Expanded Proteomic Survey of the Human Parasite Leishmania major Focusing on Changes in Null Mutants of the Golgi GDP-Mannose/Fucose/Arabinopyranose Transporter LPG2 and of the Mitochondrial Fucosyltransferase FUT1

The trypanosomatid protozoan parasite Leishmania has a significant impact on human health globally. Understanding the pathways associated with virulence within this significant pathogen is critical for identifying novel vaccination and chemotherapy targets. Within this study we leverage an ultradeep proteomic approach to improve our understanding of two virulence-associated genes in Leishmania, encoding the Golgi mannose/arabinopyranose/fucose nucleotide-sugar transporter (LPG2) and the mitochondrial fucosyltransferase (FUT1). Using deep peptide fractionation followed by complementary fragmentation approaches with higher-energy collisional dissociation (HCD) and electron transfer dissociation (ETD) allowed the identification of over 6,500 proteins, nearly doubling the experimentally known Leishmania major proteome. This deep proteomic analysis revealed significant quantitative differences in both Δlpg2- and Δfut1s mutants with FUT1-dependent changes linked to marked alterations within mitochondrion-associated proteins, while LPG2-dependent changes impacted many pathways, including the secretory pathway. While the FUT1 enzyme has been shown to fucosylate peptides in vitro, no evidence for protein fucosylation was identified within our ultradeep analysis, nor did we observe fucosylated glycans within Leishmania glycopeptides isolated using hydrophilic interaction liquid chromatography (HILIC) enrichment. This work provides a critical resource for the community on the observable Leishmania proteome as well as highlighting phenotypic changes associated with LPG2 or FUT1, ablation of which may guide the development of future therapeutics. IMPORTANCE Leishmania is a widespread trypanosomatid protozoan parasite of humans, with ~12 million cases currently, ranging from mild to fatal, and hundreds of millions asymptomatically infected. This work advances knowledge of the experimental proteome by nearly 2-fold, to more than 6,500 proteins and thus provides a great resource to investigators seeking to decode how this parasite is transmitted and causes disease and to identify new targets for therapeutic intervention. The ultradeep proteomics approach identified potential proteins underlying the "persistence-without-pathology" phenotype of mutants with deletion of the Golgi nucleotide transporter LPG2, showing many alterations and several candidates. Studies of a rare mutant with deletion of the mitochondrial fucosyltransferase FUT1 revealed changes underlying its strong mitochondrial dysfunction but did not reveal examples of fucosylation of either peptides or N-glycans. This suggests that this vital protein's elusive target(s) may be more complex than the methods used could detect or that this target may not be a protein but perhaps another glycoconjugate or glycolipid.

The Golgi-localized sphingosine-1-phosphate phosphatase is indispensable for Leishmania major

Sphingosine-1-phosphate phosphatase (SPP) catalyzes the dephosphorylation of sphingosine-1-phosphate (S1P) into sphingosine, the reverse reaction of sphingosine kinase. In mammals, S1P acts as a potent bioactive molecule regulating cell proliferation, migration, and immunity. In Leishmania, S1P production is crucial for the synthesis of ethanolamine and choline phospholipids, and cell survival under stress conditions. To better understand the roles of S1P, we characterized a SPP ortholog in Leishmania major which displays activity towards S1P but not structurally related lipids such as ceramide-1-phosphate or lysophosphatidic acid. While this enzyme is found in the endoplasmic reticulum in mammalian cells, L. major SPP is localized at the Golgi apparatus. Importantly, chromosomal SPP alleles cannot be deleted from L. major even with the addition of a complementing episome, suggesting that endogenously expressed SPP is essential. Finally, SPP overexpression in L. major leads to a slower growth rate and heightened sensitivity to brefeldin A and sodium orthovanadate. Together, these results suggest that the equilibrium between S1P and sphingosine is vital for the function of Golgi apparatus in Leishmania.

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.

Major Molecular Factors Related to Leishmania Pathogenicity

Leishmaniasis is a major health problem with 600k - 1M new cases worldwide and 1 billion at risk. It involves a wide range of clinical forms ranging from self-healing cutaneous lesions to systemic diseases that are fatal if not treated, depending on the species of Leishmania. Leishmania sp. are digenetic parasites that have two different morphological stages. Leishmania parasites possess a number of invasive/evasive and pathoantigenic determinants that seem to have critical roles in Leishmania infection of macrophages which leads to successful intracellular parasitism in the parasitophorous vacuoles. These determinants are traditionally known as “virulence factors”, and are considered to be good targets for developing specific inhibitors to attenuate virulence of Leishmania by gene deletions or modifications, thus causing infective, but non-pathogenic mutants for vaccination. Pathway of biosynthesis is critical for keeping the parasite viable and is important for drug designing against these parasites. These drugs are aimed to target enzymes that control these pathways. Accordingly, maintaining low level of parasitic infection and in some cases as a weapon to eradicate infection completely. The current paper focuses on several virulence factors as determinants of Leishmania pathogenicity, as well as the metabolites produced by Leishmania to secure its survival in the host.

Establishment of serum-free adapted Chinese hamster ovary cells with double knockout of GDP-mannose-4,6-dehydratase and GDP-fucose transporter

Although antibodies have attracted attention as next-generation biopharmaceuticals, the costs of purifying the products and of arranging the environment for cell cultivation are high. Therefore, there is a need to increase antibody efficacy and improve product quality as much as possible. Since antibodies are glycoproteins, their glycan structures have been found to affect the function of antibodies. Especially, afucosylation of the N-linked glycan in the Fc region is known to significantly increase antibody-dependent cellular cytotoxicity. In this study, we established a double-mutant ΔGMDΔGFT in which GDP-mannose 4,6-dehydratase and GDP-fucose transporter were knocked out in Chinese hamster ovary cells, a platform for biopharmaceutical protein production. By adapting ΔGMDΔGFT cells to serum-free medium and constructing suspension-cultured cells, we established host CHO cells with no detected fucosylated glycans and succeeded in production of afucosylated antibodies. We also demonstrated that, in culture in the presence of serum, fucosylation occurs due to contamination from serum components. Furthermore, we found that afucosylation of glycans does not affect cell growth after adaptation to serum-free medium as compared to wild-type CHO cells growth and does not significantly affect the expression levels of other endogenous fucose metabolism-related enzyme genes.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10616-021-00501-3.

