The role of nucleotide sugar transporters in development of eukaryotes

Golgi-localized APYRASE 1 is critical for Arabidopsis growth by affecting cell wall integrity under boron deficiency

Many nucleoside triphosphate-diphosphohydrolases (NTPDases/APYRASEs, APYs) play a key role in modulating extracellular nucleotide levels. However, the Golgi-localized APYs, which help control glycosylation, have rarely been studied. Here, we identified AtAPY1, a gene encoding an NTPDase in the Golgi apparatus, which is required for cell wall integrity and plant growth under boron (B) limited availability. Loss of function in AtAPY1 hindered cell elongation and division in root tips while increasing the number of cortical cell layers, leading to swelling of the root tip and abundant root hairs under low B stress. Further, expression pattern analysis revealed that B deficiency significantly induced AtAPY1, especially in the root meristem and stele. Fluorescent-labeled AtAPY1-GFP localized to the Golgi stack. Biochemical analysis showed that AtAPY1 exhibited a preference of UDP and GDP hydrolysis activities. Consequently, the loss of function in AtAPY1 might disturb the homoeostasis of NMP-driven NDP-sugar transport, which was closely related to the synthesis of cell wall polysaccharides. Further, cell wall-composition analysis showed that pectin content increased and borate-dimerized RG-II decreased in apy1 mutants, along with a decrease in cellulose content. Eventually, altered polysaccharide characteristics presumably cause growth defects in apy1 mutants under B deficiency. Altogether, these data strongly support a novel role for AtAPY1 in mediating responses to low B availability by regulating cell wall integrity.

Biochemical characterization of a GDP-mannose transporter from Chaetomium thermophilum

Nucleotide Sugar Transporters (NSTs) belong to the SLC35 family (human solute carrier) of membrane transport proteins and are crucial components of the glycosylation machinery. NSTs are localized in the ER and Golgi apparatus membranes, where they accumulate nucleotide sugars from the cytosol for subsequent polysaccharide biosynthesis. Loss of NST function impacts the glycosylation of cell surface molecules. Mutations in NSTs cause several developmental disorders, immune disorders, and increased susceptibility to infection. Atomic resolution structures of three NSTs have provided a blueprint for a detailed molecular interpretation of their biochemical properties. In this work, we have identified, cloned, and expressed 18 members of the SLC35 family from various eukaryotic organisms in Saccharomyces cerevisiae. Out of 18 clones, we determined Vrg4 from Chaetomium thermophilum (CtVrg4) is a GDP-mannose transporter with an enhanced melting point temperature (Tm) of 56.9°C, which increases with the addition of substrates, GMP and GDP-mannose. In addition, we report-for the first time-that the CtVrg4 shows an affinity to bind to phosphatidylinositol lipids.

Mice lacking nucleotide sugar transporter SLC35A3 exhibit lethal chondrodysplasia with vertebral anomalies and impaired glycosaminoglycan biosynthesis

SLC35A3 is considered an uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) transporter in mammals and regulates the branching of N-glycans. A missense mutation in SLC35A3 causes complex vertebral malformation (CVM) in cattle. However, the biological functions of SLC35A3 have not been fully clarified. To address these issues, we have established Slc35a3-/-mice using CRISPR/Cas9 genome editing system. The generated mutant mice were perinatal lethal and exhibited chondrodysplasia recapitulating CVM-like vertebral anomalies. During embryogenesis, Slc35a3 mRNA was expressed in the presomitic mesoderm of wild-type mice, suggesting that SLC35A3 transports UDP-GlcNAc used for the sugar modification that is essential for somite formation. In the growth plate cartilage of Slc35a3-/-embryos, extracellular space was drastically reduced, and many flat proliferative chondrocytes were reshaped. Proliferation, apoptosis and differentiation were not affected in the chondrocytes of Slc35a3-/-mice, suggesting that the chondrodysplasia phenotypes were mainly caused by the abnormal extracellular matrix quality. Because these histological abnormalities were similar to those observed in several mutant mice accompanying the impaired glycosaminoglycan (GAG) biosynthesis, GAG levels were measured in the spine and limbs of Slc35a3-/-mice using disaccharide composition analysis. Compared with control mice, the amounts of heparan sulfate, keratan sulfate, and chondroitin sulfate/dermatan sulfate, were significantly decreased in Slc35a3-/-mice. These findings suggest that SLC35A3 regulates GAG biosynthesis and the chondrodysplasia phenotypes were partially caused by the decreased GAG synthesis. Hence, Slc35a3-/- mice would be a useful model for investigating the in vivo roles of SLC35A3 and the pathological mechanisms of SLC35A3-associated diseases.

Nucleotide sugar transporter SLC35A2 is involved in promoting hepatocellular carcinoma metastasis by regulating cellular glycosylation

PURPOSE

Recently, aberrant glycosylation has been recognized to be relate to malignant behaviors of cancer and outcomes of patients with various cancers. SLC35A2 plays an indispensable role on glycosylation as a nucleotide sugar transporter. However, effects of SLC35A2 on malignant behaviors of cancer cells and alteration of cancer cells surface glycosylation profiles are still not fully understood, particularly in hepatocellular carcinoma (HCC). Hence, from a glycosylation perspective, we investigated the effects of SLC35A2 on metastatic behaviors of HCC cells.

METHODS

SLC35A2 expression in clinical samples and HCC cells was examined by immunohistochemical staining or Western blot/quantitative PCR and was regulated by RNA interference or vectors-mediated transfection. Effects of SLC35A2 expression alteration on metastatic behaviors and membrane glycan profile of HCC cells were observed by using respectively invasion, migration, cell adhesion assay, in vivo lung metastatic nude mouse model and lectins microarray. Co-location among proteins in HCC cells was observed by fluorescence microscope and detected by an in vitro co-immunoprecipitation assay.

RESULTS

SLC35A2 was upregulated in HCC tissues, and is associated with poor prognosis of HCC patients. SLC35A2 expression alteration significantly affected the invasion, adhesion, metastasis and membrane glycan profile and led to the dysregulated expressions or glycosylation of cell adhesion-related molecules in HCC cells. Mechanistically, the maintenance of SLC35A2 activity is critical for the recruitment of the key galactosyltransferase B4GalT1, which is responsible for complex glycoconjugate and lactose biosynthesis, to Golgi apparatus in HCC cells.

CONCLUSION

SLC35A2 plays important roles in promoting HCC metastasis by regulating cellular glycosylation modification and inducing the cell adhesive ability of HCC cells.

Ero1-Pdi1 module-catalysed dimerization of a nucleotide sugar transporter, FonNst2, regulates virulence of Fusarium oxysporum on watermelon

Fusarium oxysporum f. sp. niveum (Fon) is a soil-borne fungus causing vascular Fusarium wilt on watermelon; however, the molecular network regulating Fon virulence remains to be elucidated. Here, we report the function and mechanism of nucleotide sugar transporters (Nsts) in Fon. Fon genome harbours nine FonNst genes with distinct functions in vegetative growth, asexual production, cell wall stress response and virulence. FonNst2 and FonNst3 are required for full virulence of Fon on watermelon and FonNst2 is mainly involved in fungal colonization of the plant tissues. FonNst2 and FonNst3 form homo- or hetero-dimers but function independently in Fon virulence. FonNst2, which has UDP-galactose transporter activity in yeast, interacts with FonEro1 and FonPdi1, both of which are required for full virulence of Fon. FonNst2, FonPdi1 and FonEro1 target to endoplasmic reticulum (ER) and are essential for ER homeostasis and function. FonEro1-FonPdi1 module catalyses the dimerization of FonNst2, which is critical for Fon virulence. Undimerized FonNst2 is unstable and degraded via ER-associated protein degradation in vivo. These data demonstrate that FonEro1-FonPdi1 module-catalysed dimerization of FonNst2 is critical for Fon virulence on watermelon and provide new insights into the regulation of virulence in plant fungal pathogens via disulfide bond formation of key pathogenicity factors.

