniques have allowed the glycan profiles of wild-type and mutant tissues to be assessed, revealing novel glycan structures that may be functionally analogous to vertebrate glycans. This review describes a selected set of recent advances in understanding the functions of N-linked and O-linked (non-glycosaminoglycan) glycoprotein glycans in Drosophila with emphasis on their relatedness to vertebrate organisms.

Mgat1-dependent N-glycosylation of membrane components primes Drosophila melanogaster blood cells for the cellular encapsulation response

In nature, larvae of the fruitfly Drosophila melanogaster are commonly infected by parasitoid wasps, and so have evolved a robust immune response to counter wasp infection. In this response, fly immune cells form a multilayered capsule surrounding the wasp egg, leading to death of the parasite. Many of the molecular mechanisms underlying this encapsulation response are conserved with human immune responses. Our findings suggest that protein N-glycosylation, a common protein post-translational modification of human immune proteins, may be one such conserved mechanism. We found that membrane proteins on Drosophila immune cells are N-glycosylated in a temporally specific manner following wasp infection. Furthermore we have identified mutations in eight genes encoding enzymes of the N-glycosylation pathway that decrease fly resistance to wasp infection. More specifically, loss of protein N-glycosylation in immune cells following wasp infection led to the formation of defective capsules, which disintegrated over time and were thereby unsuccessful at preventing wasp development. Interestingly, we also found that one species of Drosophila parasitoid wasp, Leptopilina victoriae, targets protein N-glycosylation as part of its virulence mechanism, and that overexpression of an N-glycosylation enzyme could confer resistance against this wasp species to otherwise susceptible flies. Taken together, these findings demonstrate that protein N-glycosylation is a key player in Drosophila cellular encapsulation and suggest that this response may provide a novel model to study conserved roles of protein glycosylation in immunity.

Organisms such as the fruitfly Drosophila melanogaster have long been used as model systems to understand complex aspects of human biology. Work on Drosophila antimicrobial immunity has led to identification of mechanisms underlying human innate immunity, such as the use of Toll-like receptors for recognizing antigen and initiating humoral immune responses. Flies and humans are also infected by larger parasites against which they mount immune blood-cell based responses, but the genetic basis for cellular immunity is poorly characterized. In nature, flies are often infected by parasitoid wasps that lay their eggs in fly larvae, inducing a cellular immune response in the flies. Fly blood cells surround the wasp egg and form a tightly connected capsule leading to death of the egg in a process called encapsulation, which is similar to human granuloma formation. In this study we identified eight new genes that are important for encapsulation. These genes are part of the N-glycosylation pathway, and we found that without N-glycosylation of proteins on blood cell surfaces, capsules surrounding wasp eggs cannot consolidate into a tight capsule, allowing the wasps to escape. Interestingly, we also found a wasp that disrupts N-glycosylation so that it can evade the encapsulation response. Our work may provide a model to better understand the role of N-glycosylation in human immunity.

Structure-function analysis of endogenous lectin mind-the-gap in synaptogenesis

Mind-the-Gap (MTG) is required for neuronal induction of Drosophila neuromuscular junction (NMJ) postsynaptic domains, including glutamate receptor (GluR) localization. We have previously hypothesized that MTG is secreted from the presynaptic terminal to reside in the synaptic cleft, where it binds glycans to organize the heavily glycosylated, extracellular synaptomatrix required for transsynaptic signaling between neuron and muscle. In this study, we test this hypothesis with MTG structure-function analyses of predicted signal peptide (SP) and carbohydrate-binding domain (CBD), by introducing deletion and point-mutant transgenic constructs into mtg null mutants. We show that the SP is required for MTG secretion and localization to synapses in vivo. We further show that the CBD is required to restrict MTG diffusion in the extracellular synaptomatrix and for postembryonic viability. However, CBD mutation results in elevation of postsynaptic GluR localization during synaptogenesis, not the mtg null mutant phenotype of reduced GluRs as predicted by our hypothesis, suggesting that proper synaptic localization of MTG limits GluR recruitment. In further testing CBD requirements, we show that MTG binds N-acetylglucosamine (GlcNAc) in a Ca(2+)-dependent manner, and thereby binds HRP-epitope glycans, but that these carbohydrate interactions do not require the CBD. We conclude that the MTG lectin has both positive and negative binding interactions with glycans in the extracellular synaptic domain, which both facilitate and limit GluR localization during NMJ embryonic synaptogenesis.