LPG2 Gene Duplication in Leishmania infantum: A Case for CRISPR-Cas9 Gene Editing

On the surface of the Leishmania promastigote, phosphoglycans (PG) such as lipophosphoglycan (LPG), proteophosphoglycan (PPG), free phosphoglycan polymers (PGs), and acid phosphatases (sAP), are dominant and contribute to the invasion and survival of Leishmania within the host cell by modulating macrophage signaling and intracellular trafficking. Phosphoglycan synthesis depends on the Golgi GDP-mannose transporter encoded by the LPG2 gene. Aiming to investigate the role of PG-containing molecules in Leishmania infantum infection process, herein we describe the generation and characterization of L. infantum LPG2-deficient parasites. This gene was unexpectedly identified as duplicated in the L. infantum genome, which impaired gene targeting using the conventional homologous recombination approach. This limitation was circumvented by the use of CRISPR/Cas9 technology. Knockout parasites were selected by agglutination assays using CA7AE antibodies followed by a lectin (RCA 120). Five clones were isolated and molecularly characterized, all revealing the expected edited genome, as well as the complete absence of LPG and PG-containing molecule expression. Finally, the deletion of LPG2 was found to impair the outcome of infection in human neutrophils, as demonstrated by a pronounced reduction (~83%) in intracellular load compared to wild-type parasite infection. The results obtained herein reinforce the importance of LPG and other PGs as virulence factors in host-parasite interactions.

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.

Structural basis for mammalian nucleotide sugar transport

Nucleotide-sugar transporters (NSTs) are critical components of the cellular glycosylation machinery. They transport nucleotide-sugar conjugates into the Golgi lumen, where they are used for the glycosylation of proteins and lipids, and they then subsequently transport the nucleotide monophosphate byproduct back to the cytoplasm. Dysregulation of human NSTs causes several debilitating diseases, and NSTs are virulence factors for many pathogens. Here we present the first crystal structures of a mammalian NST, the mouse CMP-sialic acid transporter (mCST), in complex with its physiological substrates CMP and CMP-sialic acid. Detailed visualization of extensive protein-substrate interactions explains the mechanisms governing substrate selectivity. Further structural analysis of mCST’s unique lumen-facing partially-occluded conformation, coupled with the characterization of substrate-induced quenching of mCST’s intrinsic tryptophan fluorescence, reveals the concerted conformational transitions that occur during substrate transport. These results provide a framework for understanding the effects of disease-causing mutations and the mechanisms of this diverse family of transporters.

The cells in our body are tiny machines which, amongst other things, produce proteins. One of the production steps involves a compartment in the cell called the Golgi, where proteins are tagged and packaged before being sent to their final destination. In particular, sugars can be added onto an immature protein to help to fold it, stabilize it, and to affect how it works.

Before sugars can be attached to a protein, they need to be ‘activated’ outside of the Golgi by attaching to a small molecule known as a nucleotide. Then, these ‘nucleotide-sugars’ are ferried across the Golgi membrane and inside the compartment by nucleotide-sugar transporters, or NSTs. Humans have seven different kinds of NSTs, each responsible for helping specific types of nucleotide-sugars cross the Golgi membrane. Changes in NSTs are linked to several human diseases, including certain types of epilepsy; these proteins are also important for dangerous microbes to be able to infect cells. Yet, scientists know very little about how the transporters recognize their cargo, and how they transport it.

To shed light on these questions, Ahuja and Whorton set to uncover for the first time the 3D structure of a mammalian NST using a method known as X-ray crystallography. This revealed how nearly every component of this transporter is arranged when the protein is bound to two different molecules: a specific nucleotide, or a type of nucleotide-sugar. The results help to understand how changes in certain components of the NST can lead to a problem in the way the protein works. Ultimately, this knowledge may be useful to prevent diseases linked to faulty NSTs, or to stop microbes from using the transporters to their own advantage.

Proteomic Profiling of Leishmania donovani Promastigote Subcellular Organelles

To facilitate a greater understanding of the biological processes in the medically important Leishmania donovani parasite, a combination of differential and density-gradient ultracentrifugation techniques were used to achieve a comprehensive subcellular fractionation of the promastigote stage. An in-depth label-free proteomic LC-MS/MS analysis of the density gradients resulted in the identification of ∼50% of the Leishmania proteome (3883 proteins detected), which included ∼645 integral membrane proteins and 1737 uncharacterized proteins. Clustering and subcellular localization of proteins was based on a subset of training Leishmania proteins with known subcellular localizations that had been determined using biochemical, confocal microscopy, or immunoelectron microscopy approaches. This subcellular map will be a valuable resource that will help dissect the cell biology and metabolic processes associated with specific organelles of Leishmania and related kinetoplastids.

The role of membrane transporters in Leishmania virulence

Leishmania are parasitic protozoa which infect humans and cause severe morbidity and mortality. Leishmania parasitise as extracellular promastigotes in the insect vector and as intracellular amastigotes in the mammalian host. Cycling between hosts involves implementation of stringent and co-ordinated responses to shifting environmental conditions. One of the key dynamic aspects of Leishmania biology is substrate acquisition and metabolism. Genomic analyses have revealed that Leishmania encode many putative membrane transporters, many of which are differentially expressed during the parasite life cycle. Only a small fraction of these transporters, however, have been functionally characterised. Currently, most information is available about nutrient transporters, mainly involved in carbohydrate, amino acid, nucleobase and nucleoside, cofactor, and ion acquisition. Several have apparent roles in Leishmania virulence and will be discussed in this perspective.