SLC35A2-CDG: Novel variant and review

SLC35A2 encodes the X-linked transporter that carries uridine diphosphate (UDP)-galactose from the cytosol to the lumen of the Golgi apparatus and the endoplasmic reticulum. Pathogenic variants have been associated to a congenital disorder of glycosylation (CDG) with epileptic encephalopathy as a predominant feature. Among the sixty five patients described so far, a strong gender bias is observed as only seven patients are males. This work is a review and reports a SLC35A2-CDG in a male without epilepsy and with growth deficiency associated with decreased serum IGF1, minor neurological involvement, minor facial dysmorphism, and camptodactyly of fingers and toes. Sequence analysis revealed a hemizygosity for a novel de novo variant: c.233A > G (p.Lys78Arg) in SLC35A2. Further analysis of SLC35A2 sequence by comparing both orthologous and paralogous positions, revealed that not only the variant found in this study, but also most of the reported mutated positions are conserved in SLC35A2 orthologous, and many even in the paralogous SLC35A1 and SLC35A3. This is strong evidence that replacements at these positions will have a critical pathological effect and may also explain the gender bias observed among SLC35A2-CDG patients.

Tau Post-translational Modifications: Dynamic Transformers of Tau Function, Degradation, and Aggregation

Post-translational modifications (PTMs) on tau have long been recognized as affecting protein function and contributing to neurodegeneration. The explosion of information on potential and observed PTMs on tau provides an opportunity to better understand these modifications in the context of tau homeostasis, which becomes perturbed with aging and disease. Prevailing views regard tau as a protein that undergoes abnormal phosphorylation prior to its accumulation into the toxic aggregates implicated in Alzheimer's disease (AD) and other tauopathies. However, the phosphorylation of tau may, in fact, represent part of the normal but interrupted function and catabolism of the protein. In addition to phosphorylation, tau undergoes another forms of post-translational modification including (but not limited to), acetylation, ubiquitination, glycation, glycosylation, SUMOylation, methylation, oxidation, and nitration. A holistic appreciation of how these PTMs regulate tau during health and are potentially hijacked in disease remains elusive. Recent studies have reinforced the idea that PTMs play a critical role in tau localization, protein-protein interactions, maintenance of levels, and modifying aggregate structure. These studies also provide tantalizing clues into the possibility that neurons actively choose how tau is post-translationally modified, in potentially competitive and combinatorial ways, to achieve broad, cellular programs commensurate with the distinctive environmental conditions found during development, aging, stress, and disease. Here, we review tau PTMs and describe what is currently known about their functional impacts. In addition, we classify these PTMs from the perspectives of protein localization, electrostatics, and stability, which all contribute to normal tau function and homeostasis. Finally, we assess the potential impact of tau PTMs on tau solubility and aggregation. Tau occupies an undoubtedly important position in the biology of neurodegenerative diseases. This review aims to provide an integrated perspective of how post-translational modifications actively, purposefully, and dynamically remodel tau function, clearance, and aggregation. In doing so, we hope to enable a more comprehensive understanding of tau PTMs that will positively impact future studies.

Metabolism of Glycosphingolipids and Their Role in the Pathophysiology of Lysosomal Storage Disorders

Glycosphingolipids (GSLs) are a specialized class of membrane lipids composed of a ceramide backbone and a carbohydrate-rich head group. GSLs populate lipid rafts of the cell membrane of eukaryotic cells, and serve important cellular functions including control of cell-cell signaling, signal transduction and cell recognition. Of the hundreds of unique GSL structures, anionic gangliosides are the most heavily implicated in the pathogenesis of lysosomal storage diseases (LSDs) such as Tay-Sachs and Sandhoff disease. Each LSD is characterized by the accumulation of GSLs in the lysosomes of neurons, which negatively interact with other intracellular molecules to culminate in cell death. In this review, we summarize the biosynthesis and degradation pathways of GSLs, discuss how aberrant GSL metabolism contributes to key features of LSD pathophysiology, draw parallels between LSDs and neurodegenerative proteinopathies such as Alzheimer's and Parkinson's disease and lastly, discuss possible therapies for patients.

Glycobiology of Human Fungal Pathogens: New Avenues for Drug Development

Invasive fungal infections (IFI) are an increasing threat to the developing world, with fungal spores being ubiquitous and inhaled every day. Some fungal species are commensal organisms that are part of the normal human microbiota, and, as such, do not pose a threat to the immune system. However, when the natural balance of this association is disturbed or the host's immune system is compromised, these fungal pathogens overtake the organism, and cause IFI. To understand the invasiveness of these pathogens and to address the growing problem of IFI, it is essential to identify the cellular processes of the invading organism and their virulence. In this review, we will discuss the prevalence and current options available to treat IFI, including recent reports of drug resistance. Nevertheless, the main focus of this review is to describe the glycobiology of human fungal pathogens and how various components of the fungal cell wall, particularly cell wall polysaccharides and glycoconjugates, are involved in fungal pathogenicity, their biosynthesis and how they can be potentially exploited to develop novel antifungal treatment options. We will specifically describe the nucleotide sugar transporters (NSTs) that are important in fungal survival and suggest that the inhibition of fungal NSTs may potentially be useful to prevent the establishment of fungal infections.

Structural basis for substrate specificity and regulation of nucleotide sugar transporters in the lipid bilayer

Nucleotide sugars are the activated form of monosaccharides used by glycosyltransferases during glycosylation. In eukaryotes the SLC35 family of solute carriers are responsible for their selective uptake into the Endoplasmic Reticulum or Golgi apparatus. The structure of the yeast GDP-mannose transporter, Vrg4, revealed a requirement for short chain lipids and a marked difference in transport rate between the nucleotide sugar and nucleoside monophosphate, suggesting a complex network of regulatory elements control transport into these organelles. Here we report the crystal structure of the GMP bound complex of Vrg4, revealing the molecular basis for GMP recognition and transport. Molecular dynamics, combined with biochemical analysis, reveal a lipid mediated dimer interface and mechanism for coordinating structural rearrangements during transport. Together these results provide further insight into how SLC35 family transporters function within the secretory pathway and sheds light onto the role that membrane lipids play in regulating transport across the membrane.

Early Stage Glycosylation Biomarkers in Alzheimer's Disease

Alzheimer's disease (AD) is of great cause for concern in our ageing population, which currently lacks diagnostic tools to permit accurate and timely diagnosis for affected individuals. The development of such tools could enable therapeutic interventions earlier in the disease course and thus potentially reducing the debilitating effects of AD. Glycosylation is a common, and important, post translational modification of proteins implicated in a host of disease states resulting in a complex array of glycans being incorporated into biomolecules. Recent investigations of glycan profiles, in a wide range of conditions, has been made possible due to technological advances in the field enabling accurate glycoanalyses. Amyloid beta (Aβ) peptides, tau protein, and other important proteins involved in AD pathogenesis, have altered glycosylation profiles. Crucially, these abnormalities present early in the disease state, are present in the peripheral blood, and help to distinguish AD from other dementias. This review describes the aberrant glycome in AD, focusing on proteins implicated in development and progression, and elucidates the potential of glycome aberrations as early stage biomarkers of AD.

