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

Cytosolic UDP-Gal biosynthetic machinery is required for dimerization of SLC35A2 in the Golgi membrane and its interaction with B4GalT1

Glycosylation is a vital post-translational modification involving the addition of sugars to proteins and lipids, facilitated by glycosyltransferases and dependent on nucleotide sugar donors like UDP-galactose (UDP-Gal). This study examines how disruptions in UDP-Gal synthesis affect protein-protein interactions critical for glycosylation. Using CRISPR/Cas9, we generated HEK293T cell lines lacking key enzymes of the Leloir pathway: UDP-galactose 4'-epimerase (GALE), galactose-1-phosphate uridylyltransferase (GALT), or both. The knockout of GALE led to a significant reduction in intracellular UDP-Gal levels and altered N-glycan profiles, indicating impaired galactosylation. Through the NanoBiT assay, we observed that knocking out GALE alone or both GALE and GALT diminished the ability of the UDP-Gal transporter SLC35A2 to form homomers and to interact with the beta-1,4-galactosyltransferase 1 (B4GALT1). These findings suggest that the nucleotide sugar availability and/or the presence of the corresponding enzymes in the cytoplasm influences the formation of protein complexes involved in glycosylation in the Golgi apparatus, potentially affecting the glycosylation process itself. Our study highlights the dynamic nature of the glycosylation machinery and suggests that the interactions between glycosylation proteins are responsive to changes in nucleotide sugar levels. This opens new avenues for understanding the mechanisms underlying glycosylation and for investigating congenital glycosylation disorders.

Interplay between de novo and salvage pathways of GDP-fucose synthesis

GDP-fucose is synthesised via two pathways: de novo and salvage. The first uses GDP-mannose as a substrate, and the second uses free fucose. To date, these pathways have been considered to work separately and not to have an influence on each other. We report the mutual response of the de novo and salvage pathways to the lack of enzymes from a particular route of GDP-fucose synthesis. We detected different efficiencies of GDP-fucose and fucosylated structure synthesis after a single inactivation of enzymes of the de novo pathway. Our study demonstrated the unequal influence of the salvage enzymes on the production of GDP-fucose by enzymes of the de novo biosynthesis pathway. Simultaneously, we detected an elevated level of one of the enzymes of the de novo pathway in the cell line lacking the enzyme of the salvage biosynthesis pathway. Additionally, we identified dissimilarities in fucose uptake between cells lacking TSTA3 and GMDS proteins.

Mutations in the SLC35C1 gene, contributing to significant differences in fucosylation patterns, may underlie the diverse phenotypic manifestations observed in leukocyte adhesion deficiency type II patients

Congenital disorders of glycosylation (CDG) are a large family of genetic diseases resulting from defects in the synthesis of glycans and the attachment of glycans to macromolecules. The CDG known as leukocyte adhesion deficiency II (LAD II) is an autosomal, recessive disorder caused by mutations in the SLC35C1 gene, encoding a transmembrane protein of the Golgi apparatus, involved in GDP-fucose transport from the cytosol to the Golgi lumen. In this study, a cell-based model was used as a tool to characterize the molecular background of a therapy based on a fucose-supplemented diet. Such therapies have been successfully introduced in some (but not all) known cases of LAD II. In this study, the effect of external fucose was analyzed in SLC35C1 KO cell lines, expressing 11 mutated SLC35C1 proteins, previously discovered in patients with an LAD II diagnosis. For many of them, the cis-Golgi subcellular localization was affected; however, some proteins were localized properly. Additionally, although mutated SLC35C1 caused different α-1-6 core fucosylation of N-glycans, which explains previously described, more or less severe disorder symptoms, the differences practically disappeared after external fucose supplementation, with fucosylation restored to the level observed in healthy cells. This indicates that additional fucose in the diet should improve the condition of all patients. Thus, for patients diagnosed with LAD II we advocate careful analysis of particular mutations using the SLC35C1-KO cell line-based model, to predict changes in localization and fucosylation rate. We also recommend searching for additional mutations in the human genome of LAD II patients, when fucose supplementation does not influence patients' state.

The draft genome of Nitzschia closterium f. minutissima and transcriptome analysis reveals novel insights into diatom biosilicification

BACKGROUND

Nitzschia closterium f. minutissima is a commonly available diatom that plays important roles in marine aquaculture. It was originally classified as Nitzschia (Bacillariaceae, Bacillariophyta) but is currently regarded as a heterotypic synonym of Phaeodactylum tricornutum. The aim of this study was to obtain the draft genome of the marine microalga N. closterium f. minutissima to understand its phylogenetic placement and evolutionary specialization. Given that the ornate hierarchical silicified cell walls (frustules) of diatoms have immense applications in nanotechnology for biomedical fields, biosensors and optoelectric devices, transcriptomic data were generated by using reference genome-based read mapping to identify significantly differentially expressed genes and elucidate the molecular processes involved in diatom biosilicification.

