Small Molecule Inhibitors of Mucin-Type O-Linked Glycosylation from a Uridine-Based Library

Rational Design of Dual-Domain Binding Inhibitors for N-Acetylgalactosamine Transferase 2 with Improved Selectivity over the T1 and T3 Isoforms

The GalNAc-transferase (GalNAc-T) family, consisting of 20 isoenzymes, regulates the O-glycosylation process of mucin glycopeptides by transferring GalNAc units to serine/threonine residues. Dysregulation of specific GalNAc-Ts is associated with various diseases, making these enzymes attractive targets for drug development. The development of inhibitors is key to understanding the implications of GalNAc-Ts in human diseases. However, developing selective inhibitors for individual GalNAc-Ts represents a major challenge due to shared structural similarities among the isoenzymes and some degree of redundancy among the natural substrates. Herein, we report the development of a GalNAc-T2 inhibitor with higher potency compared to those of the T1 and T3 isoforms. The most promising candidate features bivalent GalNAc and thiophene moieties on a peptide chain, enabling binding to both the lectin and catalytic domains of the enzyme. The binding mode was confirmed by competitive saturation transfer difference NMR experiments and validated through molecular dynamics simulations. The inhibitor demonstrated an IC50 of 21.4 μM for GalNAc-T2, with 8- and 32-fold higher selectivity over the T3 and T1 isoforms, respectively, representing a significant step forward in the synthesis of specific GalNAc-T inhibitors tailored to the unique structural features of the targeted isoform.

Small molecule LpxC inhibitors against gram-negative bacteria: Advances and future perspectives

Uridine diphosphate-3-O-(hydroxymyristoyl)-N-acetylglucosamine deacetylase (LpxC) is a metalloenzyme with zinc ions as cofactors and is a key enzyme in the essential structural outer membrane lipid A synthesis commitment step of gram-negative bacteria. As LpxC is extremely homologous among different Gram-negative bacteria, it is conserved in almost all gram-negative bacteria, which makes LpxC a promising target. LpxC inhibitors have been reported extensively in recent years, such as PF-5081090 and CHIR-090 were found to have broad-spectrum antibiotic activity against P. aeruginosa and E. coli. They are mainly classified into hydroxamate inhibitors and non-hydroxamate inhibitors based on their structure, but no LpxC inhibitors have been marketed due to safety and activity issues. This review, therefore, focuses on small molecule inhibitors of LpxC against gram-negative pathogenic bacteria and covers recent advances in LpxC inhibitors, focusing on their structural optimization process, structure-activity relationships, and future directions, with the aim of providing ideas for the development of LpxC inhibitors and clinical research.

Identification of global inhibitors of cellular glycosylation

Small molecule inhibitors of glycosylation enzymes are valuable tools for dissecting glycan functions and potential drug candidates. Screening for inhibitors of glycosyltransferases are mainly performed by in vitro enzyme assays with difficulties moving candidates to cells and animals. Here, we circumvent this by employing a cell-based screening assay using glycoengineered cells expressing tailored reporter glycoproteins. We focused on GalNAc-type O-glycosylation and selected the GalNAc-T11 isoenzyme that selectively glycosylates endocytic low-density lipoprotein receptor (LDLR)-related proteins as targets. Our screen of a limited small molecule compound library did not identify selective inhibitors of GalNAc-T11, however, we identify two compounds that broadly inhibited Golgi-localized glycosylation processes. These compounds mediate the reversible fragmentation of the Golgi system without affecting secretion. We demonstrate how these inhibitors can be used to manipulate glycosylation in cells to induce expression of truncated O-glycans and augment binding of cancer-specific Tn-glycoprotein antibodies and to inhibit expression of heparan sulfate and binding and infection of SARS-CoV-2.