The Complex Interplay Between Extracellular Matrix and Cells in Tissues

Extracellular matrix (ECM) maintains the structural integrity of tissues and regulates cell and tissue functions. ECM is comprised of fibrillar proteins, proteoglycans (PGs), glycosaminoglycans, and glycoproteins, creating a heterogeneous but well-orchestrated network. This network communicates with resident cells via cell-surface receptors. In particular, integrins, CD44, discoidin domain receptors, and cell-surface PGs and additionally voltage-gated ion channels can interact with ECM components, regulating signaling cascades as well as cytoskeleton configuration. The interplay of ECM with recipient cells is enriched by the extracellular vesicles, as they accommodate ECM, signaling, and cytoskeleton molecules in their cargo. Along with the numerous biological properties that ECM can modify, autophagy and angiogenesis, which are critical for tissue homeostasis, are included. Throughout development and disease onset and progression, ECM endures rearrangement to fulfill cellular requirements. The main responsible molecules for tissue remodeling are ECM-degrading enzymes including matrix metalloproteinases, plasminogen activators, cathepsins, and hyaluronidases, which can modify the ECM structure and function in a dynamic mode. A brief summary of the complex interplay between ECM macromolecules and cells in tissues and the contribution of ECM in tissue homeostasis and diseases is given.

Expression of the human UDP-galactose transporter gene hUGT1 in tobacco plants' enhanced plant hardness

We reported previously that tobacco plants transformed with the human UDP-galactose transporter 1 gene (hUGT1) had enhanced growth, displayed characteristic traits, and had an increased proportion of galactose (hyper-galactosylation) in the cell wall matrix polysaccharides. Here, we report that hUGT1-transgenic plants have an enhanced hardness. As determined by breaking and bending tests, the leaves and stems of hUGT1-transgenic plants were harder than those of control plants. Transmission electron microscopy revealed that the cell walls of palisade cells in leaves, and those of cortex cells and xylem fibers in stems of hUGT1-transgenic plants, were thicker than those of control plants. The increased amounts of total cell wall materials extracted from the leaves and stems of hUGT1-transgenic plants supported the increased cell wall thickness. In addition, the cell walls of the hUGT1-transgenic plants showed an increased lignin contents, which was supported by the up-regulation of lignin biosynthetic genes. Thus, the heterologous expression of hUGT1 enhanced the accumulation of cell wall materials, which was accompanied by the increased lignin content, resulting in the increased hardness of the leaves and stems of hUGT1-trangenic plants. The enhanced accumulation of cell wall materials might be related to the hyper-galactosylation of cell wall matrix polysaccharides, most notably arabinogalactan, because of the enhanced UDP-galactose transport from the cytosol to the Golgi apparatus by hUGT1, as suggested in our previous report.

Structural basis of nucleotide sugar transport across the Golgi membrane

Glycosylation is a fundamental cellular process that in eukaryotes occurs in the lumen of both the Golgi apparatus and endoplasmic reticulum 1. Nucleotide sugar transporters (NSTs) are an essential component of the glyscosylation pathway, providing the diverse range of substrates required for the glycosyltransferases 2,3. NSTs are linked to several developmental and immune disorders in humans and in pathogenic microbes play an important role in virulence 4–8. How NSTs recognise and transport activated monosaccharides however is currently unclear. Here we present the first crystal structure of an NST, the GDP-mannose transporter Vrg4, in both the substrate free and bound states. A hitherto unobserved requirement for short chain lipids in activating the transporter supports a model for regulation within the highly dynamic membranes of the Golgi apparatus. Our results provide a structural basis for understanding nucleotide sugar recognition and provide insights into the transport and regulatory mechanism for this family of intracellular transporters.

A case of early onset epileptic encephalopathy with de novo mutation in SLC35A2: Clinical features and treatment for epilepsy

INTRODUCTION

Mutations of SLC35A2 that encodes Golgi-localized Uridine diphosphate (UDP)-galactose transporter at Xp11.23 lead to congenital disorders of glycosylation (CDG). Although patients with CDG generally have diverse systemic symptoms, patients with a SLC35A2 mutation manifest predominantly disorders of the central nervous system (CNS).

CASE REPORT

A female infant aged 12months was referred to our center because of intractable seizures. The patient was born with birth weight of 3228g after 40weeks of unremarkable gestation. At the age of 2months, she had partial seizures evolving to epileptic spasms. Her electroencephalogram showed hypsarrhythmia. Her seizures were refractory to antiepileptic drugs. At referral to our center at 12months, she had developmental delay (no head control), widely spaced inverted nipples, external strabismus, and bilateral heterochromia of irises. Blood examinations were normal. Brain magnetic resonance imaging findings included cerebral and cerebellar atrophy, thinning of the corpus callosum, and arachnoid pouch. Whole-exome sequencing detected a de novo frameshift mutation c.950delG (p.Gly317Alafs*32) at exon 4 in SLC35A2. Seizures subsided after the second adrenocorticotropic hormones (ACTH) therapy at 18months. At the age of 36months, although she had intellectual disability with no meaningful words, she was seizure-free and was able to sit without support and showed smiling face a lot.

CONCLUSION

This report reviewed the clinical features of patients with a SLC35A2 mutation. ACTH therapy may be effective for refractory epilepsy in these patients.

Eukaryotic protein glycosylation: a primer for histochemists and cell biologists

Proteins undergo co- and posttranslational modifications, and their glycosylation is the most frequent and structurally variegated type. Histochemically, the detection of glycan presence has first been performed by stains. The availability of carbohydrate-specific tools (lectins, monoclonal antibodies) has revolutionized glycophenotyping, allowing monitoring of distinct structures. The different types of protein glycosylation in Eukaryotes are described. Following this educational survey, examples where known biological function is related to the glycan structures carried by proteins are given. In particular, mucins and their glycosylation patterns are considered as instructive proof-of-principle case. The tissue and cellular location of glycoprotein biosynthesis and metabolism is reviewed, with attention to new findings in goblet cells. Finally, protein glycosylation in disease is documented, with selected examples, where aberrant glycan expression impacts on normal function to let disease pathology become manifest. The histological applications adopted in these studies are emphasized throughout the text.

Functional expression of a human GDP-L-fucose transporter in Escherichia coli

OBJECTIVES

To investigate the translocation of nucleotide-activated sugars from the cytosol across a membrane into the endoplasmatic reticulum or the Golgi apparatus which is an important step in the synthesis of glycoproteins and glycolipids in eukaryotes.

RESULTS

The heterologous expression of the recombinant and codon-adapted human GDP-L-fucose antiporter gene SLC35C1 (encoding an N-terminal OmpA-signal sequence) led to a functional transporter protein located in the cytoplasmic membrane of Escherichia coli. The in vitro transport was investigated using inverted membrane vesicles. SLC35C1 is an antiporter specific for GDP-L-fucose and depending on the concomitant reverse transport of GMP. The recombinant transporter FucT1 exhibited an activity for the transport of ³H-GDP-L-fucose with a V_max of 8 pmol/min mg with a K_m of 4 µM. The functional expression of SLC35C1 in GDP-L-fucose overproducing E. coli led to the export of GDP-L-fucose to the culture supernatant.

CONCLUSIONS

The export of GDP-L-fucose by E. coli provides the opportunity for the engineering of a periplasmatic fucosylation reaction in recombinant bacterial cells.