Mass spectrometric comparison of N-glycan profiles from Caenorhabditis elegans mutant embryos

The free-living nematode Caenorhabditis elegans is a well-characterized eukaryotic model organism. Recent glycomic analyses of the glycosylation potential of this worm revealed an extremely high structural variability of its N-glycans. Moreover, the glycan patterns of each developmental stage appeared to be unique. In this study we have determined the N-glycan profiles of wild-type embryos in comparison to mutant embryos arresting embryogenesis early before differentiation and causing extensive transformations of cell identities, which allows to follow the diversification of N-glycans during development using mass spectrometry. As a striking feature, wild-type embryos obtained from liquid culture expressed a less heterogeneous oligosaccharide pattern than embryos recovered from agar plates. N-glycan profiles of mutant embryos displayed, in part, distinct differences in comparison to wild-type embryos suggesting alterations in oligosaccharide trimming and processing, which may be linked to specific cell fate alterations in the embryos.

Substrate specificities and intracellular distributions of three N-glycan processing enzymes functioning at a key branch point in the insect N-glycosylation pathway

Man(α1-6)[GlcNAc(β1-2)Man(α1-3)]ManGlcNAc(2) is a key branch point intermediate in the insect N-glycosylation pathway because it can be either trimmed by a processing β-N-acetylglucosaminidase (FDL) to produce paucimannosidic N-glycans or elongated by N-acetylglucosaminyltransferase II (GNT-II) to produce complex N-glycans. N-acetylglucosaminyltransferase I (GNT-I) contributes to branch point intermediate production and can potentially reverse the FDL trimming reaction. However, there has been no concerted effort to evaluate the relationships among these three enzymes in any single insect system. Hence, we extended our previous studies on Spodoptera frugiperda (Sf) FDL to include GNT-I and -II. Sf-GNT-I and -II cDNAs were isolated, the predicted protein sequences were analyzed, and both gene products were expressed and their acceptor substrate specificities and intracellular localizations were determined. Sf-GNT-I transferred N-acetylglucosamine to Man(5)GlcNAc(2), Man(3)GlcNAc(2), and GlcNAc(β1-2)Man(α1-6)[Man(α1-3)]ManGlcNAc(2), demonstrating its role in branch point intermediate production and its ability to reverse FDL trimming. Sf-GNT-II only transferred N-acetylglucosamine to Man(α1-6)[GlcNAc(β1-2)Man(α1-3)]ManGlcNAc(2), demonstrating that it initiates complex N-glycan production, but cannot use Man(3)GlcNAc(2) to produce hybrid or complex structures. Fluorescently tagged Sf-GNT-I and -II co-localized with an endogenous Sf Golgi marker and Sf-FDL co-localized with Sf-GNT-I and -II, indicating that all three enzymes are Golgi resident proteins. Unexpectedly, fluorescently tagged Drosophila melanogaster FDL also co-localized with Sf-GNT-I and an endogenous Drosophila Golgi marker, indicating that it is a Golgi resident enzyme in insect cells. Thus, the substrate specificities and physical juxtapositioning of GNT-I, GNT-II, and FDL support the idea that these enzymes function at the N-glycan processing branch point and are major factors determining the net outcome of the insect cell N-glycosylation pathway.

Genetic modulation of Rpd3 expression impairs long-term courtship memory in Drosophila

There is increasing evidence that regulation of local chromatin structure is a critical mechanism underlying the consolidation of long-term memory (LTM), however considerably less is understood about the specific mechanisms by which these epigenetic effects are mediated. Furthermore, the importance of histone acetylation in Drosophila memory has not been reported. The histone deacetylase (HDAC) Rpd3 is abundant in the adult fly brain, suggesting a post-mitotic function. Here, we investigated the role of Rpd3 in long-term courtship memory in Drosophila. We found that while modulation of Rpd3 levels predominantly in the adult mushroom body had no observed impact on immediate recall or one-hour memory, 24-hour LTM was severely impaired. Surprisingly, both overexpression as well as RNAi-mediated knockdown of Rpd3 resulted in impairment of long-term courtship memory, suggesting that the dose of Rpd3 is critical for normal LTM.