Small molecule inhibitors of mammalian glycosylation

Glycans are one of the fundamental biopolymers encountered in living systems. Compared to polynucleotide and polypeptide biosynthesis, polysaccharide biosynthesis is a uniquely combinatorial process to which interdependent enzymes with seemingly broad specificities contribute. The resulting intracellular cell surface, and secreted glycans play key roles in health and disease, from embryogenesis to cancer progression. The study and modulation of glycans in cell and organismal biology is aided by small molecule inhibitors of the enzymes involved in glycan biosynthesis. In this review, we survey the arsenal of currently available inhibitors, focusing on agents which have been independently validated in diverse systems. We highlight the utility of these inhibitors and drawbacks to their use, emphasizing the need for innovation for basic research as well as for therapeutic applications.

Prognostic relevance of the hexosamine biosynthesis pathway activation in leiomyosarcoma

Metabolic reprogramming of tumor cells and the increase of glucose uptake is one of the hallmarks of cancer. In order to identify metabolic pathways activated in leiomyosarcoma (LMS), we analyzed transcriptomic profiles of distinct subtypes of LMS in several datasets. Primary, recurrent and metastatic tumors in the subtype 2 of LMS showed consistent enrichment of genes involved in hexosamine biosynthesis pathway (HBP). We demonstrated that glutamine-fructose-6-phosphate transaminase 2 (GFPT2), the rate-limiting enzyme in HBP, is expressed on protein level in a subset of LMS and the expression of this enzyme is frequently retained in patient-matched primary and metastatic tumors. In a new independent cohort of 327 patients, we showed that GFPT2 is associated with poor outcome of uterine LMS but not extra-uterine LMS. Based on the analysis of a small group of patients studied by ¹⁸F-FDG-PET imaging, we propose that strong expression of GFPT2 in primary LMS may be associated with high metabolic activity. Our data suggest that HBP is a potential new therapeutic target in one of the subtypes of LMS.

Genetic glycoengineering in mammalian cells

Advances in nuclease-based gene-editing technologies have enabled precise, stable, and systematic genetic engineering of glycosylation capacities in mammalian cells, opening up a plethora of opportunities for studying the glycome and exploiting glycans in biomedicine. Glycoengineering using chemical, enzymatic, and genetic approaches has a long history, and precise gene editing provides a nearly unlimited playground for stable engineering of glycosylation in mammalian cells to explore and dissect the glycome and its many biological functions. Genetic engineering of glycosylation in cells also brings studies of the glycome to the single cell level and opens up wider use and integration of data in traditional omics workflows in cell biology. The last few years have seen new applications of glycoengineering in mammalian cells with perspectives for wider use in basic and applied glycosciences, and these have already led to discoveries of functions of glycans and improved designs of glycoprotein therapeutics. Here, we review the current state of the art of genetic glycoengineering in mammalian cells and highlight emerging opportunities.

Phosphate-sensing and regulatory mechanism of FGF23 production

BACKGROUND

Inorganic phosphate (Pi) is an essential mineral for human. Hypophosphatemia and hyperphosphatemia cause rickets/osteomalacia and ectopic calcification, respectively, indicating that serum Pi level needs to be regulated. Fibroblast growth factor (FGF) 23 is a principal hormone to regulate serum Pi level. FGF23 is produced by the bone, especially by the osteoblasts and osteocytes, and works by binding to FGF receptor (FGFR) 1c and α-Klotho complex in the kidney. FGF23 reduces serum Pi level by inhibiting both renal phosphate reabsorption and intestinal phosphate absorption via reduction of serum 1,25-dihydroxyvitamin D level. It has been unclear how the bone senses changes of serum Pi level and how the bone regulates the production of FGF23.

RECENT FINDINGS

Our recent results indicate that the post-translational modification of FGF23 protein through a gene product of GALNT3 is the main regulatory mechanism of enhanced FGF23 production by high dietary Pi. Furthermore, high extracellular Pi directly activates FGFR1 and its downstream intracellular signaling pathway regulates the expression level of GALNT3.

CONCLUSIONS

We propose that FGFR1 works as a Pi-sensing receptor in the regulation of FGF23 production and serum Pi level. There is a negative feedback system, which is a basic mechanism of endocrine regulation, in the regulation of serum Pi involving FGFR1, and FGF23. These findings may lead to the development of new therapeutic methods to treat diseases caused by abnormal Pi level.