The conundrum of UDP-Glc entrance into the yeast ER lumen

UDP-Glc entrance into the endoplasmic reticulum (ER) of eukaryotic cells is a key step in the quality control of glycoprotein folding, a mechanism requiring transfer of a Glc residue from the nucleotide sugar (NS) to glycoprotein folding intermediates by the UDP-Glc:glycoprotein glucosyltransferase (UGGT). According to a bioinformatics search there are only eight genes in the Schizosaccharomyces pombe genome belonging to the three Pfam families to which all known nucleotide-sugar transporters (NSTs) of the secretory pathway belong. The protein products of two of them (hut1⁺ and yea4⁺) localize to the ER, those of genes gms1⁺, vrg4⁺, pet1⁺, pet2⁺ and pet3⁺ to the Golgi, whereas that of gms2⁺ has an unknown location. Here we demonstrate that (1) Δhut1 and Δgpt1 (UGGT null) mutants share several phenotypic features; (2) Δhut1 mutants show a 50% reduction in UDP-Glc transport into ER-derived membranes; (3) in vivo UDP-Glc ER entrance occurred in Δhut1Δyea4Δgms2 mutants and in cells in which Δhut1 disruption was combined with that of each of four of the genes encoding Golgi-located proteins. Therefore, disruption of all genes whose products localize to the ER or have an unknown location did not obliterate UDP-Glc ER entrance. We conclude that the hut1⁺ gene product is involved in UDP-Glc entrance into the ER, but that at least another as yet unknown NST displaying an unconventional sequence operates in the yeast secretory pathway. This conclusion agrees with our previous results showing that UDP-Glc entrance into the yeast ER does not follow the classical NST antiport mechanism.

The inside and outside: topological issues in plant cell wall biosynthesis and the roles of nucleotide sugar transporters

The cell wall is a complex extracellular matrix composed primarily of polysaccharides. Noncellulosic polysaccharides, glycoproteins and proteoglycans are synthesized in the Golgi apparatus by glycosyltransferases (GTs), which use nucleotide sugars as donors to glycosylate nascent glycan and glycoprotein acceptors that are subsequently exported to the extracellular space. Many nucleotide sugars are synthesized in the cytosol, leading to a topological issue because the active sites of most GTs are located in the Golgi lumen. Nucleotide sugar transporters (NSTs) overcome this problem by translocating nucleoside diphosphate sugars from the cytosol into the lumen of the organelle. The structures of the cell wall components synthesized in the Golgi are diverse and complex; therefore, transporter activities are necessary so that the nucleotide sugars can provide substrates for the GTs. In this review, we describe the topology of reactions involved in polysaccharide biosynthesis in the Golgi and focus on the roles of NSTs as well as their impacts on cell wall structure when they are altered.

Overview of Nucleotide Sugar Transporter Gene Family Functions Across Multiple Species

Glycoproteins and glycolipids are crucial in a number of cellular processes, such as growth, development, and responses to external cues, among others. Polysaccharides, another class of sugar-containing molecules, also play important structural and signaling roles in the extracellular matrix. The additions of glycans to proteins and lipids, as well as polysaccharide synthesis, are processes that primarily occur in the Golgi apparatus, and the substrates used in this biosynthetic process are nucleotide sugars. These proteins, lipids, and polysaccharides are also modified by the addition of sulfate groups in the Golgi apparatus in a series of reactions where nucleotide sulfate is needed. The required nucleotide sugar substrates are mainly synthesized in the cytosol and transported into the Golgi apparatus by nucleotide sugar transporters (NSTs), which can additionally transport nucleotide sulfate. Due to the critical role of NSTs in eukaryotic organisms, any malfunction of these could change glycan and polysaccharide structures, thus affecting function and altering organism physiology. For example, mutations or deletion on NST genes lead to pathological conditions in humans or alter cell walls in plants. In recent years, many NSTs have been identified and functionally characterized, but several remain unanalyzed. This study examined existing information on functionally characterized NSTs and conducted a phylogenetic analysis of 257 NSTs predicted from nine animal and plant model species, as well as from protists and fungi. From this analysis, relationships between substrate specificity and the primary NST structure can be inferred, thereby advancing understandings of nucleotide sugar gene family functions across multiple species.

UDP-galactose transporter gene hUGT1 expression in tobacco plants leads to hyper-galactosylated cell wall components

We reported previously that tobacco plants transformed with the human UDP-galactose transporter 1 gene (hUGT1-transgenic plants) displayed morphological, architectural, and physiological alterations, such as enhanced growth, increased accumulation of chlorophyll and lignin, and a gibberellin-responsive phenotype. In the present study, we demonstrated that hUGT1 expression altered the monosaccharide composition of cell wall matrix polysaccharides, such as pectic and hemicellulosic polysaccharides, which are biosynthesized in the Golgi lumen. An analysis of the monosaccharide composition of the cell wall matrix polysaccharides revealed that the ratio of galactose to total monosaccharides was significantly elevated in the hemicellulose II and pectin fractions of hUGT1-transgenic plants compared with that of control plants. A hyper-galactosylated xyloglucan structure was detected in hemicellulose II using oligosaccharide mass profiling. These results indicated that, because of the enhanced UDP-galactose transport from the cytosol to the Golgi apparatus by hUGT1, galactose incorporation in the cell wall matrix polysaccharides increased. This increased galactose incorporation may have contributed to increased galactose tolerance in hUGT1-transgenic plants.

Metabolic Glyco-Engineering in Eukaryotic Cells and Selected Applications

By metabolic glyco-engineering cellular glycoconjugates are modified through the incorporation of synthetic monosaccharides which are usually analogues of naturally present sugars. In order to get incorporated, the monosaccharides need to enter the cytoplasm and to be substrates for the enzymes necessary for their transformation into activated sugars, most often nucleotide sugars. These have to be substrates for glycosyltransferases which finally catalyze their incorporation into glycans. Such pathways are difficult to reconstitute in vitro and therefore new monosaccharide analogues have to be tested in tissue culture for their suitability in metabolic glyco-engineering. For this, glycosylation mutants are the most appropriate since they are unable to synthesize specific glycans but through the introduction of the monosaccharide analogues they may express some glycans at the cell surface with the unnatural sugar incorporated. The presence of those glycans can be easily and quantitatively detected by lectin binding or by chemical methods identifying specific sugars. Monosaccharide analogues can also block the pathways leading to sugar incorporation, thus inhibiting the synthesis of glycan structures which is also easily detectable at the cell surface by lectin labeling. The most useful and most frequently employed application of metabolic glyco-engineering is the introduction of reactive groups which can undergo bio-orthogonal click reactions for the efficient labeling of glycans at the surface of live cells.

Identification of a Golgi-localized UDP-N-acetylglucosamine transporter in Trypanosoma cruzi

BACKGROUND

Nucleotide sugar transporters (NSTs) play an essential role in translocating nucleotide sugars into the lumen of the endoplasmic reticulum and Golgi apparatus to be used as substrates in glycosylation reactions. This intracellular transport is an essential step in the biosynthesis of glycoconjugates.

RESULTS

We have identified a family of 11 putative NSTs in Trypanosoma cruzi, the etiological agent of Chagas' disease. A UDP-N-acetylglucosamine transporter, TcNST1, was identified by a yeast complementation approach. Based on a phylogenetic analysis four candidate genes were selected and used for complementation assays in a Kluyveromyces lactis mutant strain. The transporter is likely expressed in all stages of the parasite life cycle and during differentiation of epimastigotes to infective metacyclics. Immunofluorescence analyses of a GFP-TcNST1 fusion protein indicate that the transporter is localized to the Golgi apparatus. As many NSTs are multisubstrate transporters, we also tested the capacity of TcNST1 to transport GDP-Man.