GTPase Sar1 regulates the trafficking and secretion of the virulence factor gp63 in Leishmania

Metalloprotease gp63 (Leishmania donovani gp63 (Ldgp63)) is a critical virulence factor secreted by Leishmania However, how newly synthesized Ldgp63 exits the endoplasmic reticulum (ER) and is secreted by this parasite is unknown. Here, we cloned, expressed, and characterized the GTPase LdSar1 and other COPII components like LdSec23, LdSec24, LdSec13, and LdSec31 from Leishmania to understand their role in ER exit of Ldgp63. Using dominant-positive (LdSar1:H74L) and dominant-negative (LdSar1:T34N) mutants of LdSar1, we found that GTP-bound LdSar1 specifically binds to LdSec23, which binds, in turn, with LdSec24(1-702) to form a prebudding complex. Moreover, LdSec13 specifically interacted with His₆-LdSec31(1-603), and LdSec31 bound the prebudding complex via LdSec23. Interestingly, dileucine 594/595 and valine 597 residues present in the Ldgp63 C-terminal domain were critical for binding with LdSec24(703-966), and GFP-Ldgp63^L594A/L595A or GFP-Ldgp63^V597S mutants failed to exit from the ER. Moreover, Ldgp63-containing COPII vesicle budding from the ER was inhibited by LdSar1:T34N in an in vitro budding assay, indicating that GTP-bound LdSar1 is required for budding of Ldgp63-containing COPII vesicles. To directly demonstrate the function of LdSar1 in Ldgp63 trafficking, we coexpressed RFP-Ldgp63 along with LdSar1:WT-GFP or LdSar1:T34N-GFP and found that LdSar1:T34N overexpression blocks Ldgp63 trafficking and secretion in Leishmania Finally, we noted significantly compromised survival of LdSar1:T34N-GFP-overexpressing transgenic parasites in macrophages. Taken together, these results indicated that Ldgp63 interacts with the COPII complex via LdSec24 for Ldgp63 ER exit and subsequent secretion.

Genetic metabolic complementation establishes a requirement for GDP-fucose in Leishmania

To survive in its sand fly vector, the trypanosomatid protozoan parasite Leishmania first attaches to the midgut to avoid excretion, but eventually it must detach for transmission by the next bite. In Leishmania major strain Friedlin, this is controlled by modifications of the stage-specific adhesin lipophosphoglycan (LPG). During differentiation to infective metacyclics, d-arabinopyranose (d-Arap) caps the LPG side-chain galactose residues, blocking interaction with the midgut lectin PpGalec, thereby leading to parasite detachment and transmission. Previously, we characterized two closely related L. major genes (FKP40 and AFKP80) encoding bifunctional proteins with kinase/pyrophosphorylase activities required for salvage and conversion of l-fucose and/or d-Arap into the nucleotide-sugar substrates required by glycosyltransferases. Whereas only AFKP80 yielded GDP-d-Arap from exogenous d-Arap, both proteins were able to salvage l-fucose to GDP-fucose. We now show that Δafkp80⁻ null mutants ablated d-Arap modifications of LPG as predicted, whereas Δfkp40⁻ null mutants resembled wild type (WT). Fucoconjugates had not been reported previously in L. major, but unexpectedly, we were unable to generate fkp40⁻/afkp80⁻ double mutants, unless one of the A/FKPs was expressed ectopically. To test whether GDP-fucose itself was essential for Leishmania viability, we employed “genetic metabolite complementation.” First, the trypanosome de novo pathway enzymes GDP-mannose dehydratase (GMD) and GDP-fucose synthetase (GMER) were expressed ectopically; from these cells, the Δfkp40⁻/Δafkp80⁻ double mutant was now readily obtained. As expected, the Δfkp40⁻/Δafkp80⁻/+TbGMD-GMER line lacked the capacity to generate GDP-Arap, while synthesizing abundant GDP-fucose. These results establish a requirement for GDP-fucose for L. major viability and predict the existence of an essential fucoconjugate(s).

Virulence Factors as Targets for Anticryptococcal Therapy

The global mortality due to cryptococcosis caused by Cryptococcus neoformans or C. gattii is unacceptably high. Currently available therapies are decades old and may be impacted by drug resistance. Therefore, the need for more effective antifungal drugs for cryptococcosis is evident. A number of Cryptococcus virulence factors have been studied in detail, providing crucial information about the fungal biology and putative molecular targets for antifungals. This review focuses on the use of well-described virulence factors of Cryptococcus as potential anticryptococcal agents.

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.

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.

VvGONST-A and VvGONST-B are Golgi-localised GDP-sugar transporters in grapevine (Vitis vinifera L.)

Plant nucleotide-sugar transporters (NSTs) are responsible for the import of nucleotide-sugar substrates into the Golgi lumen, for subsequent use in glycosylation reactions. NSTs are specific for either GDP- or UDP-sugars, and almost all transporters studied to date have been isolated from Arabidopsis thaliana L. In order to determine the conservation of the import mechanism in other higher plant species, here we report the identification and characterisation of VvGONST-A and VvGONST-B from grapevine (Vitis vinifera L. cv. Thompson Seedless), which are the orthologues of the GDP-sugar transporters GONST3 and GONST4 in Arabidopsis. Both grapevine NSTs possess the molecular features characteristic of GDP-sugar transporters, including a GDP-binding domain (GXL/VNK) towards the C-terminal. VvGONST-A and VvGONST-B expression is highest at berry setting and decreases throughout berry development and ripening. Moreover, we show using green fluorescent protein (GFP) tagged versions and brefeldin A treatments, that both are localised in the Golgi apparatus. Additionally, in vitro transport assays after expression of both NSTs in tobacco leaves indicate that VvGONST-A and VvGONST-B are capable of transporting GDP-mannose and GDP-glucose, respectively, but not a range of other UDP- and GDP-sugars. The possible functions of these NSTs in glucomannan synthesis and/or glycosylation of sphingolipids are discussed.

Leishmania lipophosphoglycan: how to establish structure-activity relationships for this highly complex and multifunctional glycoconjugate?