RESULTS

In this study, we generated 13.81 Gb of pass reads from the PromethION sequencer. The draft genome of N. closterium f. minutissima has a total length of 29.28 Mb, and contains 28 contigs with an N50 value of 1.23 Mb. The GC content was 48.55%, and approximately 18.36% of the genome assembly contained repeat sequences. Gene annotation revealed 9,132 protein-coding genes. The results of comparative genomic analysis showed that N. closterium f. minutissima was clustered as a sister lineage of Phaeodactylum tricornutum and the divergence time between them was estimated to be approximately 17.2 million years ago (Mya). CAFF analysis demonstrated that 220 gene families that significantly changed were unique to N. closterium f. minutissima and that 154 were specific to P. tricornutum, moreover, only 26 gene families overlapped between these two species. A total of 818 DEGs in response to silicon were identified in N. closterium f. minutissima through RNA sequencing, these genes are involved in various molecular processes such as transcription regulator activity. Several genes encoding proteins, including silicon transporters, heat shock factors, methyltransferases, ankyrin repeat domains, cGMP-mediated signaling pathways-related proteins, cytoskeleton-associated proteins, polyamines, glycoproteins and saturated fatty acids may contribute to the formation of frustules in N. closterium f. minutissima.

CONCLUSIONS

Here, we described a draft genome of N. closterium f. minutissima and compared it with those of eight other diatoms, which provided new insight into its evolutionary features. Transcriptome analysis to identify DEGs in response to silicon will help to elucidate the underlying molecular mechanism of diatom biosilicification in N. closterium f. minutissima.

The glycosylation defect in solute carrier SLC35A2/SLC35A3 double knockout cells is rescued by SLC35A2-SLC35A3 and SLC35A3-SLC35A2 hybrids

SLC35A2 and SLC35A3 are homologous proteins with postulated nucleotide sugar transporting activities. Unlike SLC35A2, whose specificity for UDP-Gal is well-established, the UDP-GlcNAc transporting activity initially attributed to SLC35A3 has recently been put into question. In this study, we constructed two hybrid proteins (SLC35A2-SLC35A3 and SLC35A3-SLC35A2) and expressed them in a previously generated SLC35A2/SLC35A3 double knockout HEK293T cell line. Our idea was to force equivalent stoichiometry of the two proteins in the cells in order to reproduce the behavior of the SLC35A2/SLC35A3 complexes in the Golgi membrane. The hybrid proteins were able to fully restore glycosylation in the double knockout. In contrast, the expression of SLC35A3 alone in these cells improved galactosylation only to a limited extent. Our study shows that the proper glycosylation requires a balanced cooperation between SLC35A2 and SLC35A3.

Validation of a metabolite-GWAS network for Populus trichocarpa family 1 UDP-glycosyltransferases

Metabolite genome-wide association studies (mGWASs) are increasingly used to discover the genetic basis of target phenotypes in plants such as Populus trichocarpa, a biofuel feedstock and model woody plant species. Despite their growing importance in plant genetics and metabolomics, few mGWASs are experimentally validated. Here, we present a functional genomics workflow for validating mGWAS-predicted enzyme-substrate relationships. We focus on uridine diphosphate-glycosyltransferases (UGTs), a large family of enzymes that catalyze sugar transfer to a variety of plant secondary metabolites involved in defense, signaling, and lignification. Glycosylation influences physiological roles, localization within cells and tissues, and metabolic fates of these metabolites. UGTs have substantially expanded in P. trichocarpa, presenting a challenge for large-scale characterization. Using a high-throughput assay, we produced substrate acceptance profiles for 40 previously uncharacterized candidate enzymes. Assays confirmed 10 of 13 leaf mGWAS associations, and a focused metabolite screen demonstrated varying levels of substrate specificity among UGTs. A substrate binding model case study of UGT-23 rationalized observed enzyme activities and mGWAS associations, including glycosylation of trichocarpinene to produce trichocarpin, a major higher-order salicylate in P. trichocarpa. We identified UGTs putatively involved in lignan, flavonoid, salicylate, and phytohormone metabolism, with potential implications for cell wall biosynthesis, nitrogen uptake, and biotic and abiotic stress response that determine sustainable biomass crop production. Our results provide new support for in silico analyses and evidence-based guidance for in vivo functional characterization.

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.

SLC35A2 deficiency reduces protein levels of core 1 β-1,3-galactosyltransferase 1 (C1GalT1) and its chaperone Cosmc and affects their subcellular localization

Nucleotide sugar transporters (NSTs) are multitransmembrane proteins, localized in the Golgi apparatus and/or endoplasmic reticulum, which provide glycosylation enzymes with their substrates. It has been demonstrated that NSTs may form complexes with functionally related glycosyltransferases, especially in the N-glycosylation pathway. However, potential interactions of NSTs with enzymes mediating the biosynthesis of mucin-type O-glycans have not been addressed to date. Here we report that UDP-galactose transporter (UGT; SLC35A2) associates with core 1 β-1,3-galactosyltransferase 1 (C1GalT1; T-synthase). This provides the first example of an interaction between an enzyme that acts exclusively in the O-glycosylation pathway and an NST. We also found that SLC35A2 associated with the C1GalT1-specific chaperone Cosmc, and that the endogenous Cosmc was localized in both the endoplasmic reticulum and Golgi apparatus of wild-type HEK293T cells. Furthermore, in SLC35A2-deficient cells protein levels of C1GalT1 and Cosmc were decreased and their Golgi localization was less pronounced. Finally, we identified SLC35A2 as a novel molecular target for the antifungal agent itraconazole. Based on our findings we propose that NSTs may contribute to the stabilization of their interaction partners and help them to achieve target localization in the cell, most likely by facilitating their assembly into larger functional units.