CONCLUSIONS

We have identified a UDP-N-acetylglucosamine transporter in T. cruzi, which is specifically localized to the Golgi apparatus and seems to be expressed, at the mRNA level, throughout the parasite life cycle. Functional studies of TcNST1 will be important to unravel the role of NSTs and specific glycoconjugates in T. cruzi survival and infectivity.

A new case of UDP-galactose transporter deficiency (SLC35A2-CDG): molecular basis, clinical phenotype, and therapeutic approach

Congenital disorders of glycosylation (CDG) are a group of hereditary metabolic diseases characterized by abnormal glycosylation of proteins and lipids. Often, multisystem disorders with central nervous system involvement and a large variety of clinical symptoms occur. The main characteristics are developmental delay, seizures, and ataxia. In this paper we report the clinical and biochemical characteristics of a 5-year-old girl with a defective galactosylation of N-glycans, resulting in developmental delay, muscular hypotonia, epileptic seizures, inverted nipples, and visual impairment. Next generation sequencing revealed a de novo mutation (c.797G > T, p.G266V) in the X-chromosomal gene SLC35A2 (solute carrier family 35, UDP-galactose transporter, member A2; MIM 300896). While this mutation was found heterozygous, random X-inactivation of the normal allele will lead to loss of normal SLC35A2 activity in respective cells. The functional relevance of the mutation was demonstrated by complementation of UGT-deficient MDCK-RCA(r) and CHO-Lec8 cells by normal UGT-expression construct but not by the mutant version. The effect of dietary galactose supplementation on glycosylation was investigated, showing a nearly complete normalization of transferrin glycosylation.

Glycan variation and evolution in the eukaryotes

In this review, we document the evolution of common glycan structures in the eukaryotes, and illustrate the considerable variety of oligosaccharides existing in these organisms. We focus on the families of N- and O-glycans, glycosphingolipids, glycosaminoglycans, glycosylphosphatidylinositol (GPI) anchors, sialic acids (Sias), and cytoplasmic and nuclear glycans. We also outline similar and divergent aspects of the glycans during evolution within the groups, which include inter- and intraspecies differences, molecular mimicry, viral glycosylation adaptations, glycosyltransferase specificity relating to function, and the natural dynamism powering these events. Finally, we present an overview of the patterns of glycosylation found within the groups comprising the Eukaryota, namely the Deuterostomia, Fungi, Viridiplantae, Nematoda, and Arthropoda.

Dynamic regulation of innate immune responses in Drosophila by Senju-mediated glycosylation

The innate immune system is the first line of defense encountered by invading pathogens. Delayed and/or inadequate innate immune responses can result in failure to combat pathogens, whereas excessive and/or inappropriate responses cause runaway inflammation. Therefore, immune responses are tightly regulated from initiation to resolution and are repressed during the steady state. It is well known that glycans presented on pathogens play important roles in pathogen recognition and the interactions between host molecules and microbes; however, the function of glycans of host organisms in innate immune responses is less well known. Here, we show that innate immune quiescence and strength of the immune response are controlled by host glycosylation involving a novel UDP-galactose transporter called Senju. In senju mutants, reduced expression of galactose-containing glycans resulted in hyperactivation of the Toll signaling pathway in the absence of immune challenges. Genetic epistasis and biochemical analyses revealed that Senju regulates the Toll signaling pathway at a step that converts Toll ligand Spatzle to its active form. Interestingly, Toll activation in immune-challenged wild type (WT) flies reduced the expression of galactose-containing glycans. Suppression of the degalactosylation by senju overexpression resulted in reduced induction of Toll-dependent expression of an antimicrobial peptide, Drosomycin, and increased susceptibility to infection with Gram-positive bacteria. These data suggest that Senju-mediated galactosylation suppresses undesirable Toll signaling activation during the steady state; however, Toll activation in response to infection leads to degalactosylation, which raises the immune response to an adequate level and contributes to the prompt elimination of pathogens.

Golgi-located NTPDase1 of Leishmania major is required for lipophosphoglycan elongation and normal lesion development whereas secreted NTPDase2 is dispensable for virulence

Parasitic protozoa, such as Leishmania species, are thought to express a number of surface and secreted nucleoside triphosphate diphosphohydrolases (NTPDases) which hydrolyze a broad range of nucleoside tri- and diphosphates. However, the functional significance of NTPDases in parasite virulence is poorly defined. The Leishmania major genome was found to contain two putative NTPDases, termed LmNTPDase1 and 2, with predicted NTPDase catalytic domains and either an N-terminal signal sequence and/or transmembrane domain, respectively. Expression of both proteins as C-terminal GFP fusion proteins revealed that LmNTPDase1 was exclusively targeted to the Golgi apparatus, while LmNTPDase2 was predominantly secreted. An L. major LmNTPDase1 null mutant displayed increased sensitivity to serum complement lysis and exhibited a lag in lesion development when infections in susceptible BALB/c mice were initiated with promastigotes, but not with the obligate intracellular amastigote stage. This phenotype is characteristic of L. major strains lacking lipophosphoglycan (LPG), the major surface glycoconjugate of promastigote stages. Biochemical studies showed that the L. major NTPDase1 null mutant synthesized normal levels of LPG that was structurally identical to wild type LPG, with the exception of having shorter phosphoglycan chains. These data suggest that the Golgi-localized NTPase1 is involved in regulating the normal sugar-nucleotide dependent elongation of LPG and assembly of protective surface glycocalyx. In contrast, deletion of the gene encoding LmNTPDase2 had no measurable impact on parasite virulence in BALB/c mice. These data suggest that the Leishmania major NTPDase enzymes have potentially important roles in the insect stage, but only play a transient or non-major role in pathogenesis in the mammalian host.

Nucleoside triphosphate diphosphohydrolases (NTPDases) are a family of enzymes expressed in many eukaryotes, ranging from single-celled parasites to mammals. In mammals, NTPDases can have an immunomodulatory role, while in pathogenic protists cell-surface and secreted NTPDases are thought to be important virulence factors, although this has never been explicitly tested. In this study we have investigated the function of two NTPDases, termed LmNTPDase1 and LmNTPDase2, in Leishmania major parasites. We show that LmNTPDase 1 and LmNTPDase 2 are differentially targeted to the Golgi apparatus and secreted, respectively. A Leishmania major mutant lacking the Golgi LmNTPDase1 exhibited a delayed capacity to induce lesions in susceptible mice when promastigote (insect) stages were used to initiate infection, but not when amastigote (mammalian-infective) stages were used. Loss of promastigote infectivity in the LmNTPDase1 null mutant was associated with the synthesis and surface expression of lipophosphoglycan (LPG), with shorter glycan chains and increased sensitivity to complement-mediated lysis. In contrast, a null mutant lacking the secreted LmNTPDase2 did not exhibit any difference in virulence. Our results suggest that Leishmania major NTPDases have specific roles in regulating Golgi glycosylation pathways, and nucleoside salvage pathways in the insect stages, but do not appear to be required for virulence of the mammalian-infective stages.