Connectivity map-based drug repositioning of bortezomib to reverse the metastatic effect of GALNT14 in lung cancer

Despite the continual discovery of promising new cancer targets, drug discovery is often hampered by the poor druggability of these targets. As such, repurposing FDA-approved drugs based on cancer signatures is a useful alternative to cancer precision medicine. Here, we adopted an in silico approach based on large-scale gene expression signatures to identify drug candidates for lung cancer metastasis. Our clinicogenomic analysis identified GALNT14 as a putative driver of lung cancer metastasis, leading to poor survival. To overcome the poor druggability of GALNT14 in the control of metastasis, we utilized the Connectivity Map and identified bortezomib (BTZ) as a potent metastatic inhibitor, bypassing the direct inhibition of the enzymatic activity of GALNT14. The antimetastatic effect of BTZ was verified both in vitro and in vivo. Notably, both BTZ treatment and GALNT14 knockdown attenuated TGFβ-mediated gene expression and suppressed TGFβ-dependent metastatic genes. These results demonstrate that our in silico approach is a viable strategy for the use of undruggable targets in cancer therapies and for revealing the underlying mechanisms of these targets.

Targeting Glycosylation: A New Road for Cancer Drug Discovery

Cancer is a deadly disease that encompasses numerous cellular modifications. Among them, alterations in glycosylation are a proven reliable hallmark of cancer, with most biomarkers used in the clinic detecting cancer-associated glycans. Despite their clear potential as therapy targets, glycans have been overlooked in drug discovery strategies. The complexity associated with the glycosylation process, and lack of specific methodologies to study it, have long hampered progress. However, recent advances in new methodologies, such as glycoengineering of cells and high-throughput screening (HTS), have opened new avenues of discovery. We envision that glycan-based targeting has the potential to start a new era of cancer therapy. In this article, we discuss the promise of cancer-associated glycosylation for the discovery of effective cancer drugs.

The GALNT6‑LGALS3BP axis promotes breast cancer cell growth

Polypeptide N‑acetylgalactosaminyltransferase 6 (GALNT6), which is involved in the initiation of O‑glycosylation, has been reported to play crucial roles in mammary carcinogenesis through binding to several substrates; however, its biological roles in mediating growth‑promoting effects remain unknown. The present study demonstrated a crucial pathophysiological role of GALNT6 through its O‑glycosylation of lectin galactoside‑binding soluble 3 binding protein (LGALS3BP), a secreted growth‑promoting glycoprotein, in breast cancer growth. The Cancer Genome Atlas data analysis revealed that high expression levels of GALNT6 were significantly associated with poor prognosis of breast cancer. GALNT6 O‑glycosylated LGALS3BP in breast cancer cells, whereas knockdown of GALNT6 by siRNA led to the inhibition of both the O‑glycosylation and secretion of LGALS3BP, resulting in the suppression of breast cancer cell growth. Notably, LGALS3BP is potentially O‑glycosylated at three sites (T556, T571 and S582) by GALNT6, thereby promoting autocrine cell growth, whereas the expression of LGALS3BP with three Ala substitutions (T556A, T571A and S582A) in cells drastically reduced GALNT6‑dependent LGALS3BP O‑glycosylation and secretion, resulting in suppression of autocrine growth‑promoting effect. The findings of the present study suggest that the GALNT6‑LGALS3BP axis is crucial for breast cancer cell proliferation and may be a therapeutic target and biomarker for mammary tumors.

How do we sense phosphate to regulate serum phosphate level?

Abnormal phosphate levels result in several pathological conditions such as rickets/osteomalacia and ectopic calcification indicating that there must be a system that regulates phosphate level within a narrow range. FGF23 has been shown to be an essential hormone regulating serum phosphate level. FGF23 binds to Klotho-FGF receptor complex to reduce serum phosphate level. Several reports suggested that FGF receptor is involved in the regulation of FGF23 production. It has been also shown that high extracellular phosphate can activate several intracellular signaling pathways. However, it has been unclear whether and how phosphate regulates FGF23 production in vivo. Our recent results indicate that high extracellular phosphate directly activates FGF receptor 1 and the downstream intracellular signaling enhances FGF23 production. Thus, there is a negative feedback system for the regulation of serum phosphate level involving FGF receptor and FGF23. We propose that FGF receptor works at least as one of phosphate sensors in the maintenance of serum phosphate level.