Delivery of Nucleotide Sugars to the Mammalian Golgi: A Very Well (un)Explained Story

Nucleotide sugars (NSs) serve as substrates for glycosylation reactions. The majority of these compounds are synthesized in the cytoplasm, whereas glycosylation occurs in the endoplasmic reticulum (ER) and Golgi lumens, where catalytic domains of glycosyltransferases (GTs) are located. Therefore, translocation of NS across the organelle membranes is a prerequisite. This process is thought to be mediated by a group of multi-transmembrane proteins from the SLC35 family, i.e., nucleotide sugar transporters (NSTs). Despite many years of research, some uncertainties/inconsistencies related with the mechanisms of NS transport and the substrate specificities of NSTs remain. Here we present a comprehensive review of the NS import into the mammalian Golgi, which consists of three major parts. In the first part, we provide a historical view of the experimental approaches used to study NS transport and evaluate the most important achievements. The second part summarizes various aspects of knowledge concerning NSTs, ranging from subcellular localization up to the pathologies related with their defective function. In the third part, we present the outcomes of our research performed using mammalian cell-based models and discuss its relevance in relation to the general context.

SLC35A2 Deficiency Promotes an Epithelial-to-Mesenchymal Transition-like Phenotype in Madin–Darby Canine Kidney Cells

In mammalian cells, SLC35A2 delivers UDP–galactose for galactosylation reactions that take place predominantly in the Golgi lumen. Mutations in the corresponding gene cause a subtype of a congenital disorder of glycosylation (SLC35A2-CDG). Although more and more patients are diagnosed with SLC35A2-CDG, the link between defective galactosylation and disease symptoms is not fully understood. According to a number of reports, impaired glycosylation may trigger the process of epithelial-to-mesenchymal transition (EMT). We therefore examined whether the loss of SLC35A2 activity would promote EMT in a non-malignant epithelial cell line. For this purpose, we knocked out the SLC35A2 gene in Madin–Darby canine kidney (MDCK) cells. The resulting clones adopted an elongated, spindle-shaped morphology and showed impaired cell–cell adhesion. Using qPCR and western blotting, we revealed down-regulation of E-cadherin in the knockouts, while the fibronectin and vimentin levels were elevated. Moreover, the knockout cells displayed reorganization of vimentin intermediate filaments and altered subcellular distribution of a vimentin-binding protein, formiminotransferase cyclodeaminase (FTCD). Furthermore, depletion of SLC35A2 triggered Golgi compaction. Finally, the SLC35A2 knockouts displayed increased motility and invasiveness. In conclusion, SLC35A2-deficient MDCK cells showed several hallmarks of EMT. Our findings point to a novel role for SLC35A2 as a gatekeeper of the epithelial phenotype.

O-GlcNAcylation regulates β1,4-GlcNAc-branched N-glycan biosynthesis via the OGT/SLC35A3/GnT-IV axis

N-Linked glycosylation and O-linked N-acetylglucosamine (O-GlcNAc) are important protein post-translational modifications that are orchestrated by a diverse set of gene products. Thus far, the relationship between these two types of glycosylation has remained elusive, and it is unclear whether one influences the other via UDP-GlcNAc, which is a common donor substrate. Theoretically, a decrease in O-GlcNAcylation may increase the products of GlcNAc-branched N-glycans. In this study, via examination by lectin blotting, HPLC, and mass spectrometry analysis, however, we found that the amounts of GlcNAc-branched tri-antennary N-glycans catalyzed by N-acetylglucosaminyltransferase IV (GnT-IV) and tetra-antennary N-glycans were significantly decreased in O-GlcNAc transferase knockdown cells (OGT-KD) compared with those in wild type cells. We examined this specific alteration by focusing on SLC35A3, which is the main UDP-GlcNAc transporter in mammals that is believed to modulate GnT-IV activation. It is interesting that a deficiency of SLC35A3 specifically leads to a decrease in the amounts of GlcNAc-branched tri- and tetra-antennary N-glycans. Furthermore, co-immunoprecipitation experiments have shown that SLC35A3 interacts with GnT-IV, but not with N-acetylglucosaminyltransferase V. Western blot and chemoenzymatic labeling assay have confirmed that OGT modifies SLC35A3 and that O-GlcNAcylation contributes to its stability. Furthermore, we found that SLC35A3-KO enhances cell spreading and suppresses both cell migration and cell proliferation, which is similar to the phenomena observed in the OGT-KD cells. Taken together, these data are the first to demonstrate that O-GlcNAcylation specifically governs the biosynthesis of tri- and tetra-antennary N-glycans via the OGT-SLC35A3-GnT-IV axis.

Identification of novel potential interaction partners of UDP-galactose (SLC35A2), UDP-N-acetylglucosamine (SLC35A3) and an orphan (SLC35A4) nucleotide sugar transporters