Cryptococcus neoformans dual GDP-mannose transporters and their role in biology and virulence

Cryptococcus neoformans is an opportunistic yeast responsible for lethal meningoencephalitis in humans. This pathogen elaborates a polysaccharide capsule, which is its major virulence factor. Mannose constitutes over one-half of the capsule mass and is also extensively utilized in cell wall synthesis and in glycosylation of proteins and lipids. The activated mannose donor for most biosynthetic reactions, GDP-mannose, is made in the cytosol, although it is primarily consumed in secretory organelles. This compartmentalization necessitates specific transmembrane transporters to make the donor available for glycan synthesis. We previously identified two cryptococcal GDP-mannose transporters, Gmt1 and Gmt2. Biochemical studies of each protein expressed in Saccharomyces cerevisiae showed that both are functional, with similar kinetics and substrate specificities in vitro. We have now examined these proteins in vivo and demonstrate that cells lacking Gmt1 show significant phenotypic differences from those lacking Gmt2 in terms of growth, colony morphology, protein glycosylation, and capsule phenotypes. Some of these observations may be explained by differential expression of the two genes, but others suggest that the two proteins play overlapping but nonidentical roles in cryptococcal biology. Furthermore, gmt1 gmt2 double mutant cells, which are unexpectedly viable, exhibit severe defects in capsule synthesis and protein glycosylation and are avirulent in mouse models of cryptococcosis.

Glycobiology: progress, problems, and perspectives

This review highlights different aspects of glycobiology with analysis of recent progress in the study of biosynthesis, degradation, and biological role of glycoconjugates and of hereditary diseases related to the metabolism of these compounds. In addition, the review presents some analysis of the papers of other authors who have contributed to this special issue.

Transforming growth factor β signaling upregulates the expression of human GDP-fucose transporter by activating transcription factor Sp1

GDP-fucose transporter plays a crucial role in fucosylation of glycoproteins by providing activated fucose donor, GDP-fucose, for fucosyltransferases in the lumen of the Golgi apparatus. Fucose-containing glycans are involved in many biological processes, which are essential for growth and development. Mutations in the GDP-fucose transporter gene cause leukocyte adhesion deficiency syndrome II, a disease characterized by slow growth, mental retardation and immunodeficiency. However, no information is available regarding its transcriptional regulation. Here, by using human cells, we show that TGF-β1 specifically induces the GDP-fucose transporter expression, but not other transporters tested such as CMP-sialic acid transporter, suggesting a diversity of regulatory pathways for the expression of these transporters. The regulatory elements that are responsive to the TGF-β1 stimulation are present in the region between bp -330 and -268 in the GDP-fucose transporter promoter. We found that this region contains two identical octamer GC-rich motifs (GGGGCGTG) that were demonstrated to be essential for the transporter expression. We also show that the transcription factor Sp1 specifically binds to the GC-rich motifs in vitro and Sp1 coupled with phospho-Smad2 is associated with the promoter region covering the Sp1-binding motifs in vivo using chromatin immunoprecipitation (ChIP) assays. In addition, we further confirmed that Sp1 is essential for the GDP-fucose transporter expression stimulated by TGF-β1 using a luciferase reporter system. These results highlight the role of TGF-β signaling in regulation of the GDP-fucose transporter expression via activating Sp1. This is the first transcriptional study for any nucleotide sugar transporters that have been identified so far. Notably, TGF-β1 receptor itself is known to be modified by fucosylation. Given the essential role of GDP-fucose transporter in fucosylation, the finding that TGF-β1 stimulates the expression of this transporter, suggests a possible intracellular link between the function of nucleotide sugar transporter and the TGF-β signaling pathway.

UDP-N-acetylglucosamine transporter (SLC35A3) regulates biosynthesis of highly branched N-glycans and keratan sulfate

SLC35A3 is considered the main UDP-N-acetylglucosamine transporter (NGT) in mammals. Detailed analysis of NGT is restricted because mammalian mutant cells defective in this activity have not been isolated. Therefore, using the siRNA approach, we developed and characterized several NGT-deficient mammalian cell lines. CHO, CHO-Lec8, and HeLa cells deficient in NGT activity displayed a decrease in the amount of highly branched tri- and tetraantennary N-glycans, whereas monoantennary and diantennary ones remained unchanged or even were accumulated. Silencing the expression of NGT in Madin-Darby canine kidney II cells resulted in a dramatic decrease in the keratan sulfate content, whereas no changes in biosynthesis of heparan sulfate were observed. We also demonstrated for the first time close proximity between NGT and mannosyl (α-1,6-)-glycoprotein β-1,6-N-acetylglucosaminyltransferase (Mgat5) in the Golgi membrane. We conclude that NGT may be important for the biosynthesis of highly branched, multiantennary complex N-glycans and keratan sulfate. We hypothesize that NGT may specifically supply β-1,3-N-acetylglucosaminyl-transferase 7 (β3GnT7), Mgat5, and possibly mannosyl (α-1,3-)-glycoprotein β-1,4-N-acetylglucosaminyltransferase (Mgat4) with UDP-GlcNAc.

UDP-Gal/UDP-GlcNAc chimeric transporter complements mutation defect in mammalian cells deficient in UDP-Gal transporter

The role of UDP-galactose transporter (UGT; SLC35A2) and UDP-N-acetylglucosamine transporter (NGT; SLC35A3) in glycosylation of macromolecules may be coupled and either of the transporters may partially replace the function played by its partner. The aim of this study was to construct chimeric transporters composed of the N-terminal portion of human NGT and the C-terminal portion of human UGT1 or UGT2 (NGT-UGT1 or NGT-UGT2, respectively), as well as of the N-terminal portion of UGT and C-terminal portion of NGT (UGT-NGT), overexpress them stably in UGT-deficient CHO-Lec8 and MDCK-RCA(r) cells, and characterize their involvement in glycosylation. Two chimeric proteins, NGT-UGT1 and NGT-UGT2, did not overexpress properly. In contrast, UGT-NGT chimeric protein was successfully overexpressed and localized properly to the Golgi apparatus. UGT-NGT chimeric transporter delivered UDP-Gal to the Golgi vesicles of UGT-deficient cells, which resulted in the increased content of galactosylated structures to such an extent that the wild-type phenotypes were completely restored. Our data further support our hypothesis that UGT and NGT cooperate in the UDP-Gal delivery for glycosyltransferases located in the Golgi apparatus. In a wider context, the results gained in this study add to our understanding of glycosylation, one of the basic posttranslational modifications, which affects the majority of macromolecules.

Inhibition of nucleotide sugar transport in Trypanosoma brucei alters surface glycosylation

Nucleotide sugar transporters (NSTs) are indispensible for the biosynthesis of glycoproteins by providing the nucleotide sugars needed for glycosylation in the lumen of the Golgi apparatus. Mutations in NST genes cause human and cattle diseases and impaired cell walls of yeast and fungi. Information regarding their function in the protozoan parasite, Trypanosoma brucei, a causative agent of African trypanosomiasis, is unknown. Here, we characterized the substrate specificities of four NSTs, TbNST1-4, which are expressed in both the insect procyclic form (PCF) and mammalian bloodstream form (BSF) stages. TbNST1/2 transports UDP-Gal/UDP-GlcNAc, TbNST3 transports GDP-Man, and TbNST4 transports UDP-GlcNAc, UDP-GalNAc, and GDP-Man. TbNST4 is the first NST shown to transport both pyrimidine and purine nucleotide sugars and is demonstrated here to be localized at the Golgi apparatus. RNAi-mediated silencing of TbNST4 in the procyclic form caused underglycosylated surface glycoprotein EP-procyclin. Similarly, defective glycosylation of the variant surface glycoprotein (VSG221) as well as the lysosomal membrane protein p67 was observed in Δtbnst4 BSF T. brucei. Relative infectivity analysis showed that defects in glycosylation of the surface coat resulting from tbnst4 deletion were insufficient to impact the ability of this parasite to infect mice. Notably, the fact that inactivation of a single NST gene results in measurable defects in surface glycoproteins in different life cycle stages of the parasite highlights the essential role of NST(s) in glycosylation of T. brucei. Thus, results presented in this study provide a framework for conducting functional analyses of other NSTs identified in T. brucei.