Inhibition of polypeptide N-acetyl-α-galactosaminyltransferases is an underlying mechanism of dietary polyphenols preventing colorectal tumorigenesis

Ellagitannin-derived ellagic acid (EA) and colonic metabolite urolithins are functional dietary ingredients for cancer prevention, but the underlying mechanism need elucidation. Mucin-type O-glycosylation, initiated by polypeptide N-acetyl-α-galactosaminyltransferases (ppGalNAc-Ts), fine-tunes multiple biological processes and is closely associated with cancer progression. Herein, we aim to explore how specific tannin-based polyphenols affect tumor behavior of colorectal cancer cells (CRC) by modulating O-glycosylation. Utilizing HPLC-based enzyme assay, we find urolithin D (UroD), EA and gallic acid (GA) potently inhibit ppGalNAc-Ts. In particular, UroD inhibits ppGalNAc-T2 through a peptide/protein-competitive manner with nanomolar affinity. Computational simulations combined with site-directed mutagenesis further support the inhibitors' mode of action. Moreover, lectin analysis and metabolic labelling reveal that UroD can reduce cell O-glycans but not N-glycans. Transwell experiments prove that UroD inhibits migration and invasion of CRC cells. Our work proves that specific tannin-based polyphenols can potently inhibit ppGalNAc-Ts activity to reduce cell O-glycosylation and lead to lowering the migration and invasion of CRC cells, suggesting that disturbance of mucin-type O-glycosylation is an important mechanism for the function of dietary polyphenols.

The Multiplicity of Polypeptide GalNAc-Transferase: Assays, Inhibitors, and Structures

Mucin-type O-glycosylation is the dominant form of glycosylation in eukaryotes and plays an important role in various physiological processes. The polypeptide GalNAc-transferase (GalNAc-T) catalyzes the first step in the attachment of mucin-type O-glycosylation. GalNAc-T was recently uncovered to be linked with cancer, atherogenic dyslipidemia, and X-linked hypophosphatemic rickets. Therefore, it has attracted increasing interest as a new target for exploring the underlying mechanism and developing new treatments for related diseases. Decades of studies on GalNAc-T have laid a stable foundation for understanding the catalytic mechanism, determining atom-resolution three-dimensional structures, and developing various types of biochemical assays as well as small-molecule inhibitor leads. Here, we systematically summarize this invaluable knowledge on GalNAc-T and cultivate new perspectives to foster breakthrough points for mucin-type O-glycosylation.

The small molecule luteolin inhibits N-acetyl-α-galactosaminyltransferases and reduces mucin-type O-glycosylation of amyloid precursor protein

Mucin-type O-glycosylation is the most abundant type of O-glycosylation. It is initiated by the members of the polypeptide N-acetyl-α-galactosaminyltransferase (ppGalNAc-T) family and closely associated with both physiological and pathological conditions, such as coronary artery disease or Alzheimer's disease. The lack of direct and selective inhibitors of ppGalNAc-Ts has largely impeded research progress in understanding the molecular events in mucin-type O-glycosylation. Here, we report that a small molecule, the plant flavonoid luteolin, selectively inhibits ppGalNAc-Ts in vitro and in cells. We found that luteolin inhibits ppGalNAc-T2 in a peptide/protein-competitive manner but not promiscuously (e.g. via aggregation-based activity). X-ray structural analysis revealed that luteolin binds to the PXP motif-binding site found in most protein substrates, which was further validated by comparing the interactions of luteolin with wild-type enzyme and with mutants using 1H NMR-based binding experiments. Functional studies disclosed that luteolin at least partially reduced production of β-amyloid protein by selectively inhibiting the activity of ppGalNAc-T isoforms. In conclusion, our study provides key structural and functional details on luteolin inhibiting ppGalNAc-T activity, opening up the way for further optimization of more potent and specific ppGalNAc-T inhibitors. Moreover, our findings may inform future investigations into site-specific O-GalNAc glycosylation and into the molecular mechanism of luteolin-mediated ppGalNAc-T inhibition.