Nucleotide sugar transporters (NSTs) are ER and Golgi-resident members of the solute carrier 35 (SLC35) family which supply substrates for glycosylation by exchanging lumenal nucleotide monophosphates for cytosolic nucleotide sugars. Defective NSTs have been associated with congenital disorders of glycosylation (CDG), however, molecular basis of many types of CDG remains poorly characterized. To better understand the biology of NSTs, we identified potential interaction partners of UDP-galactose transporter (SLC35A2), UDP-N-acetylglucosamine transporter (SLC35A3) and an orphan nucleotide sugar transporter SLC35A4 of to date unassigned specificity. For this purpose, each of the SLC35A2-A4 proteins was used as a bait in four independent pull-down experiments and the identity of the immunoprecipitated material was discovered using MS techniques. From the candidate list obtained, we selected a few for which the interaction was confirmed in vitro using the NanoBiT system, a split luciferase-based luminescent technique. NSTs have been shown to interact with two ATPases (ATP2A2, ATP2C1), Golgi pH regulator B (GPR89B) and calcium channel (TMCO1), which may reflect the regulation of glycosylation by ion homeostasis, and with basigin (BSG). Our findings provide a starting point for the NST interaction network discovery in order to better understand how glycosylation is regulated and linked to other cellular processes. SIGNIFICANCE: Despite the facts that nucleotide sugar transporters are a key component of the protein glycosylation machinery, and deficiencies in their activity underlie serious metabolic diseases, biology, function and regulation of these essential proteins remain enigmatic. In this study we have advanced the field by identifying sets of new potential interaction partners for UDP-galactose transporter (SLC35A2), UDP-N-acetylglucosamine transporter (SLC35A3) and an orphan transporter SLC35A4 of yet undefined role. Several of these new interactions were additionally confirmed in vitro using the NanoBiT system, a split luciferase complementation assay. This work is also significant in that it addresses the overall challenge of discovering membrane protein interaction partners by a detailed comparison of 4 different co-immunoprecipitation strategies and by custom sample preparation and data processing workflows.

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.

Frequent SLC35A2 brain mosaicism in mild malformation of cortical development with oligodendroglial hyperplasia in epilepsy (MOGHE)

Focal malformations of cortical development (MCD) are linked to somatic brain mutations occurring during neurodevelopment. Mild malformation of cortical development with oligodendroglial hyperplasia in epilepsy (MOGHE) is a newly recognized clinico-pathological entity associated with pediatric drug-resistant focal epilepsy, and amenable to neurosurgical treatment. MOGHE is histopathologically characterized by clusters of increased oligodendroglial cell densities, patchy zones of hypomyelination, and heterotopic neurons in the white matter. The molecular etiology of MOGHE remained unknown so far. We hypothesized a contribution of mosaic brain variants and performed deep targeted gene sequencing on 20 surgical MOGHE brain samples from a single-center cohort of pediatric patients. We identified somatic pathogenic SLC35A2 variants in 9/20 (45%) patients with mosaic rates ranging from 7 to 52%. SLC35A2 encodes a UDP-galactose transporter, previously implicated in other malformations of cortical development (MCD) and a rare type of congenital disorder of glycosylation. To further clarify the histological features of SLC35A2-brain tissues, we then collected 17 samples with pathogenic SLC35A2 variants from a multicenter cohort of MCD cases. Histopathological reassessment including anti-Olig2 staining confirmed a MOGHE diagnosis in all cases. Analysis by droplet digital PCR of pools of microdissected cells from one MOGHE tissue revealed a variant enrichment in clustered oligodendroglial cells and heterotopic neurons. Through an international consortium, we assembled an unprecedented series of 26 SLC35A2-MOGHE cases providing evidence that mosaic SLC35A2 variants, likely occurred in a neuroglial progenitor cell during brain development, are a genetic marker for MOGHE.

Novel Insights into Selected Disease-Causing Mutations within the SLC35A1 Gene Encoding the CMP-Sialic Acid Transporter

Congenital disorders of glycosylation (CDG) are a group of rare genetic and metabolic diseases caused by alterations in glycosylation pathways. Five patients bearing CDG-causing mutations in the SLC35A1 gene encoding the CMP-sialic acid transporter (CST) have been reported to date. In this study we examined how specific mutations in the SLC35A1 gene affect the protein’s properties in two previously described SLC35A1-CDG cases: one caused by a substitution (Q101H) and another involving a compound heterozygous mutation (T156R/E196K). The effects of single mutations and the combination of T156R and E196K mutations on the CST’s functionality was examined separately in CST-deficient HEK293T cells. As shown by microscopic studies, none of the CDG-causing mutations affected the protein’s proper localization in the Golgi apparatus. Cellular glycophenotypes were characterized using lectins, structural assignment of N- and O-glycans and analysis of glycolipids. Single Q101H, T156R and E196K mutants were able to partially restore sialylation in CST-deficient cells, and the deleterious effect of a single T156R or E196K mutation on the CST functionality was strongly enhanced upon their combination. We also revealed differences in the ability of CST variants to form dimers. The results of this study improve our understanding of the molecular background of SLC35A1-CDG cases.

Serum bikunin isoforms in congenital disorders of glycosylation and linkeropathies

Bikunin (Bkn) isoforms are serum chondroitin sulfate (CS) proteoglycans synthesized by the liver. They include two light forms, that is, the Bkn core protein and the Bkn linked to the CS chain (urinary trypsin inhibitor [UTI]), and two heavy forms, that is, pro-α-trypsin inhibitor and inter-α-trypsin inhibitor, corresponding to UTI esterified by one or two heavy chains glycoproteins, respectively. We previously showed that the Western-blot analysis of the light forms could allow the fast and easy detection of patients with linkeropathy, deficient in enzymes involved in the synthesis of the initial common tetrasaccharide linker of glycosaminoglycans. Here, we analyzed all serum Bkn isoforms in a context of congenital disorders of glycosylation (CDG) and showed very specific abnormal patterns suggesting potential interests for their screening and diagnosis. In particular, genetic deficiencies in V-ATPase (ATP6V0A2-CDG, CCDC115-CDG, ATP6AP1-CDG), in Golgi manganese homeostasis (TMEM165-CDG) and in the N-acetyl-glucosamine Golgi transport (SLC35A3-CDG) all share specific abnormal Bkn patterns. Furthermore, for each studied linkeropathy, we show that the light abnormal Bkn could be further in-depth characterized by two-dimensional electrophoresis. Moreover, besides being interesting as a specific biomarker of both CDG and linkeropathies, Bkn isoforms' analyses can provide new insights into the pathophysiology of the aforementioned diseases.