A key feature of many pathogenic microorganisms is the presence of a dense glycocalyx at their surface, composed of lipid-anchored glycoproteins and non-protein-bound polysaccharides. These surface glycolipids are important virulence factors for bacterial, fungal and protozoan pathogens. The highly complex glycoconjugate lipophosphoglycan (LPG) is one of the dominant surface macromolecules of the promastigote stage of all Leishmania parasitic species. LPG plays critical pleiotropic roles in parasite survival and infectivity in both the sandfly vector and the mammalian host. Here, we review the composition of the Leishmania glycocalyx, the chemical structure of LPG and what is currently known about its effects in the mammalian host, specifically. We will then discuss the current approaches employed to elucidate LPG functions. Finally, we will provide a viewpoint on future directions that this area of investigation could take to unravel in detail the biological activity of the specific molecular elements composing the structurally complex LPG.

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.

Characterization of a ricin-resistant mutant of Leishmania donovani that expresses lipophosphoglycan

The abundant cell-surface lipophosphoglycan (LPG) of Leishmania parasites plays a central role throughout the eukaryote's life cycle. A number of LPG-defective mutants and their complementing genes have been isolated and have proven invaluable in assessing the importance of LPG and related glycoconjugates in parasite virulence. While ricin agglutination selection protocols frequently result in lpg- mutants, one  Leishmania donovani variant we isolated, named JABBA, was found to be lpg+. Procyclic (logarithmic) JABBA expresses significant amounts of a large-sized LPG, larger than observed from procyclic wild type but similar in size to LPG from wild type from metacyclic (stationary) phase. Structural analysis of the LPG from logarithmically grown JABBA by capillary electrophoresis protocols revealed that it averaged 30 repeat units composed of the unsubstituted Gal(β1,4)Man(α1)-PO4 typical of wild-type L. donovani. Analysis of JABBA LPG caps indicated that 20% is branched trisaccharide Gal(β1,4)[Glc(β1,2)]Man and tetrasaccharide Gal(β1,4)[Glc(β1,2)Man(α1,2)]Man instead of the usual Gal(β1,4)Man and Man(α1,2)Man terminating caps. Consistent with these structural observations, analyses of the relevant glycosyltransferases in JABBA microsomes involved in LPG biosynthesis showed a 2-fold increase in elongating mannosylphosphoryltransferase activity and up-regulation of a β-glucosyltransferase activity. Furthermore, the caps of JABBA LPG are cryptic in presentation as shown by the loss of binding by the lectins, ricin, peanut agglutinin and concanavalin A and reduced accessibility of the terminal galactose residues to oxidation by galactose oxidase. These results indicate that LPG from JABBA is intriguingly similar to the larger LPG in wild-type parasites that arises following the differentiation of the non-infectious procyclic promastigotes to infectious, metacyclic forms.

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.

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.

Therapeutic efficacy of stable analogues of vasoactive intestinal peptide against pathogens

Vasoactive intestinal peptide (VIP) is an anti-inflammatory neuropeptide recently identified as a potential antimicrobial peptide. To overcome the metabolic limitations of VIP, we modified the native peptide sequence and generated two stable synthetic analogues (VIP51 and VIP51(6-30)) with better antimicrobial profiles. Herein we investigate the effects of both VIP analogues on cell viability, membrane integrity, and ultrastructure of various bacterial strains and Leishmania species. We found that the two VIP derivatives kill various non-pathogenic and pathogenic Gram-positive and Gram-negative bacteria as well as the parasite Leishmania major through a mechanism that depends on the interaction with certain components of the microbial surface, the formation of pores, and the disruption of the surface membrane. The cytotoxicity of the VIP derivatives is specific for pathogens, because they do not affect the viability of mammalian cells. Docking simulations indicate that the chemical changes made in the analogues are critical to increase their antimicrobial activities. Consequently, we found that the native VIP is less potent as an antibacterial and fails as a leishmanicidal. Noteworthy from a therapeutic point of view is that treatment with both derivatives increases the survival and reduces bacterial load and inflammation in mice with polymicrobial sepsis. Moreover, treatment with VIP51(6-30) is very effective at reducing lesion size and parasite burden in a model of cutaneous leishmaniasis. These results indicate that the VIP analogues emerge as attractive alternatives for treating drug-resistant infectious diseases and provide key insights into a rational design of novel agents against these pathogens.

Protective role of the neuropeptide urocortin II against experimental sepsis and leishmaniasis by direct killing of pathogens

We currently face an alarming resurgence in infectious diseases characterized by antimicrobial resistance and therapeutic failure. This has generated the urgent need of developing new therapeutic approaches that include agents with nontraditional modes of action. A recent interest focused on approaches based on our natural immune defenses, especially on peptides that combine innate antimicrobial activity against diverse pathogens and immunoregulatory functions. In this study, to our knowledge, we describe for the first time the antimicrobial activity of the neuropeptide urocortin II (UCNII) against a panel of Gram-positive and Gram-negative bacteria and tropical parasites of the genus Leishmania. Importantly, this cytotoxicity was selective for pathogens, because UCNII did not affect mammalian cell viability. Structurally, UCNII has a cationic and amphipathic design that resembles antimicrobial peptides. Using mutants and UCNII fragments, we determined the structural requirements for the interaction between the peptide and the surface of pathogen. Following its binding to pathogen, UCNII caused cell death through different membrane-disrupting mechanisms that involve aggregation and membrane depolarization in bacteria and pore formation in Leishmania. Noteworthily, UCNII killed the infective form of Leishmania major even inside the infected macrophages. Consequently, UCNII prevented mortality caused by polymicrobial sepsis and ameliorated pathological signs of cutaneous leishmaniasis. Besides its presence in body physical and mucosal barriers, we found that innate immune cells produce UCNII in response to infections. Therefore, UCNII could be considered as an ancient highly-conserved host peptide involved in the natural antimicrobial defense and emerge as an attractive alternative to current treatments for microbial disorders with associated drug resistances.

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.