AtAPY1 and AtAPY2 function as Golgi-localized nucleoside diphosphatases in Arabidopsis thaliana

Nucleoside triphosphate diphosphohydrolases (NTPDases; apyrases) (EC 3.6.1.5) hydrolyze di- and triphosphate nucleotides, but not monophosphate nucleotides. They are categorized as E-type ATPases, have a broad divalent cation (Mg(2+), Ca(2+)) requirement for activation and are insensitive to inhibitors of F-type, P-type and V-type ATPases. Among the seven NTPDases identified in Arabidopsis, only APYRASE 1 (AtAPY1) and APYRASE 2 (AtAPY2) have been previously characterized. In this work, either AtAPY1 or AtAPY2 tagged with C-terminal green fluorescent protein (GFP) driven by their respective native promoter can rescue the apy1 apy2 double knockout (apy1 apy2 dKO) successfully, and confocal microscopy reveals that these two Arabidopsis apyrases reside in the Golgi apparatus. In Saccharomyces cerevisiae, both AtAPY1 and AtAPY2 can complement the Golgi-localized GDA1 mutant, rescuing its aberrant protein glycosylation phenotype. In Arabidopsis, microsomes of the wild type show higher substrate preferences toward UDP compared with other NDP substrates. Loss-of-function Arabidopsis AtAPY1 mutants exhibit reduced microsomal UDPase activity, and this activity is even more significantly reduced in the loss-of-function AtAPY2 mutant and in the AtAPY1/AtAPY2 RNA interference (RNAi) technology repressor lines. Microsomes from wild-type plants also have detectable GDPase activity, which is significantly reduced in apy2 but not apy1 mutants. The GFP-tagged AtAPY1 or AtAPY2 constructs in the apy1 apy2 dKO plants can restore microsomal UDP/GDPase activity, confirming that they both also have functional competency. The cell walls of apy1, apy2 and the RNAi-silenced lines all have an increased composition of galactose, but the transport efficiency of UDP-galactose across microsomal membranes was not altered. Taken together, these results reveal that AtAPY1 and AtAPY2 are Golgi-localized nucleotide diphosphatases and are likely to have roles in regulating UDP/GDP concentrations in the Golgi lumen.

UDP-N-acetylglucosamine transporter and UDP-galactose transporter form heterologous complexes in the Golgi membrane

UDP-galactose transporter (UGT; SLC35A2) and UDP-N-acetylglucosamine transporter (NGT; SLC35A3) are evolutionarily related. We hypothesize that their role in glycosylation may be coupled through heterologous complex formation. Coimmunoprecipitation analysis and FLIM-FRET measurements performed on living cells showed that NGT and UGT form complexes when overexpressed in MDCK-RCA(r) cells. We also postulate that the interaction of NGT and UGT may explain the dual localization of UGT2. For the first time we demonstrated in vivo homodimerization of the NGT nucleotide sugar transporter. In conclusion, we suggest that NGT and UGT function in glycosylation is combined via their mutual interaction.

O-glycosylation modulates integrin and FGF signalling by influencing the secretion of basement membrane components

Extracellular microenvironments have crucial roles in modulating cell interactions during development. Here we discover that a conserved protein modification (O-glycosylation) influences extracellular matrix composition during mammalian organogenesis, affecting integrin signalling and fibroblast growth factor-mediated cell proliferation. Specifically, mice deficient for an enzyme (Galnt1) that adds sugars to proteins during early stages of organogenesis resulted in intracellular accumulation of major basement membrane proteins and endoplasmic reticulum stress, with resultant effects on fibroblast growth factor signalling, epithelial cell proliferation and organ growth. Exogenous addition of basement membrane components rescued fibroblast growth factor signalling and the growth defects in a β1-integrin-dependent manner. Our work demonstrates for the first time that O-glycosylation influences the composition of the extracellular matrix during mammalian organ development, influencing specific aspects of the endoplasmic reticulum stress response, cell signalling, cell proliferation and organ growth. Our work provides insight into the role of this conserved protein modification in both development and disease.

Developmental diseases caused by impaired nucleotide sugar transporters

Nucleotide sugar transporters play critical roles in glycosylation of proteins, lipids and proteoglycans, which are essential for organogenesis, development, mammalian cellular immunity and pathogenicity of human pathogenic agents. Functional deficiencies of these transporters result in global defects of glycoconjugates, which in turn lead to a diversity of biochemical, physiological and pathological phenotypes. In this short review, we will highlight human and bovine diseases caused by mutations of these transporters.

Identification of functional elements of the GDP-fucose transporter SLC35C1 using a novel Chinese hamster ovary mutant

The GDP-fucose transporter SLC35C1 critically regulates the fucosylation of glycans. Elucidation of its structure-function relationships remains a challenge due to the lack of an appropriate mutant cell line. Here we report a novel Chinese hamster ovary (CHO) mutant, CHO-gmt5, generated by the zinc-finger nuclease technology, in which the Slc35c1 gene was knocked out from a previously reported CHO mutant that has a dysfunctional CMP-sialic acid transporter (CST) gene (Slc35a1). Consequently, CHO-gmt5 harbors double genetic defects in Slc35a1 and Slc35c1 and produces N-glycans deficient in both sialic acid and fucose. The structure-function relationships of SLC35C1 were studied using CHO-gmt5 cells. In contrast to the CST and UDP-galactose transporter, the C-terminal tail of SLC35C1 is not required for its Golgi localization but is essential for generating glycans that are recognized by a fucose-binding lectin, Aleuria aurantia lectin (AAL), suggesting an important role in the transport activity of SLC35C1. Furthermore, we found that this impact can be independently contributed by a cluster of three lysine residues and a Glu-Met (EM) sequence within the C terminus. We also showed that the conserved glycine residues at positions 180 and 277 of SLC35C1 have significant impacts on AAL binding to CHO-gmt5 cells, suggesting that these conserved glycine residues are required for the transport activity of Slc35 proteins. The absence of sialic acid and fucose on Fc N-glycan has been independently shown to enhance the antibody-dependent cellular cytotoxicity (ADCC) effect. By combining these features into one cell line, we postulate that CHO-gmt5 may represent a more advantageous cell line for the production of recombinant antibodies with enhanced ADCC effect.