Biosynthesis of GlcNAc-rich N- and O-glycans in the Golgi apparatus does not require the nucleotide sugar transporter SLC35A3

Nucleotide sugar transporters, encoded by the SLC35 gene family, deliver nucleotide sugars throughout the cell for various glycosyltransferase-catalyzed glycosylation reactions. GlcNAc, in the form of UDP-GlcNAc, and galactose, as UDP-Gal, are delivered into the Golgi apparatus by SLC35A3 and SLC35A2 transporters, respectively. However, although the UDP-Gal transporting activity of SLC35A2 has been clearly demonstrated, UDP-GlcNAc delivery by SLC35A3 is not fully understood. Therefore, we analyzed a panel of CHO, HEK293T, and HepG2 cell lines including WT cells, SLC35A2 knockouts, SLC35A3 knockouts, and double-knockout cells. Cells lacking SLC35A2 displayed significant changes in N- and O-glycan synthesis. However, in SLC35A3-knockout CHO cells, only limited changes were observed; GlcNAc was still incorporated into N-glycans, but complex type N-glycan branching was impaired, although UDP-GlcNAc transport into Golgi vesicles was not decreased. In SLC35A3-knockout HEK293T cells, UDP-GlcNAc transport was significantly decreased but not completely abolished. However, N-glycan branching was not impaired in these cells. In CHO and HEK293T cells, the effect of SLC35A3 deficiency on N-glycan branching was potentiated in the absence of SLC35A2. Moreover, in SLC35A3-knockout HEK293T and HepG2 cells, GlcNAc was still incorporated into O-glycans. However, in the case of HepG2 cells, no qualitative changes in N-glycans between WT and SLC35A3 knockout cells nor between SLC35A2 knockout and double-knockout cells were observed. These findings suggest that SLC35A3 may not be the primary UDP-GlcNAc transporter and/or different mechanisms of UDP-GlcNAc transport into the Golgi apparatus may exist.

Analysis of homologous and heterologous interactions between UDP-galactose transporter and beta-1,4-galactosyltransferase 1 using NanoBiT

Split luciferase complementation assay is one of the approaches enabling identification and analysis of protein-protein interactions in vivo. The NanoBiT technology is the most recent improvement of this strategy. Nucleotide sugar transporters and glycosyltransferases of the Golgi apparatus are the key players in glycosylation. Here we demonstrate the applicability of the NanoBiT system for studying homooligomerization of these proteins. We also report and discuss a novel heterologous interaction between UDP-galactose transporter and beta-1,4-galactosyltransferase 1.

Nucleotide Sugar Transporter SLC35 Family Structure and Function

The covalent attachment of sugars to growing glycan chains is heavily reliant on a specific family of solute transporters (SLC35), the nucleotide sugar transporters (NSTs) that connect the synthesis of activated sugars in the nucleus or cytosol, to glycosyltransferases that reside in the lumen of the endoplasmic reticulum (ER) and/or Golgi apparatus. This review provides a timely update on recent progress in the NST field, specifically we explore several NSTs of the SLC35 family whose substrate specificity and function have been poorly understood, but where recent significant progress has been made. This includes SLC35 A4, A5 and D3, as well as progress made towards understanding the association of SLC35A2 with SLC35A3 and how this relates to their potential regulation, and how the disruption to the dilysine motif in SLC35B4 causes mislocalisation, calling into question multisubstrate NSTs and their subcellular localisation and function. We also report on the recently described first crystal structure of an NST, the SLC35D2 homolog Vrg-4 from yeast. Using this crystal structure, we have generated a new model of SLC35A1, (CMP-sialic acid transporter, CST), with structural and mechanistic predictions based on all known CST-related data, and includes an overview of reported mutations that alter transport and/or substrate recognition (both de novo and site-directed). We also present a model of the CST-del177 isoform that potentially explains why the human CST isoform remains active while the hamster CST isoform is inactive, and we provide a possible alternate access mechanism that accounts for the CST being functional as either a monomer or a homodimer. Finally we provide an update on two NST crystal structures that were published subsequent to the submission and during review of this report.

N-acetylglucosaminyltransferases and nucleotide sugar transporters form multi-enzyme-multi-transporter assemblies in golgi membranes in vivo

Branching and processing of N-glycans in the medial-Golgi rely both on the transport of the donor UDP-N-acetylglucosamine (UDP-GlcNAc) to the Golgi lumen by the SLC35A3 nucleotide sugar transporter (NST) as well as on the addition of the GlcNAc residue to terminal mannoses in nascent N-glycans by several linkage-specific N-acetyl-glucosaminyltransferases (MGAT1-MGAT5). Previous data indicate that the MGATs and NSTs both form higher order assemblies in the Golgi membranes. Here, we investigate their specific and mutual interactions using high-throughput FRET- and BiFC-based interaction screens. We show that MGAT1, MGAT2, MGAT3, MGAT4B (but not MGAT5) and Golgi alpha-mannosidase IIX (MAN2A2) form several distinct molecular assemblies with each other and that the MAN2A2 acts as a central hub for the interactions. Similar assemblies were also detected between the NSTs SLC35A2, SLC35A3, and SLC35A4. Using in vivo BiFC-based FRET interaction screens, we also identified novel ternary complexes between the MGATs themselves or between the MGATs and the NSTs. These findings suggest that the MGATs and the NSTs self-assemble into multi-enzyme/multi-transporter complexes in the Golgi membranes in vivo to facilitate efficient synthesis of complex N-glycans.