Biosynthetic Machinery Involved in Aberrant Glycosylation: Promising Targets for Developing of Drugs Against Cancer

Cancer cells depend on altered metabolism and nutrient uptake to generate and keep the malignant phenotype. The hexosamine biosynthetic pathway is a branch of glucose metabolism that produces UDP-GlcNAc and its derivatives, UDP-GalNAc and CMP-Neu5Ac and donor substrates used in the production of glycoproteins and glycolipids. Growing evidence demonstrates that alteration of the pool of activated substrates might lead to different glycosylation and cell signaling. It is already well established that aberrant glycosylation can modulate tumor growth and malignant transformation in different cancer types. Therefore, biosynthetic machinery involved in the assembly of aberrant glycans are becoming prominent targets for anti-tumor drugs. This review describes three classes of glycosylation, O-GlcNAcylation, N-linked, and mucin type O-linked glycosylation, involved in tumor progression, their biosynthesis and highlights the available inhibitors as potential anti-tumor drugs.

A strategy for O-glycoproteomics of enveloped viruses--the O-glycoproteome of herpes simplex virus type 1

Glycosylation of viral envelope proteins is important for infectivity and interaction with host immunity, however, our current knowledge of the functions of glycosylation is largely limited to N-glycosylation because it is difficult to predict and identify site-specific O-glycosylation. Here, we present a novel proteome-wide discovery strategy for O-glycosylation sites on viral envelope proteins using herpes simplex virus type 1 (HSV-1) as a model. We identified 74 O-linked glycosylation sites on 8 out of the 12 HSV-1 envelope proteins. Two of the identified glycosites found in glycoprotein B were previously implicated in virus attachment to immune cells. We show that HSV-1 infection distorts the secretory pathway and that infected cells accumulate glycoproteins with truncated O-glycans, nonetheless retaining the ability to elongate most of the surface glycans. With the use of precise gene editing, we further demonstrate that elongated O-glycans are essential for HSV-1 in human HaCaT keratinocytes, where HSV-1 produced markedly lower viral titers in HaCaT with abrogated O-glycans compared to the isogenic counterpart with normal O-glycans. The roles of O-linked glycosylation for viral entry, formation, secretion, and immune recognition are poorly understood, and the O-glycoproteomics strategy presented here now opens for unbiased discovery on all enveloped viruses.

Information on site-specific O-glycosylation of viral envelope glycoproteins is generally very limited despite important functions. We present a powerful mass-spectrometry based strategy to globally identify O-glycosylation sites on viral envelope proteins of a given virus in the context of a productive infection. We successfully utilized the strategy to map O-linked glycosylation sites on the complex HSV-1 virus demonstrating that O-glycosylation is widely distributed on most envelope proteins. Moreover, we used genetically engineered keratinocytes lacking O-glycan elongation capacity to demonstrate that O-linked glycans are indeed important for HSV-1 biology as HSV-1 particles produced in these cells had significantly lower titers compared to wild-type keratinocytes. These tools enable wider discovery and detailed analysis of the role of site-specific O-glycosylation in virology.

Recent advances toward the development of inhibitors to attenuate tumor metastasis via the interruption of lectin-ligand interactions

Aberrant glycosylation is a well-recognized phenomenon that occurs on the surface of tumor cells, and the overexpression of a number of ligands (such as TF, sialyl Tn, and sialyl Lewis X) has been correlated to a worse prognosis for the patient. These unique carbohydrate structures play an integral role in cell-cell communication and have also been associated with more metastatic cancer phenotypes, which can result from binding to lectins present on cell surfaces. The most well studied metastasis-associated lectins are the galectins and selectins, which have been correlated to adhesion, neoangiogenesis, and immune-cell evasion processes. In order to slow the rate of metastatic lesion formation, a number of approaches have been successfully developed which involve interfering with the tumor lectin-substrate binding event. Through the generation of inhibitors, or by attenuating lectin and/or carbohydrate expression, promising results have been observed both in vitro and in vivo. This article briefly summarizes the involvement of lectins in the metastatic process and also describes different approaches used to prevent these undesirable carbohydrate-lectin binding events, which should ultimately lead to improvement in current cancer therapies.