Biosynthesis of the fungal cell wall polysaccharide galactomannan requires intraluminal GDP-mannose

Fungal cell walls frequently contain a polymer of mannose and galactose called galactomannan. In the pathogenic filamentous fungus Aspergillus fumigatus, this polysaccharide is made of a linear mannan backbone with side chains of galactofuran and is anchored to the plasma membrane via a glycosylphosphatidylinositol or is covalently linked to the cell wall. To date, the biosynthesis and significance of this polysaccharide are unknown. The present data demonstrate that deletion of the Golgi UDP-galactofuranose transporter GlfB or the GDP-mannose transporter GmtA leads to the absence of galactofuran or galactomannan, respectively. This indicates that the biosynthesis of galactomannan probably occurs in the lumen of the Golgi apparatus and thus contrasts with the biosynthesis of other fungal cell wall polysaccharides studied to date that takes place at the plasma membrane. Transglycosylation of galactomannan from the membrane to the cell wall is hypothesized because both the cell wall-bound and membrane-bound polysaccharide forms are affected in the generated mutants. Considering the severe growth defect of the A. fumigatus GmtA-deficient mutant, proving this paradigm might provide new targets for antifungal therapy.

Vesicles as carriers of virulence factors in parasitic protozoan diseases

Different types of shed vesicles as, for example, exosomes, plasma-membrane-derived vesicles or microparticles, are the focus of intense research in view of their potential role in cell-cell communication and under the perspective that they might be good tools for immunotherapy, vaccination or diagnostic purposes. This review discusses ways employed by pathogenic trypanosomatids to interact with the host by shedding vesicles that contain molecules important for the establishment of infection, as opposed to previous beliefs considering them as a waste of cellular metabolism. Trypanosomatids are compared with Apicomplexa, which circulate parasite antigens bound to vesicles shed by host cells. The knowledge of the origin and chemical composition of these different vesicles might lead to the understanding of the mechanisms that determine their biological function.

The Drosophila neurally altered carbohydrate mutant has a defective Golgi GDP-fucose transporter

Studying genetic disorders in model organisms can provide insights into heritable human diseases. The Drosophila neurally altered carbohydrate (nac) mutant is deficient for neural expression of the HRP epitope, which consists of N-glycans with core α1,3-linked fucose residues. Here, we show that a conserved serine residue in the Golgi GDP-fucose transporter (GFR) is substituted by leucine in nac(1) flies, which abolishes GDP-fucose transport in vivo and in vitro. This loss of function is due to a biochemical defect, not to destabilization or mistargeting of the mutant GFR protein. Mass spectrometry and HPLC analysis showed that nac(1) mutants lack not only core α1,3-linked, but also core α1,6-linked fucose residues on their N-glycans. Thus, the nac(1) Gfr mutation produces a previously unrecognized general defect in N-glycan core fucosylation. Transgenic expression of a wild-type Gfr gene restored the HRP epitope in neural tissues, directly demonstrating that the Gfr mutation is solely responsible for the neural HRP epitope deficiency in the nac(1) mutant. These results validate the Drosophila nac(1) mutant as a model for the human congenital disorder of glycosylation, CDG-IIc (also known as LAD-II), which is also the result of a GFR deficiency.

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.

Innate immune activation and subversion of Mammalian functions by leishmania lipophosphoglycan

Leishmania promastigotes express several prominent glycoconjugates, either secreted or anchored to the parasite surface. Of these lipophosphoglycan (LPG) is the most abundant, and along with other phosphoglycan-bearing molecules, plays important roles in parasite infectivity and pathogenesis in both the sand fly and the mammalian host. Besides its contribution for parasite survival in the sand fly vector, LPG is important for modulation the host immune responses to favor the establishment of mammalian infection. This review will summarize the current knowledge regarding the role of LPG in Leishmania infectivity, focusing on the interaction of LPG and innate immune cells and in the subversion of mammalian functions by this molecule.

Inhibition of Golgi apparatus glycosylation causes endoplasmic reticulum stress and decreased protein synthesis

Nucleotide sugar transporters of the Golgi apparatus play an essential role in the glycosylation of proteins, lipids, and proteoglycans. Down-regulation of expression of the transporters for CMP-sialic acid, GDP-fucose, or both unexpectedly resulted in accumulation of glycoconjugates in the Golgi apparatus rather than in the plasma membrane. Pulse-chase experiments with radiolabeled sugars and amino acids showed decreased synthesis and secretion of both nonglycoproteins and glycoproteins. Further studies revealed that the above silencing induced endoplasmic reticulum stress and inhibited protein translation initiation. Together these results suggest that global inhibition of Golgi apparatus glycosylation may lead to important secondary metabolic changes, unrelated to glycosylation.

Leishmania major phosphoglycans influence the host early immune response by modulating dendritic cell functions

The precise role of Leishmania glycoconjugate molecules including phosphoglycans (PGs) and lipophosphoglycan (LPG) on host cellular responses is still poorly defined. Here, we investigated the interaction of Leishmania major LPG2 null mutant (lpg2(-)), which lacks both PGs and LPG, with dendritic cells (DCs) and the subsequent early immune response in infected mice. Surprisingly, the absence of phosphoglycans did not influence expression pattern of major histocompatibility complex class II (MHC II), CD40, CD80, and CD86 on DCs in vitro and in vivo. However, lpg2(-) L. major induced significantly higher production of interleukin-12p40 (IL-12p40) by infected bone marrow-derived DCs (BMDCs) than wild-type (WT) parasites in vitro. Furthermore, the production of IL-12p40 by draining lymph node cells from lpg2(-) mutant-infected mice was higher than those from WT L. major-infected mice. In model antigen presentation experiments, DCs from lpg2(-) mutant-infected mice induced more gamma interferon (IFN-gamma) and IL-2 production by Leishmania-specific T cells than those from WT-infected mice. Lymphocytes isolated from mice infected for 3 days with lpg2(-) parasites produce similar levels of IFN-gamma, but significantly less IL-4 and IL-10 than WT controls. Decreased IL-4 production was also seen in another general PG-deficient mutant lacking the Golgi UDP-galactose transporters (lpg5A(-) lpg5B(-)), but not with the lpg1(-) mutant lacking only LPG, thereby implicating PGs generally in the reduction of IL-4 production. Thus, Leishmania PGs influence host early immune response by modulating DC functions in a way that inhibits antigen presentation and promotes early IL-4 response, and their absence may impact the balance between Th1 and Th2 responses.