The UDP-Glycosyltransferase (UGT) Superfamily: New Members, New Functions, and Novel Paradigms

UDP-glycosyltransferases (UGTs) catalyze the covalent addition of sugars to a broad range of lipophilic molecules. This biotransformation plays a critical role in elimination of a broad range of exogenous chemicals and by-products of endogenous metabolism, and also controls the levels and distribution of many endogenous signaling molecules. In mammals, the superfamily comprises four families: UGT1, UGT2, UGT3, and UGT8. UGT1 and UGT2 enzymes have important roles in pharmacology and toxicology including contributing to interindividual differences in drug disposition as well as to cancer risk. These UGTs are highly expressed in organs of detoxification (e.g., liver, kidney, intestine) and can be induced by pathways that sense demand for detoxification and for modulation of endobiotic signaling molecules. The functions of the UGT3 and UGT8 family enzymes have only been characterized relatively recently; these enzymes show different UDP-sugar preferences to that of UGT1 and UGT2 enzymes, and to date, their contributions to drug metabolism appear to be relatively minor. This review summarizes and provides critical analysis of the current state of research into all four families of UGT enzymes. Key areas discussed include the roles of UGTs in drug metabolism, cancer risk, and regulation of signaling, as well as the transcriptional and posttranscriptional control of UGT expression and function. The latter part of this review provides an in-depth analysis of the known and predicted functions of UGT3 and UGT8 enzymes, focused on their likely roles in modulation of levels of endogenous signaling pathways.

SLC35A5 Protein-A Golgi Complex Member with Putative Nucleotide Sugar Transport Activity

Solute carrier family 35 member A5 (SLC35A5) is a member of the SLC35A protein subfamily comprising nucleotide sugar transporters. However, the function of SLC35A5 is yet to be experimentally determined. In this study, we inactivated the SLC35A5 gene in the HepG2 cell line to study a potential role of this protein in glycosylation. Introduced modification affected neither N- nor O-glycans. There was also no influence of the gene knock-out on glycolipid synthesis. However, inactivation of the SLC35A5 gene caused a slight increase in the level of chondroitin sulfate proteoglycans. Moreover, inactivation of the SLC35A5 gene resulted in the decrease of the uridine diphosphate (UDP)-glucuronic acid, UDP-N-acetylglucosamine, and UDP-N-acetylgalactosamine Golgi uptake, with no influence on the UDP-galactose transport activity. Further studies demonstrated that SLC35A5 localized exclusively to the Golgi apparatus. Careful insight into the protein sequence revealed that the C-terminus of this protein is extremely acidic and contains distinctive motifs, namely DXEE, DXD, and DXXD. Our studies show that the C-terminus is directed toward the cytosol. We also demonstrated that SLC35A5 formed homomers, as well as heteromers with other members of the SLC35A protein subfamily. In conclusion, the SLC35A5 protein might be a Golgi-resident multiprotein complex member engaged in nucleotide sugar transport.

Lysine at position 329 within a C-terminal dilysine motif is crucial for the ER localization of human SLC35B4

SLC35B4 belongs to the solute carrier 35 (SLC35) family whose best-characterized members display a nucleotide sugar transporting activity. Using an experimental model of HepG2 cells and indirect immunofluorescent staining, we verified that SLC35B4 was localized to the endoplasmic reticulum (ER). We demonstrated that dilysine motif, especially lysine at position 329, is crucial for the ER localization of this protein in human cells and therefore one should use protein C-tagging with caution. To verify the importance of the protein in glycoconjugates synthesis, we generated SLC35B4-deficient HepG2 cell line using CRISPR-Cas9 approach. Our data showed that knock-out of the SLC35B4 gene does not affect major UDP-Xyl- and UDP-GlcNAc-dependent glycosylation pathways.

CDG Therapies: From Bench to Bedside

Congenital disorders of glycosylation (CDG) are a group of genetic disorders that affect protein and lipid glycosylation and glycosylphosphatidylinositol synthesis. More than 100 different disorders have been reported and the number is rapidly increasing. Since glycosylation is an essential post-translational process, patients present a large range of symptoms and variable phenotypes, from very mild to extremely severe. Only for few CDG, potentially curative therapies are being used, including dietary supplementation (e.g., galactose for PGM1-CDG, fucose for SLC35C1-CDG, Mn²⁺ for TMEM165-CDG or mannose for MPI-CDG) and organ transplantation (e.g., liver for MPI-CDG and heart for DOLK-CDG). However, for the majority of patients, only symptomatic and preventive treatments are in use. This constitutes a burden for patients, care-givers and ultimately the healthcare system. Innovative diagnostic approaches, in vitro and in vivo models and novel biomarkers have been developed that can lead to novel therapeutic avenues aiming to ameliorate the patients’ symptoms and lives. This review summarizes the advances in therapeutic approaches for CDG.