Extracellular matrix and its receptors in Drosophila neural development

Extracellular matrix (ECM) and matrix receptors are intimately involved in most biological processes. The ECM plays fundamental developmental and physiological roles in health and disease, including processes underlying the development, maintenance, and regeneration of the nervous system. To understand the principles of ECM-mediated functions in the nervous system, genetic model organisms like Drosophila provide simple, malleable, and powerful experimental platforms. This article provides an overview of ECM proteins and receptors in Drosophila. It then focuses on their roles during three progressive phases of neural development: (1) neural progenitor proliferation, (2) axonal growth and pathfinding, and (3) synapse formation and function. Each section highlights known ECM and ECM-receptor components and recent studies done in mutant conditions to reveal their in vivo functions, all illustrating the enormous opportunities provided when merging work on the nervous system with systematic research into ECM-related gene functions.

N-glycans of Phaeodactylum tricornutum diatom and functional characterization of its N-acetylglucosaminyltransferase I enzyme

N-glycosylation, a major co- and post-translational event in the synthesis of proteins in eukaryotes, is unknown in aquatic photosynthetic microalgae. In this paper, we describe the N-glycosylation pathway in the diatom Phaeodactylum tricornutum. Bio-informatic analysis of its genome revealed the presence of a complete set of sequences potentially encoding for proteins involved in the synthesis of the lipid-linked Glc(3)Man(9)GlcNAc(2)-PP-dolichol N-glycan, some subunits of the oligosaccharyltransferase complex, as well as endoplasmic reticulum glucosidases and chaperones required for protein quality control and, finally, the α-mannosidase I involved in the trimming of the N-glycan precursor into Man-5 N-glycan. Moreover, one N-acetylglucosaminyltransferase I, a Golgi glycosyltransferase that initiates the synthesis of complex type N-glycans, was predicted in the P. tricornutum genome. We demonstrated that this gene encodes for an active N-acetylglucosaminyltransferase I, which is able to restore complex type N-glycans maturation in the Chinese hamster ovary Lec1 mutant, defective in its endogeneous N-acetylglucosaminyltransferase I. Consistent with these data, the structural analyses of N-linked glycans demonstrated that P. tricornutum proteins carry mainly high mannose type N-glycans ranging from Man-5 to Man-9. Although representing a minor glycan population, paucimannose N-glycans were also detected, suggesting the occurrence of an N-acetylglucosaminyltransferase I-dependent maturation of N-glycans in this diatom.

Sugar-free frosting, a homolog of SAD kinase, drives neural-specific glycan expression in the Drosophila embryo

Precise glycan structures on specific glycoproteins impart functionalities essential for neural development. However, mechanisms controlling embryonic neural-specific glycosylation are unknown. A genetic screen for relevant mutations in Drosophila generated the sugar-free frosting (sff) mutant that reveals a new function for protein kinases in regulating substrate flux through specific Golgi processing pathways. Sff is the Drosophila homolog of SAD kinase, which regulates synaptic vesicle tethering and neuronal polarity in nematodes and vertebrates. Our Drosophila sff mutant phenotype has features in common with SAD kinase mutant phenotypes in these other organisms, but we detect altered neural glycosylation well before the initiation of embryonic synaptogenesis. Characterization of Golgi compartmentation markers indicates altered colocalization that is consistent with the detected shift in glycan complexity in sff mutant embryos. Therefore, in analogy to synaptic vesicle tethering, we propose that Sff regulates vesicle tethering at Golgi membranes in the developing Drosophila embryo. Furthermore, neuronal sff expression is dependent on transcellular signaling through a non-neural toll-like receptor, linking neural-specific glycan expression to a kinase activity that is induced in response to environmental cues.