Design of glycosyltransferase inhibitors: pyridine as a pyrophosphate surrogate

A series of ten glycosyltransferase inhibitors has been designed and synthesized by using pyridine as a pyrophosphate surrogate. The series was prepared by conjugation of carbohydrate, pyridine, and nucleoside building blocks by using a combination of glycosylation, the Staudinger-Vilarrasa amide-bond formation, and azide-alkyne click chemistry. The compounds were evaluated as inhibitors of five metal-dependent galactosyltransferases. Crystallographic analyses of three inhibitors complexed in the active site of one of the enzymes confirmed that the pyridine moiety chelates the Mn(2+) ion causing a slight displacement (2 Å) from its original position. The carbohydrate head group occupies a different position than in the natural uridine diphosphate (UDP)-Gal substrate with little interaction with the enzyme.

Inhibition of mucin-type O-glycosylation through metabolic processing and incorporation of N-thioglycolyl-D-galactosamine peracetate (Ac5GalNTGc)

Mucin-type O-glycans form one of the most abundant and complex post-translational modifications (PTM) on cell surface proteins that govern adhesion, migration, and trafficking of hematopoietic cells. Development of targeted approaches to probe functions of O-glycans is at an early stage. Among several approaches, small molecules with unique chemical functional groups that could modulate glycan biosynthesis form a critical tool. Herein, we show that metabolism of peracetyl N-acyl-D-galactosamine derivatives carrying an N-thioglycolyl (Ac5GalNTGc, 1) moiety-but not N-glycolyl (Ac5GalNGc, 2) and N-acetyl (Ac4GalNAc, 3)-through the N-acetyl-D-galactosamine (GalNAc) salvage pathway induced abrogation of MAL-II and PNA epitopes in Jurkat cells. Mass spectrometry of permethylated O-glycans from Jurkat cells confirmed the presence of significant amounts of elaborated O-glycans (sialyl-T and disialyl-T) which were inhibited upon treatment with 1. O-Glycosylation of CD43, a cell surface antigen rich in O-glycans, was drastically reduced by 1 in a thiol-dependent manner. By contrast, only mild effects were observed for CD45 glycoforms. Direct metabolic incorporation of 1 was confirmed by thiol-selective Michael addition reaction of immunoprecipitated CD43-myc/FLAG. Mechanistically, CD43 glycoforms were unperturbed by peracetylated N-(3-acetylthiopropanoyl) (4), N-(4-acetylthiobutanoyl) (5), and N-methylthioacetyl (6) galactosamine derivatives, N-thioglycolyl-D-glucosamine (7, C-4 epimer of 1), and α-O-benzyl 2-acetamido-2-deoxy-D-galactopyranoside (8), confirming the critical requirement of both free sulfhydryl and galactosamine moieties for inhibition of mucin-type O-glycans. Similar, yet differential, effects of 1 were observed for CD43 glycoforms in multiple hematopoietic cells. Development of small molecules that could alter glycan patterns in an antigen-selective and cell-type selective manner might provide avenues for understanding biological functions of glycans.

Identification of a family of four UDP-polypeptide N-acetylgalactosaminyl transferases in Cryptosporidium species

Although mucin-type O-glycans are critical for Cryptosporidium infection, the enzymes catalyzing their synthesis have not been studied. Here, we report four UDP N-acetyl-α-D-galactosamine:polypeptide N-acetylgalactosaminyl transferases (ppGalNAc-Ts) from the genomes of C. parvum, C. hominis and C. muris. All are Type II membrane proteins which include a cytoplasmic tail, a transmembrane domain, a stem region, a glycosyltransferase family 2 domain and a C-terminal ricin B lectin domain. All are expressed during C. parvum infection in vitro, with Cp-ppGalNAc-T1 and -T4 expressed at 24 h and Cp-ppGalNAc-T2 and -T3 at 48 and 72 h post-infection, suggesting that their expression may be developmentally regulated. C. parvum sporozoite lysates display ppGalNAc-T enzymatic activity against non-glycosylated and pre-glycosylated peptides suggesting that they contain enzymes capable of glycosylating both types of substrates. The importance of mucin-type O-glycans in Cryptosporidium-host cell interactions raises the possibility that Cp-ppGalNAc-Ts may serve as targets for intervention in cryptosporidiosis.