Leishmania donovani lacking the Golgi GDP-Man transporter LPG2 exhibit attenuated virulence in mammalian hosts

Surface phosophoglycans such as lipophosphoglycan (LPG) or proteophosphoglycan (PPG) and glycosylinositol phospholipids (GIPLs) modulate essential interactions between Leishmania and mammalian macrophages. Phosphoglycan synthesis depends on the Golgi GDP-mannose transporter encoded by LPG2. LPG2-null (lpg2(-)) Leishmania major cannot establish macrophage infections or induce acute pathology, whereas lpg2(-)Leishmania mexicana retain virulence. lpg2(-)Leishmania donovani has been reported to survive poorly in cultured macrophages but in vivo survival has not been explored. Herein we discovered that, similar to lpg2(-)L. major, lpg2(-)L. donovani promastigotes exhibited diminished virulence in mice, but persisted at consistently low levels. lpg2(-)L. donovani promastigotes could not establish infection in macrophages and could not transiently inhibit phagolysosomal fusion. Furthermore, lpg2(-) promastigotes of L. major, L. donovani and L. mexicana were highly susceptible to complement-mediated lysis. We conclude that phosphoglycan assembly and expression mediated by L. donovani LPG2 are important for promastigote and amastigote virulence, unlike L. mexicana but similar to L. major.

The ortholog of human solute carrier family 35 member B1 (UDP-galactose transporter-related protein 1) is involved in maintenance of ER homeostasis and essential for larval development in Caenorhabditis elegans

Although the solute carrier 35B1 (SLC35B1) is evolutionarily conserved, its functions in metazoans remain unknown. To elucidate its function, we examined developmental roles of an SLC35B1 family gene (HUT-1: homolog of UDP-Gal transporter) in Caenorhabditis elegans. We isolated a deletion mutant of the gene and characterized phenotypes of the mutant and hut-1 RNAi-treated worms. GFP-HUT-1 reporter analysis was performed to examine gene expression patterns. We also tested whether several nucleotide sugar transporters can compensate for hut-1 deficiency. The hut-1 deletion mutant and RNAi worms showed larval growth defect and lethality with disrupted intestinal morphology. Inactivation of hut-1 induced chronic endoplasmic reticulum (ER) stress, and hut-1 showed genetic interactions with the atf-6, pek-1, and ire-1 genes involved in unfolded protein response signaling. ER ultrastructure and ER marker distribution in hut-1-deficient animals showed that HUT-1 is required for maintenance of ER structure. Reporter analysis revealed that HUT-1 is an ER protein ubiquitously expressed in tissues, including the intestine. Lethality and the ER stress phenotype of the mutant were rescued with the human hut-1 ortholog UGTrel1. These results indicate important roles for hut-1 in development and maintenance of ER homeostasis in C. elegans.

Drugs and transporters in kinetoplastid protozoa

Kinetoplastid protozoa express hundreds of membrane transport proteins that allow them to take up nutrients, establish ion gradients, efflux metabolites, translocate compounds from one intracellular compartment to another, and take up or export drugs. The combination of molecular cloning, genetic approaches, and the completed genome projects for Trypanosoma brucei, Leishmania major, and Trypanosoma cruzi have allowed detailed functional analysis of various transporters and predictions about the likely functions of others. Thus many opportunities exist to define the biological and pharmacological properties of parasite transporters whose genes were often difficult to identify in the pregenomic era. A subset of these transporters that are essential for parasite viability could serve as targets for novel drug therapies by identifying compounds that interfere with their uptake functions. Other permeases provide routes for uptake of selectively cytotoxic compounds and can thus be useful for delivery of drugs. Drug resistance may develop in strains where such drug uptake transporters are nonfunctional or in parasites that over-express other permeases that export a drug. A summary of recent work on Leishmania transporters for glucose and for purines is provided as an example of permeases that are being studied in molecular detail.

A single Caenorhabditis elegans Golgi apparatus-type transporter of UDP-glucose, UDP-galactose, UDP-N-acetylglucosamine, and UDP-N-acetylgalactosamine

The genome of Caenorhabditis elegans encodes for 18 putative nucleotide sugar transporters even though its glycome only contains 7 different monosaccharides. To understand the biological significance of this phenomenon, we have begun a systematic substrate characterization of the above putative transporters and have determined that the gene ZK896.9 encodes a Golgi apparatus transporter for UDP-glucose, UDP-galactose, UDP- N-acetylglucosamine, and UDP- N-acetylgalactosamine. This is the first tetrasubstrate nucleotide sugar transporter characterized for any organism and is also the first nonplant transporter for UDP-glucose. Evidence for the above substrate specificity and substrate transport saturation kinetics was obtained by expression of ZK896.9 in Saccharomyces cerevisiae followed by Golgi enriched vesicle isolation and assays in vitro. Further evidence for UDP-glucose transport was obtained by expression of ZK 896.9 in Giardia lamblia, an organism recently characterized as having endogenous transport activity for only UDP- N-acetylglucosamine. Expression of ZK896.9 was also able to correct the phenotype of a mutant Chinese ovary cell line specifically defective in the transport of UDP-galactose into the Golgi apparatus and of a mutant of the yeast Kluyveromyces lactis specifically defective in the transport of UDP- N-acetylglucosamine into its Golgi apparatus. Because up to now all three other characterized nucleotide sugar transporters of C. elegans have been found to transport two or three substrates, the substrate specificity of ZK896.9 raises questions as to the evolutionary ancestry of this group of proteins in this nematode.