Gene Expression in Leaves of Susceptible Glycine max during Infection with Phakopsora pachyrhizi Using Next Generation Sequencing

Soybean rust is caused by the obligate biotrophic fungus Phakopsora pachyrhizi, an exotic pathogen causing important yield losses in soybean production. We used an mRNA-Seq strategy to analyze the expression pattern of soybean genes and better understand molecular events occurring in soybean following the infection. cDNA libraries were constructed from RNA isolated from whole infected soybean leaves 10 days after inoculation with P. pachyrhizi and sequenced using an Illumina platform to identify soybean genes that are affected by pathogen growth. We obtained 15 million sequences corresponding to soybean genes. Forty-two percent of the genes were downregulated including genes encoding proteins involved in amino acid metabolism, carbohydrate metabolism, and transport facilitation; 31% were upregulated including genes encoding proteins involved in lipid metabolism, glycan biosynthesis, and signal transduction. Candidate host genes identified in this study will be manipulated to assay their potential to control soybean rust disease.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for the period 2005-2006

This review is the fourth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2006. The review covers fundamental studies, fragmentation of carbohydrate ions, method developments, and applications of the technique to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, glycated proteins, glycolipids from bacteria, glycosides, and various other natural products. There is a short section on the use of MALDI-TOF mass spectrometry for the study of enzymes involved in glycan processing, a section on industrial processes, particularly the development of biopharmaceuticals and a section on the use of MALDI-MS to monitor products of chemical synthesis of carbohydrates. Large carbohydrate-protein complexes and glycodendrimers are highlighted in this final section.

Life is sweet! A novel role for N-glycans in Drosophila lifespan

N-glycans are post-translational modifications in which the sugar chain is covalently linked to protein by a GlcNAcβ1-N-asparagine linkage. Drosophila melanogaster and other invertebrates, but not vertebrates, synthesize large amounts of "paucimannose" N-glycans that contain only three or four mannose residues. The enzyme UDP-GlcNAc:α3-D-mannoside β1,2-N-acetylglucosaminyltransferase I (GnTI, encoded by the Mgat1 gene) controls the synthesis of paucimannose N-glycans. Either deletion or neuron-specific knockdown of Mgat1 in wild type flies results in pronounced defects in locomotion, structural defects in the adult central nervous system and a severely reduced lifespan. We have recently shown that neuronal expression of a wild-type Mgat1 transgene in Mgat1-null flies rescues the structural defects in the brain (fused β-lobes) and the shortened lifespan and, surprisingly, results in a dramatic 135% increase in mean lifespan relative to genetically identical controls that do not express the transgene. In this review, we discuss various approaches that can be used to determine the roles of paucimannose N-glycans in Drosophila longevity and in the adult CNS.

Identification of genes required for neural-specific glycosylation using functional genomics

Glycosylation plays crucial regulatory roles in various biological processes such as development, immunity, and neural functions. For example, α1,3-fucosylation, the addition of a fucose moiety abundant in Drosophila neural cells, is essential for neural development, function, and behavior. However, it remains largely unknown how neural-specific α1,3-fucosylation is regulated. In the present study, we searched for genes involved in the glycosylation of a neural-specific protein using a Drosophila RNAi library. We obtained 109 genes affecting glycosylation that clustered into nine functional groups. Among them, members of the RNA regulation group were enriched by a secondary screen that identified genes specifically regulating α1,3-fucosylation. Further analyses revealed that an RNA-binding protein, second mitotic wave missing (Swm), upregulates expression of the neural-specific glycosyltransferase FucTA and facilitates its mRNA export from the nucleus. This first large-scale genetic screen for glycosylation-related genes has revealed novel regulation of fucTA mRNA in neural cells.

Neural-specific α3-fucosylation of N-linked glycans in the Drosophila embryo requires fucosyltransferase A and influences developmental signaling associated with O-glycosylation

Addition of fucose (Fuc) to glycoprotein N-linked glycans or in O-linkage directly to Ser/Thr residues modulates specific cell-cell interactions and cell signaling events. Vertebrates and invertebrates add Fuc in α6-linkage to the reducing terminal N-acetylglucosamine residue of N-glycans. In Drosophila and other invertebrates, Fuc can also be added in α3-linkage to the same residue. These difucosylated N-glycans are recognized by anti-horseradish peroxidase (anti-HRP) antisera, providing a well-established marker for insect neural tissue. To understand the mechanisms and consequences of tissue-specific glycan expression, we identified a single α3-fucosyltransferase (FucTA) that produces the anti-HRP epitope in Drosophila embryos. FucTA transcripts are temporally and spatially restricted to cells that express the anti-HRP epitope and are missing in a mutant that lacks neural α3-fucosylation. Transgenic expression of FucTA, but not of any other candidate α3-fucosyltransferase, rescues the anti-HRP epitope in the embryonic nervous system of this mutant. Mass spectrometric characterization of the N-glycans of Drosophila embryos overexpressing FucTA confirms that this enzyme is indeed responsible for the biosynthesis of difucosylated glycans in vivo. Whereas ectopic expression of FucTA in the larval wing disc produces mild wing notching, the heterochronic, pan-neural expression of FucTA in early differentiating neurons generates neurogenic and cell migration phenotypes; this latter effect is associated with reduced GDP-Fuc levels in the embryo and indicates that the diversion of fucosylation resources towards fucosylation of N-glycans has an impact on developmental signaling associated with O-fucosylation.