Realizing the Promise of Chemical Glycobiology

Chemical glycobiology is emerging as one of the most uniquely powerful sub-disciplines of chemical biology. The previous scarcity of chemical strategies and the unparalleled structural diversity have created a uniquely fertile ground that is both rich in challenges and potentially very profound in implications. Glycans (oligosaccharides, polysaccharides, and glycoconjugates) are everywhere in biological systems and yet remain disproportionately neglected - reviews highlighting this 'Cinderella status' abound. Yet, the last two decades have witnessed tremendous progress, notably in chemical and chemoenzymatic synthesis, 'sequencing' and arraying, metabolic engineering and imaging. These vital steps serve to highlight not only the great potential but just how much more remains to be done. The vast chemical and functional space of glycans remains to be truly explored. Top-down full-scale glycomic and glycoproteomic studies coupled with hypothesis-driven, bottom-up innovative chemical strategies will be required to properly realize the potential impact of glycoscience on human health, energy, and economy. In this review, we cherry-pick far-sighted advances and use these to identify possible challenges, opportunities and avenues in chemical glycobiology.

Global metabolic inhibitors of sialyl- and fucosyltransferases remodel the glycome

Despite the fundamental roles of sialyl- and fucosyltransferases in mammalian physiology, there are few pharmacological tools to manipulate their function in a cellular setting. Although fluorinated analogs of the donor substrates are well-established transition state inhibitors of these enzymes, they are not membrane permeable. By exploiting promiscuous monosaccharide salvage pathways, we show that fluorinated analogs of sialic acid and fucose can be taken up and metabolized to the desired donor substrate-based inhibitors inside the cell. Because of the existence of metabolic feedback loops, they also act to prevent the de novo synthesis of the natural substrates, resulting in a global, family-wide shutdown of sialyl- and/or fucosyltransferases and remodeling of cell-surface glycans. As an example of the functional consequences, the inhibitors substantially reduce expression of the sialylated and fucosylated ligand sialyl Lewis X on myeloid cells, resulting in loss of selectin binding and impaired leukocyte rolling.

Discovery of inhibitors of Leishmania β-1,2-mannosyltransferases using a click-chemistry-derived guanosine monophosphate library

Leishmania spp. are a medically important group of protozoan parasites that synthesize a novel intracellular carbohydrate reserve polymer termed mannogen. Mannogen is a soluble homopolymer of β-1,2-linked mannose residues that accumulates in the major pathogenic stages in the sandfly vector and mammalian host. While several steps in mannogen biosynthesis have been defined, none of the enzymes have been isolated or characterized. We report the development of a simple assay for the GDP-mannose–dependent β-1,2-mannosyltransferases involved in mannogen synthesis. This assay utilizes octyl α-d-mannopyranoside to prime the formation of short mannogen oligomers up to 5 mannose residues. This assay was used to screen a focussed library of 44 GMP-triazole adducts for inhibitors. Several compounds provided effective inhibition of mannogen β-1,2-mannosyltransferases in a cell-free membrane preparation. This assay and inhibitor compounds will be useful for dissecting the role of different mannosyltransferases in regulating de novo biosynthesis and elongation reactions in mannogen metabolism.