The leishmania ARL-1 and Golgi traffic

We present here the characterisation of the Leishmania small G protein ADP-Ribosylation Factor-Like protein 1 (ARL-1). The ARL-1 gene is present in one copy per haploid genome and conserved among trypanosomatids. It encodes a protein of 20 kDa, which is equally expressed in the insect promastigote and mammalian amastigote forms of the parasite. ARL-1 localises to the Trans-Golgi Network (TGN); N-terminal myristoylation is essential for TGN localisation. In vivo expression of the LdARL-1/Q74L and LdARL-1/T51N mutants (GTP- and GDP-bound blocked forms respectively) shows that GDP/GTP cycling occurs entirely within the TGN. This is contrary to previous reports in yeast and mammals, where the mutant empty form devoid of nucleotide has been considered as the GDP-blocked form. The dominant-negative empty form mutant LdARL-1/T34N inhibits endocytosis and intracellular trafficking from the TGN to the Lysosome/Multivesicular Tubule and to the acidocalcisomes; these defects are probably related to a mislocalisation of the GRIP domain-containing vesicle tethering factors which cannot be recruited to the TGN by the cytoplasmic LdARL-1/T34N. Thus, besides the functional characterization of a new mutant and a better understanding of ARL-1 GDP/GTP cycling, this work shows that Leishmania ARL-1 is a key component of an essential pathway worth future study.

Use of Giardia, which appears to have a single nucleotide-sugar transporter for UDP-GlcNAc, to identify the UDP-Glc transporter of Entamoeba

Nucleotide-sugar transporters (NSTs) transport activated sugars (e.g. UDP-GlcNAc) from the cytosol to the lumen of the endoplasmic reticulum or Golgi apparatus where they are used to make glycoproteins and glycolipids. UDP-Glc is an important component of the N-glycan-dependent quality control (QC) system for protein folding. Because Entamoeba has this QC system while Giardia does not, we hypothesized that transfected Giardia might be used to identify the UDP-Glc transporter of Entamoeba. Here we show Giardia membranes transport UDP-GlcNAc and have apyrases, which hydrolyze nucleoside-diphosphates to make the antiporter nucleoside-monophosphate. The only NST of Giardia (GlNst), which we could identify, transports UDP-GlcNAc in transfected Saccharomyces and is present in perinuclear and peripheral vesicles and increases in expression during encystation. Entamoeba membranes transport three nucleotide-sugars (UDP-Gal, UDP-GlcNAc, and UDP-Glc), and Entamoeba has three NSTs, one of which has been shown previously to transport UDP-Gal (EhNst1). Here we show recombinant EhNst2 transports UDP-Glc in transfected Giardia, while recombinant EhNst3 transports UDP-GlcNAc in transfected Saccharomyces. In summary, all three NSTs of Entamoeba and the single NST of Giardia have been molecularly characterized, and transfected Giardia provides a new system for testing heterologous UDP-Glc transporters.

Nucleotide sugar transporters of the Golgi apparatus

The Golgi apparatus is the major site of protein, lipid and proteoglycan glycosylation. The glycosylation enzymes, as well as kinases and sulfatases that catalyze phosphorylation and sulfation, are localized within the Golgi cisternae in characteristic distributions that frequently reflect their order in a particular pathway (Kornfeld and Kornfeld 1985; Colley 1997). The glycosyl-transferases, sulfotransferases and kinases are “transferases” that require activated donor molecules for the reactions they catalyze. For eukaryotic, fungal and protozoan glycosyltransferases these are the nucleotide sugars UDP-N-acetylglucosamine (UDP-GlcNAc), UDP-galactose (UDP-Gal), GDP-fucose (GDP-Fuc), CMP-sialicacid (CMP-Sia), UDP-glucuronicacid (UDP-GlcA), GDP-mannose (GDP-Man), and UDP-xylose (UDP-Xyl) (Hirschberg et al. 1998). For the kinases, ATP functions as the donor, while for the sulfotransferases, adenosine 3′-phosphate 5′-phosphate (PAPS) acts as the donor (Hirschberg et al. 1998). The active sites of all these enzymes are oriented towards the lumen of the Golgi cisternae. This necessitates the translocation of their donors from the cytosol into the lumenal Golgi compartments. In this chapter we will focus on the structure, function and localization of the Golgi nucleotide sugar transporters (NSTs), and highlight the diseases and developmental defects associated with defective transporters. We direct the reader to several excellent reviews on Golgi transporters for additional details and references (Hirschberg et al. 1998; Berninsone and Hirschberg 2000; Gerardy-Schahn et al. 2001; Handford et al. 2006; Caffaro and Hirschberg 2006).

Gene expression profiling, chromosome assignment and mutational analysis of the porcine Golgi-resident UDP-N-acetylglucosamine transporter SLC35A3

SLC35A3 encodes a Golgi-resident UDP-N-acetylglucosamine transporter. Here, the porcine SLC35A3 gene was assigned to Sus scrofa chromosome 4 (SSC4) by a combination of radiation hybrid and linkage analysis. Expression profiling using real time RT-PCR showed ubiquitous but variable transcription of SLC35A3 in a selection of tissues. The deduced 325 amino acid sequence revealed a hydrophobic protein with 10 predicted transmembrane helices and the N- and C-terminal tails facing the cytosolic side of the Golgi apparatus. In addition, mutated versions of the UDP-GlcNAc transporter were analyzed in a yeast complementation assay, which allowed us to identify important domains and amino acid residues. Thus, the N-terminal tail was inessential for activity, whereas removal of the first transmembrane domain inhibited yeast complementation. The hydrophilic C-terminus was dispensable while mutant proteins either fully or partially deprived of the last membrane-spanning helix were functionally impaired. The third luminal loop showed modest sequence conservation and appeared structurally flexible as certain deletions were acceptable. In contrast, the fourth luminal loop was more sensitive to changes since the competence of the mutant protein was lowered by mutations. Substitutions of glycines 190, 215 and 254, which are invariant positions in the SLC35A subfamilies affected activity negatively. Interestingly, inhibition of function by a valine to phenylalanine mutation, which has been associated with skeletal malformations, is likely caused by structural incompatibility of the bulky aromatic phenylalanine side chain with the integrity of the transmembrane helix, since substitutions with the smaller aliphatic side chains of leucine and isoleucine were acceptable changes.