Age-related behavioral changes in Drosophila

Normal aging can be defined as the natural physiological changes that occur in an organism over time in the absence of any disease. Among the many age-related changes that can be observed are those that result in the progressive decline of a variety of behavioral responses, including locomotor activity and cognitive function. During the past decade, model organisms, such as the fruit fly Drosophila melanogaster, have been used extensively to study aging. These simpler model systems have been particularly useful for genetic studies of aging because of their small genome size, short generation time, and mean life span compared to either mice or humans. Drosophila also exhibits complex behaviors, many of which undergo age-related decline. Here, we describe the age-related changes in behavior that have been observed in Drosophila and discuss how these are affected in long- and short-lived strains of flies.

Neuronal expression of Mgat1 rescues the shortened life span of Drosophila Mgat11 null mutants and increases life span

The enzyme UDP-GlcNAc:alpha3-D-mannoside beta1,2-N-acetylglucosaminyltransferase I (GnT1, encoded by Mgat1) controls the synthesis of paucimannose N-glycans in Drosophila. We have previously reported that null mutations in Drosophila Mgat1 are viable but exhibit defects in locomotion, brain abnormalities, and a severely reduced life span. Here, we show that knockdown of Mgat1 in the central nervous system (CNS) of wild-type flies decreases locomotor activity and life span. This phenotype is similar to that observed in Drosophila Mgat1(1) null mutants, demonstrating that Mgat1 is required in the CNS. We also found that neuronal expression of a wild-type Mgat1 transgene rescued the shortened life span of Mgat1(1) null mutants and resulted in a dramatic 135% increase in mean life span relative to genetically identical controls. Neuronal expression of a wild-type Mgat1 transgene in wild-type flies resulted in a modest 9% increase in mean life span relative to genetically identical controls. In both Mgat1(1) null mutants and wild-type flies, neuronal expression of wild-type Mgat1 transgene resulted in a significant increase in GnT1 activity and resistance to oxidative stress. Whereas dietary restriction is not absolutely essential for the increased life span, it plays a role in the process. Interestingly, we observe a direct correlation between GnT1 activity and mean life span up to a maximum of appropriately 136 days, showing that the ability of GnT1 activity to increase life span is limited. Altogether, these observations suggest that Mgat1-dependent N-glycosylation plays an important role in the control of Drosophila life span.

Metabolism, cell surface organization, and disease

Genetic information flows from DNA to macromolecular structures-the dominant force in the molecular organization of life. However, recent work suggests that metabolite availability to the hexosamine and Golgi N-glycosylation pathways exerts control over the assembly of macromolecular complexes on the cell surface and, in this capacity, acts upstream of signaling and gene expression. The structure and number of N-glycans per protein molecule cooperate to regulate lectin binding and thereby the distribution of glycoproteins at the cell surface. Congenital disorders of glycosylation provide insight as extreme hypomorphisms, whereas milder deficiencies may encompass many common chronic conditions, including autoimmunity, metabolic syndrome, and aging.

A mutation in GDP-mannose pyrophosphorylase causes conditional hypersensitivity to ammonium, resulting in Arabidopsis root growth inhibition, altered ammonium metabolism, and hormone homeostasis