UDP-Glucuronic Acid Transport Is Required for Virulence of Cryptococcus neoformans

Glycans play diverse biological roles, ranging from structural and regulatory functions to mediating cellular interactions. For pathogens, they are also often required for virulence and survival in the host. In Cryptococcus neoformans, an opportunistic pathogen of humans, the acidic monosaccharide glucuronic acid (GlcA) is a critical component of multiple essential glycoconjugates. One of these glycoconjugates is the polysaccharide capsule, a major virulence factor that enables this yeast to modulate the host immune response and resist antimicrobial defenses. This allows cryptococci to colonize the lung and brain, leading to hundreds of thousands of deaths each year worldwide. Synthesis of most glycans, including capsule polysaccharides, occurs in the secretory pathway. However, the activated precursors for this process, nucleotide sugars, are made primarily in the cytosol. This topological problem is resolved by the action of nucleotide sugar transporters (NSTs). We discovered that Uut1 is the sole UDP-GlcA transporter in C. neoformans and is unique among NSTs for its narrow substrate range and high affinity for UDP-GlcA. Mutant cells with UUT1 deleted lack capsule polysaccharides and are highly sensitive to environmental stress. As a result, the deletion mutant is internalized and cleared by phagocytes more readily than wild-type cells are and is completely avirulent in mice. These findings expand our understanding of the requirements for capsule synthesis and cryptococcal virulence and elucidate a critical protein family.IMPORTANCECryptococcus neoformans causes lethal meningitis in almost two hundred thousand immunocompromised patients each year. Much of this fungal pathogen's ability to resist host defenses and cause disease is mediated by carbohydrate structures, including a complex polysaccharide capsule around the cell. Like most eukaryotic glycoconjugates, capsule polysaccharides are made within the secretory pathway, although their precursors are generated in the cytosol. Specific transporters are therefore required to convey these raw materials to the site of synthesis. One precursor of particular interest is UDP-glucuronic acid, which donates glucuronic acid to growing capsule polysaccharides. We discovered a highly specific, high-affinity transporter for this molecule. Deletion of the gene encoding this unusual protein abolishes capsule synthesis, alters stress resistance, and eliminates fungal virulence. In this work, we have identified a novel transporter, elucidated capsule synthesis and thereby aspects of fungal pathogenesis, and opened directions for potential antifungal therapy.

Xylose donor transport is critical for fungal virulence

Cryptococcus neoformans, an AIDS-defining opportunistic pathogen, is the leading cause of fungal meningitis worldwide and is responsible for hundreds of thousands of deaths annually. Cryptococcal glycans are required for fungal survival in the host and for pathogenesis. Most glycans are made in the secretory pathway, although the activated precursors for their synthesis, nucleotide sugars, are made primarily in the cytosol. Nucleotide sugar transporters are membrane proteins that solve this topological problem, by exchanging nucleotide sugars for the corresponding nucleoside phosphates. The major virulence factor of C. neoformans is an anti-phagocytic polysaccharide capsule that is displayed on the cell surface; capsule polysaccharides are also shed from the cell and impede the host immune response. Xylose, a neutral monosaccharide that is absent from model yeast, is a significant capsule component. Here we show that Uxt1 and Uxt2 are both transporters specific for the xylose donor, UDP-xylose, although they exhibit distinct subcellular localization, expression patterns, and kinetic parameters. Both proteins also transport the galactofuranose donor, UDP-galactofuranose. We further show that Uxt1 and Uxt2 are required for xylose incorporation into capsule and protein; they are also necessary for C. neoformans to cause disease in mice, although surprisingly not for fungal viability in the context of infection. These findings provide a starting point for deciphering the substrate specificity of an important class of transporters, elucidate a synthetic pathway that may be productively targeted for therapy, and contribute to our understanding of fundamental glycobiology.

An insight into the orphan nucleotide sugar transporter SLC35A4

SLC35A4 has been classified in the SLC35A subfamily based on amino acid sequence homology. Most of the proteins belonging to the SLC35 family act as transporters of nucleotide sugars. In this study, the subcellular localization of endogenous SLC35A4 was determined via immunofluorescence staining, and it was demonstrated that SLC35A4 localizes mainly to the Golgi apparatus. In silico topology prediction suggests that SLC35A4 has an uneven number of transmembrane domains and its N-terminus is directed towards the Golgi lumen. However, an experimental assay refuted this prediction: SLC35A4 has an even number of transmembrane regions with both termini facing the cytosol. In vivo interaction analysis using the FLIM-FRET approach revealed that SLC35A4 neither forms homomers nor associates with other members of the SLC35A subfamily except SLC35A5. Additional assays demonstrated that endogenous SLC35A4 is 10 to 40nm proximal to SLC35A2 and SLC35A3. To determine SLC35A4 function SLC35A4 knock-out cells were generated with the CRISPR-Cas9 approach. Although no significant changes in glycosylation were observed, the introduced mutation influenced the subcellular distribution of the SLC35A2/SLC35A3 complexes. Additional FLIM-FRET experiments revealed that overexpression of SLC35A4-BFP together with SLC35A3 and the SLC35A2-Golgi splice variant negatively affects the interaction between the two latter proteins. The results presented here strongly indicate a modulatory role for SLC35A4 in intracellular trafficking of SLC35A2/SLC35A3 complexes.