Functional redundancy between two Caenorhabditis elegans nucleotide sugar transporters with a novel transport mechanism

Transporters of nucleotide sugars regulate the availability of these substrates required for glycosylation reactions in the lumen of the Golgi apparatus and play an important role in the development of multicellular organisms. Caenorhabditis elegans has seven different sugars in its glycoconjugates, although 18 putative nucleotide sugar transporters are encoded in the genome. Among these, SQV-7, SRF-3, and CO3H5.2 exhibit partially overlapping substrate specificity and expression patterns. We now report evidence of functional redundancy between transporters CO3H5.2 and SRF-3. Reducing the activity of the CO3H5.2 gene product by RNA interference (RNAi) in SRF-3 mutants results in oocyte accumulation and abnormal gonad morphology, whereas comparable RNAi treatment of wild type or RNAi hypersensitive C. elegans strains does not cause detectable defects. We hypothesize this genetic enhancement to be a mechanism to ensure adequate glycoconjugate biosynthesis required for normal tissue development in multicellular organisms. Furthermore, we show that transporters SRF-3 and CO3H5.2, which are closely related in the phylogenetic tree, share a simultaneous and independent substrate transport mechanism that is different from the competitive one previously demonstrated for transporter SQV-7, which shares a lower amino acid sequence identity with CO3H5.2 and SRF-3. Therefore, different mechanisms for transporting multiple nucleotide sugars may have evolved parallel to transporter amino acid divergence.

Comparisons of mutants lacking the Golgi UDP-galactose or GDP-mannose transporters establish that phosphoglycans are important for promastigote but not amastigote virulence in Leishmania major

Abundant surface Leishmania phosphoglycans (PGs) containing [Gal(beta1,4)Man(alpha1-PO(4))]-derived repeating units are important at several points in the infectious cycle of this protozoan parasite. PG synthesis requires transport of activated nucleotide-sugar precursors from the cytoplasm to the Golgi apparatus. Correspondingly, null mutants of the L. major GDP-mannose transporter LPG2 lack PGs and are severely compromised in macrophage survival and induction of acute pathology in susceptible mice, yet they are able to persist indefinitely and induce protective immunity. However, lpg2(-) L. mexicana amastigotes similarly lacking PGs but otherwise normal in known glycoconjugates remain able to induce acute pathology. To explore this further, we tested the infectivity of a new PG-null L. major mutant, which is inactivated in the two UDP-galactose transporter genes LPG5A and LPG5B. Surprisingly this mutant did not recapitulate the phenotype of L. major lpg2(-), instead resembling the L. major lipophosphoglycan-deficient lpg1(-) mutant. Metacyclic lpg5A(-)/lpg5B(-) promastigotes showed strong defects in the initial steps of macrophage infection and survival. However, after a modest delay, the lpg5A(-)/lpg5B(-) mutant induced lesion pathology in infected mice, which thereafter progressed normally. Amastigotes recovered from these lesions were fully infective in mice and in macrophages despite the continued absence of PGs. This suggests that another LPG2-dependent metabolite is responsible for the L. major amastigote virulence defect, although further studies ruled out cytoplasmic mannans. These data thus resolve the distinct phenotypes seen among lpg2(-) Leishmania species by emphasizing the role of glycoconjugates other than PGs in amastigote virulence, while providing further support for the role of PGs in metacyclic promastigote virulence.

Two functionally divergent UDP-Gal nucleotide sugar transporters participate in phosphoglycan synthesis in Leishmania major

In the protozoan parasite Leishmania, abundant surface and secreted molecules, such as lipophosphoglycan (LPG) and proteophosphoglycans (PPGs), contain extensive galactose in the form of phosphoglycans (PGs) based on (Gal-Man-PO(4)) repeating units. PGs are synthesized in the parasite Golgi apparatus and require transport of cytoplasmic nucleotide sugar precursors to the Golgi lumen by nucleotide sugar transporters (NSTs). GDP-Man transport is mediated by the LPG2 gene product, and here we focused on transporters for UDP-Gal. Data base mining revealed 12 candidate NST genes in the L. major genome, including LPG2 as well as a candidate endoplasmic reticulum UDP-glucose transporter (HUT1L) and several pseudogenes. Gene knock-out studies established that two genes (LPG5A and LPG5B) encoded UDP-Gal NSTs. Although the single lpg5A(-) and lpg5B(-) mutants produced PGs, an lpg5A(-)/5B(-) double mutant was completely deficient. PG synthesis was restored in the lpg5A(-)/5B(-) mutant by heterologous expression of the human UDP-Gal transporter, and heterologous expression of LPG5A and LPG5B rescued the glycosylation defects of the mammalian Lec8 mutant, which is deficient in UDP-Gal uptake. Interestingly, the LPG5A and LPG5B functions overlap but are not equivalent, since the lpg5A(-) mutant showed a partial defect in LPG but not PPG phosphoglycosylation, whereas the lpg5B(-) mutant showed a partial defect in PPG but not LPG phosphoglycosylation. Identification of these key NSTs in Leishmania will facilitate the dissection of glycoconjugate synthesis and its role(s) in the parasite life cycle and further our understanding of NSTs generally.

Nucleotide-sugar transporters: structure, function and roles in vivo

The glycosylation of glycoconjugates and the biosynthesis of polysaccharides depend on nucleotide-sugars which are the substrates for glycosyltransferases. A large proportion of these enzymes are located within the lumen of the Golgi apparatus as well as the endoplasmic reticulum, while many of the nucleotide-sugars are synthesized in the cytosol. Thus, nucleotide-sugars are translocated from the cytosol to the lumen of the Golgi apparatus and endoplasmic reticulum by multiple spanning domain proteins known as nucleotide-sugar transporters (NSTs). These proteins were first identified biochemically and some of them were cloned by complementation of mutants. Genome and expressed sequence tag sequencing allowed the identification of a number of sequences that may encode for NSTs in different organisms. The functional characterization of some of these genes has shown that some of them can be highly specific in their substrate specificity while others can utilize up to three different nucleotide-sugars containing the same nucleotide. Mutations in genes encoding for NSTs can lead to changes in development in Drosophila melanogaster or Caenorhabditis elegans, as well as alterations in the infectivity of Leishmania donovani. In humans, the mutation of a GDP-fucose transporter is responsible for an impaired immune response as well as retarded growth. These results suggest that, even though there appear to be a fair number of genes encoding for NSTs, they are not functionally redundant and seem to play specific roles in glycosylation.