Engineering mammalian mucin-type O-glycosylation in plants

Mucin-type O-glycosylation is an important post-translational modification that confers a variety of biological properties and functions to proteins. This post-translational modification has a particularly complex and differentially regulated biosynthesis rendering prediction and control of where O-glycans are attached to proteins, and which structures are formed, difficult. Because plants are devoid of GalNAc-type O-glycosylation, we have assessed requirements for establishing human GalNAc O-glycosylation de novo in plants with the aim of developing cell systems with custom-designed O-glycosylation capacity. Transient expression of a Pseudomonas aeruginosa Glc(NAc) C4-epimerase and a human polypeptide GalNAc-transferase in leaves of Nicotiana benthamiana resulted in GalNAc O-glycosylation of co-expressed human O-glycoprotein substrates. A chimeric YFP construct containing a 3.5 tandem repeat sequence of MUC1 was glycosylated with up to three and five GalNAc residues when co-expressed with GalNAc-T2 and a combination of GalNAc-T2 and GalNAc-T4, respectively, as determined by mass spectrometry. O-Glycosylation was furthermore demonstrated on a tandem repeat of MUC16 and interferon α2b. In plants, prolines in certain classes of proteins are hydroxylated and further substituted with plant-specific O-glycosylation; unsubstituted hydroxyprolines were identified in our MUC1 construct. In summary, this study demonstrates that mammalian type O-glycosylation can be established in plants and that plants may serve as a host cell for production of recombinant O-glycoproteins with custom-designed O-glycosylation. The observed hydroxyproline modifications, however, call for additional future engineering efforts.

CHO glycosylation mutants as potential host cells to produce therapeutic proteins with enhanced efficacy

CHO glycosylation mutants, pioneered by Stanley and co-workers, have proven to be valuable tools in glycobiology and biopharmaceutical research. Here we aim to provide a summary of our efforts to isolate industrially applicable CHO glycosylation mutants, termed CHO-gmt cells, using cytotoxic lectins and zinc-finger nuclease technology. The genetic defects in the glycosylation machinery in these cells lead to the production of recombinant glycoproteins with consistent and unique glycan structures. In addition, these mutant cells can be easily adapted to serum-free medium in suspension cultures, the condition used by the biotech industry for large-scale production of recombinant therapeutics. In light of the critical impact of glycosylation on biopharmaceutical performances, namely, safety and efficacy, the CHO-gmt lines have enormous potential in producing glycoprotein therapeutics with optimal glycosylation profiles, thus, representing a panel of ideal host cell lines for producing recombinant biopharmaceuticals with improved safety profiles and enhanced efficacy.

Multiple members of the UDP-GalNAc: polypeptide N-acetylgalactosaminyltransferase family are essential for viability in Drosophila

Mucin-type O-glycosylation represents a major form of post-translational modification that is conserved across most eukaryotic species. This type of glycosylation is initiated by a family of enzymes (GalNAc-Ts in mammals and PGANTs in Drosophila) whose members are expressed in distinct spatial and temporal patterns during development. Previous work from our group demonstrated that one member of this family is essential for viability and another member modulates extracellular matrix composition and integrin-mediated cell adhesion during development. To investigate whether other members of this family are essential, we employed RNA interference (RNAi) to each gene in vivo. Using this approach, we identified 4 additional pgant genes that are required for viability. Ubiquitous RNAi to pgant4, pgant5, pgant7, or the putative glycosyltransferase CG30463 resulted in lethality. Tissue-specific RNAi was also used to define the specific organ systems and tissues in which each essential family member is required. Interestingly, each essential pgant had a unique complement of tissues in which it was required. Additionally, certain tissues (mesoderm, digestive system, and tracheal system) required more than one pgant, suggesting unique functions for specific enzymes in these tissues. Expanding upon our RNAi results, we found that conventional mutations in pgant5 resulted in lethality and specific defects in specialized cells of the digestive tract, resulting in loss of proper digestive system acidification. In summary, our results highlight essential roles for O-glycosylation and specific members of the pgant family in many aspects of development and organogenesis.

Golgi glycosylation and human inherited diseases

The Golgi factory receives custom glycosylates and dispatches its cargo to the correct cellular locations. The process requires importing donor substrates, moving the cargo, and recycling machinery. Correctly glycosylated cargo reflects the Golgi's quality and efficiency. Genetic disorders in the specific equipment (enzymes), donors (nucleotide sugar transporters), or equipment recycling/reorganization components (COG, SEC, golgins) can all affect glycosylation. Dozens of human glycosylation disorders fit these categories. Many other genes, with or without familiar names, well-annotated pedigrees, or likely homologies will join the ranks of glycosylation disorders. Their broad and unpredictable case-by-case phenotypes cross the traditional medical specialty boundaries. The gene functions in patients may be elusive, but their common feature may include altered glycosylation that provide clues to Golgi function. This article focuses on a group of human disorders that affect protein or lipid glycosylation. Readers may find it useful to generalize some of these patient-based, translational observations to their own research.

Roles of the Aspergillus nidulans UDP-galactofuranose transporter, UgtA in hyphal morphogenesis, cell wall architecture, conidiation, and drug sensitivity

Galactofuranose (Galf) is the 5-member-ring form of galactose found in the walls of fungi including Aspergillus, but not in mammals. UDP-galactofuranose mutase (UgmA, ANID_3112.1) generates UDP-Galf from UDP-galactopyranose (6-member ring form). UgmA-GFP is cytoplasmic, so the UDP-Galf residues it produces must be transported into an endomembrane compartment prior to incorporation into cell wall components. ANID_3113.1 (which we call UgtA) was identified as being likely to encode the A. nidulans UDP-Galf transporter, based on its high amino acid sequence identity with A. fumigatus GlfB. The ugtAΔ phenotype resembled that of ugmAΔ, which had compact colonies, wide, highly branched hyphae, and reduced sporulation. Like ugmAΔ, the ugtAΔ hyphal walls were threefold thicker than wild type strains (but different in appearance in TEM), and accumulated exogenous material in liquid culture. AfglfB restored wild type growth in the ugtAΔ strain, showing that these genes have homologous function. Immunostaining with EBA2 showed that ugtAΔ hyphae and conidiophores lacked Galf, which was restored in the AfglfB-complemented strain. Unlike wild type and ugmAΔ strains, some ugtAΔ metulae produced triplets of phialides, rather than pairs. Compared to wild type strains, spore production for ugtAΔ was reduced to 1%, and spore germination was reduced to half. UgtA-GFP had a punctate distribution in hyphae, phialides, and young spores. Notably, the ugtAΔ strain was significantly more sensitive than wild type to Caspofungin, which inhibits beta-glucan synthesis, suggesting that drugs that could be developed to target UgtA function would be useful in combination antifungal therapy.

Role of glycans and glycosyltransferases in the regulation of Notch signaling

The evolutionarily conserved Notch signaling pathway plays broad and important roles during embryonic development and in adult tissue homeostasis. Unlike most other pathways used during animal development, Notch signaling does not rely on second messengers and intracellular signaling cascades. Instead, pathway activation results in the cleavage of the Notch intracellular domain and its translocation into the nucleus, where it functions as a transcriptional co-activator of the Notch target genes. To ensure tight spatial and temporal regulation of a pathway with such an unusually direct signaling transduction, animal cells have devised a variety of specialized modulatory mechanisms. One such mechanism takes advantage of decorating the Notch extracellular domain with rare types of O-linked glycans. In this review, we will discuss the genetic and biochemical data supporting the notion that carbohydrate modification is essential for Notch signaling and attempt to provide a brief historical overview of how we have learned what we know about the glycobiology of Notch. We will also summarize what is known about the contribution of specific nucleotide-sugar transporters to Notch biology and the roles-enzymatic and non-enzymatic-played by specific glycosyltransferases in the regulation of this pathway. Mutations in the Notch pathway components cause a variety of human diseases, and manipulation of Notch signaling is emerging as a powerful tool in regenerative medicine. Therefore, studying how sugar modification modulates Notch signaling provides a framework for better understanding the role of glycosylation in animal development and might offer new tools to manipulate Notch signaling for therapeutic purposes.