Cryptococcus neoformans UGT1 encodes a UDP-Galactose/UDP-GalNAc transporter

Cryptococcus neoformans, an opportunistic fungal pathogen, produces a glycan capsule to evade the immune system during infection. This definitive virulence factor is composed mainly of complex polysaccharides, which are made in the secretory pathway by reactions that utilize activated nucleotide sugar precursors. Although the pathways that synthesize these precursors are known, the identity and the regulation of the nucleotide sugar transporters (NSTs) responsible for importing them into luminal organelles remain elusive. The UDP-galactose transporter, Ugt1, was initially identified by homology to known UGTs and glycan composition analysis of ugt1Δ mutants. However, sequence is an unreliable predictor of NST substrate specificity, cells may express multiple NSTs with overlapping specificities, and NSTs may transport multiple substrates. Determining NST activity thus requires biochemical demonstration of function. We showed that Ugt1 transports both UDP-galactose and UDP-N-acetylgalactosamine in vitro. Deletion of UGT1 resulted in growth and mating defects along with altered capsule and cellular morphology. The mutant was also phagocytosed more readily by macrophages than wild-type cells and cleared more quickly in vivo and in vitro, suggesting a mechanism for the lack of virulence observed in mouse models of infection.

UDP-galactose (SLC35A2) and UDP-N-acetylglucosamine (SLC35A3) Transporters Form Glycosylation-related Complexes with Mannoside Acetylglucosaminyltransferases (Mgats)

UDP-galactose transporter (UGT; SLC35A2) and UDP-N-acetylglucosamine transporter (NGT; SLC35A3) form heterologous complexes in the Golgi membrane. NGT occurs in close proximity to mannosyl (α-1,6-)-glycoprotein β-1,6-N-acetylglucosaminyltransferase (Mgat5). In this study we analyzed whether NGT and both splice variants of UGT (UGT1 and UGT2) are able to interact with four different mannoside acetylglucosaminyltransferases (Mgat1, Mgat2, Mgat4B, and Mgat5). Using an in situ proximity ligation assay, we found that all examined glycosyltransferases are in the vicinity of these UDP-sugar transporters both at the endogenous level and upon overexpression. This observation was confirmed via the FLIM-FRET approach for both NGT and UGT1 complexes with Mgats. This study reports for the first time close proximity between endogenous nucleotide sugar transporters and glycosyltransferases. We also observed that among all analyzed Mgats, only Mgat4B occurs in close proximity to UGT2, whereas the other three Mgats are more distant from UGT2, and it was only possible to visualize their vicinity using proximity ligation assay. This strongly suggests that the distance between these protein pairs is longer than 10 nm but at the same time shorter than 40 nm. This study adds to the understanding of glycosylation, one of the most important post-translational modifications, which affects the majority of macromolecules. Our research shows that complex formation between nucleotide sugar transporters and glycosyltransferases might be a more common phenomenon than previously thought.

Short N-terminal region of UDP-galactose transporter (SLC35A2) is crucial for galactosylation of N-glycans

UDP-galactose transporter (UGT) and UDP-N-acetylglucosamine transporter (NGT) form heterologous complexes in the Golgi apparatus (GA) membrane. We aimed to identify UGT region responsible for galactosylation of N-glycans. Chimeric proteins composed of human UGT and either NGT or CMP-sialic acid transporter (CST) localized to the GA, and all but UGT/CST chimera corrected galactosylation defect in UGT-deficient cell lines, although at different efficiency. Importantly, short N-terminal region composed of 35 N-terminal amino-acid residues of UGT was crucial for galactosylation of N-glycans. The remaining molecule must be derived from NGT not CST, confirming that the role played by UGT and NGT is coupled.

Structure and function of nucleotide sugar transporters: Current progress

The proteomes of eukaryotes, bacteria and archaea are highly diverse due, in part, to the complex post-translational modification of protein glycosylation. The diversity of glycosylation in eukaryotes is reliant on nucleotide sugar transporters to translocate specific nucleotide sugars that are synthesised in the cytosol and nucleus, into the endoplasmic reticulum and Golgi apparatus where glycosylation reactions occur. Thirty years of research utilising multidisciplinary approaches has contributed to our current understanding of NST function and structure. In this review, the structure and function, with reference to various disease states, of several NSTs including the UDP-galactose, UDP-N-acetylglucosamine, UDP-N-acetylgalactosamine, GDP-fucose, UDP-N-acetylglucosamine/UDP-glucose/GDP-mannose and CMP-sialic acid transporters will be described. Little is known regarding the exact structure of NSTs due to difficulties associated with crystallising membrane proteins. To date, no three-dimensional structure of any NST has been elucidated. What is known is based on computer predictions, mutagenesis experiments, epitope-tagging studies, in-vitro assays and phylogenetic analysis. In this regard the best-characterised NST to date is the CMP-sialic acid transporter (CST). Therefore in this review we will provide the current state-of-play with respect to the structure–function relationship of the (CST). In particular we have summarised work performed by a number groups detailing the affect of various mutations on CST transport activity, efficiency, and substrate specificity.

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.

Mosaicism of the UDP-galactose transporter SLC35A2 causes a congenital disorder of glycosylation

Biochemical analysis and whole-exome sequencing identified mutations in the Golgi-localized UDP-galactose transporter SLC35A2 that define an undiagnosed X-linked congenital disorder of glycosylation (CDG) in three unrelated families. Each mutation reduced UDP-galactose transport, leading to galactose-deficient glycoproteins. Two affected males were somatic mosaics, suggesting that a wild-type SLC35A2 allele may be required for survival. In infancy, the commonly used biomarker transferrin showed abnormal glycosylation, but its appearance became normal later in childhood, without any corresponding clinical improvement. This may indicate selection against cells carrying the mutant allele. To detect other individuals with such mutations, we suggest transferrin testing in infancy. Here, we report somatic mosaicism in CDG, and our work stresses the importance of combining both genetic and biochemical diagnoses.