Ascorbic acid (AA) is an antioxidant fulfilling a multitude of cellular functions. Given its pivotal role in maintaining the rate of cell growth and division in the quiescent centre of the root, it was hypothesized that the AA-deficient Arabidopsis thaliana mutants vtc1-1, vtc2-1, vtc3-1, and vtc4-1 have altered root growth. To test this hypothesis, root development was studied in the wild type and vtc mutants grown on Murashige and Skoog medium. It was discovered, however, that only the vtc1-1 mutant has strongly retarded root growth, while the other vtc mutants exhibit a wild-type root phenotype. It is demonstrated that the short-root phenotype in vtc1-1 is independent of AA deficiency and oxidative stress. Instead, vtc1-1 is conditionally hypersensitive to ammonium (NH(4)(+)). To provide new insights into the mechanism of NH(4)(+) sensitivity in vtc1-1, root development, NH(4)(+) content, glutamine synthetase (GS) activity, glutamate dehydrogenase activity, and glutamine content were assessed in wild-type and vtc1-1 mutant plants grown in the presence and absence of high NH(4)(+) and the GS inhibitor MSO. Since VTC1 encodes a GDP-mannose pyrophosphorylase, an enzyme generating GDP-mannose for AA biosynthesis and protein N-glycosylation, it was also tested whether protein N-glycosylation is affected in vtc1-1. Furthermore, since root development requires the action of a variety of hormones, it was investigated whether hormone homeostasis is linked to NH(4)(+) sensitivity in vtc1-1. Our data suggest that NH(4)(+) hypersensitivity in vtc1-1 is caused by disturbed N-glycosylation and that it is associated with auxin and ethylene homeostasis and/or nitric oxide signalling.

Insight into the regulation of glycan synthesis in Drosophila chaoptin based on mass spectrometry

Background

A variety of N-glycans attached to protein are known to involve in many important biological functions. Endoplasmic reticulum (ER) and Golgi localized enzymes are responsible to this template-independent glycan synthesis resulting glycoforms at each asparagine residues. The regulation mechanism such glycan synthesis remains largely unknown.

Methodology/Principal Findings

In order to investigate the relationship between glycan structure and protein conformation, we analyzed a glycoprotein of Drosophila melanogaster, chaoptin (Chp), which is localized in photoreceptor cells and is bound to the cell membrane via a glycosylphosphatidylinositol anchor. Detailed analysis based on mass spectrometry revealed the presence of 13 N-glycosylation sites and the composition of the glycoform at each site. The synthetic pathway of glycans was speculated from the observed glycan structures and the composition at each N-glycosylation site, where the presence of novel routes were suggested. The distribution of glycoforms on a Chp polypeptide suggested that various processing enzymes act on the exterior of Chp in the Golgi apparatus, although virtually no enzyme can gain access to the interior of the horseshoe-shaped scaffold, hence explaining the presence of longer glycans within the interior. Furthermore, analysis of Chp from a mutant (RNAi against dolichyl-phosphate α-d-mannosyltransferase), which affects N-glycan synthesis in the ER, revealed that truncated glycan structures were processed. As a result, the distribution of glycoforms was affected for the high-mannose-type glycans only, whereas other types of glycans remained similar to those observed in the control and wild-type.

Conclusions/Significance

These results indicate that glycan processing depends largely on the backbone structure of the parent polypeptide. The information we obtained can be applied to other members of the LRR family of proteins.

Glycosylation diseases: quo vadis?

About 250 to 500 glycogenes (genes that are directly involved in glycan assembly) are in the human genome representing about 1-2% of the total genome. Over 40 human congenital diseases associated with glycogene mutations have been described to date. It is almost certain that the causative glycogene mutations for many more congenital diseases remain to be discovered. Some glycogenes are involved in the synthesis of only a specific protein and/or a specific class of glycan whereas others play a role in the biosynthesis of more than one glycan class. Mutations in the latter type of glycogene result in complex clinical phenotypes that present difficult diagnostic problems to the clinician. In order to understand in biochemical terms the clinical signs and symptoms of a patient with a glycogene mutation, one must understand how the glycogene works. That requires, first of all, determination of the target protein or proteins of the glycogene followed by an understanding of the role, if any, of the glycogene-dependent glycan in the functions of the protein. Many glycogenes act on thousands of glycoproteins. There are unfortunately no general methods to identify all the potentially large number of glycogene target proteins and which of these proteins are responsible for the mutant phenotypes. Whereas biochemical methods have been highly successful in the discovery of glycogenes responsible for many congenital diseases, it has more recently been necessary to use other methods such as homozygosity mapping. Accurate diagnosis of many recently discovered diseases has become difficult and new diagnostic procedures must be developed. Last but not least is the lack of effective treatment for most of these children and of animal models that can be used to test new therapies.