A neutral diphosphate mimic crosslinks the active site of human O-GlcNAc transferase

Glycosyltransferases (Gtfs) catalyze the formation of a diverse array of glycoconjugates. Small molecule inhibitors to manipulate Gtf activity in cells have long been sought as tools to understand Gtf function. Success has been limited due to challenges in designing inhibitors that mimic the negatively-charged diphosphate substrates. Here we report the mechanism of action of a small molecule that inhibits O-GlcNAc transferase (OGT), an essential human enzyme that modulates cell signaling pathways by catalyzing a unique intracellular post translational modification, β-O-GlcNAcylation. The molecule contains a five heteroatom dicarbamate core that functions as a neutral diphosphate mimic. One dicarbamate carbonyl reacts with an essential active site lysine that anchors the diphosphate of the nucleotide-sugar substrate. The lysine adduct reacts again with a nearby cysteine to crosslink the OGT active site. While this unprecedented mechanism reflects the unique architecture of the OGT active site, related dicarbamate scaffolds may inhibit other enzymes that bind diphosphate containing substrates.

High-throughput screening for inhibitors of sialyl- and fucosyltransferases

Sweet screens: A high-throughput screening platform for identification of inhibitors of sialyl- and fucosyltransferases based on fluorescence polarization (FP) has been developed. An analogue of the natural donor substrate carrying a fluorescent label (green star) is transferred to a glycoprotein acceptor, which results in robust FP. The screening of 16,000 compounds against different glycosyltransferases has identified various interesting inhibitors.

Uridine-based inhibitors as new leads for antibiotics targeting Escherichia coli LpxC

The UDP-3-O-(R-3-hydroxyacyl)-N-acetylglucosamine deacetylase LpxC catalyzes the committed reaction of lipid A (endotoxin) biosynthesis in Gram-negative bacteria and is a validated antibiotic target. Although several previously described compounds bind to the unique acyl chain binding passage of LpxC with high affinity, strategies to target the enzyme's UDP-binding site have not been reported. Here the identification of a series of uridine-based LpxC inhibitors is presented. The most potent examined, 1-68A, is a pH-dependent, two-step, covalent inhibitor of Escherichia coli LpxC that competes with UDP to bind the enzyme in the first step of inhibition. Compound 1-68A exhibits a K(I) of 54 muM and a maximal rate of inactivation (k(inact)) of 1.7 min(-1) at pH 7.4. Dithiothreitol, glutathione and the C207A mutant of E. coli LpxC prevent the formation of a covalent complex by 1-68A, suggesting a role for Cys-207 in inhibition. The inhibitory activity of 1-68A and a panel of synthetic analogues identified moieties necessary for inhibition. 1-68A and a 2-dehydroxy analogue, 1-68Aa, inhibit several purified LpxC orthologues. These compounds may provide new scaffolds for extension of existing LpxC-inhibiting antibiotics to target the UDP binding pocket.

Glycan antagonists and inhibitors: a fount for drug discovery

Glycans, the carbohydrate chains of glycoproteins, proteoglycans, and glycolipids, represent a relatively unexploited area for drug development compared with other macromolecules. This review describes the major classes of glycans synthesized by animal cells, their mode of assembly, and available inhibitors for blocking their biosynthesis and function. Many of these agents have proven useful for studying the biological activities of glycans in isolated cells, during embryological development, and in physiology. Some are being used to develop drugs for treating metabolic disorders, cancer, and infection, suggesting that glycans are excellent targets for future drug development.

Howard Hang: posttranslational pathogenesis. Interview by Nicole LeBrasseur

Howard Hang is cutting through the forest of posttranslational modifications in an attempt to better understand how human pathogens alter this terrain.

Solid-phase synthesis of a thymidinyl dipeptide urea library

A thymidinyl dipeptide urea library with structural similarity to the nucleoside peptide class of antibiotics was designed and synthesized. To generate the library, a solid-phase synthesis was developed starting from 5'-azidothymidine attached to a polystyrene butyl diethylsilane (PS-DES) resin support. This study describes the prelibrary solid-phase synthesis development including maximum loading capacity optimization, selection of orthogonal functionalized side-chain protection strategies, synthesis of a 64-member test library, and optimization of the final cleavage step. Using the optimized procedures, we synthesized a 1000-member library in a 50 micromol quantity using IRORI-directed sorting technology in MiniKans, producing the target library in good yields and purity.

Chemical technologies for probing glycans

Glycans are central to many biological processes, but efforts to define their functions at the molecular level have been frustrated by a lack of suitable technologies. Here we highlight chemical tools that are beginning to address this need.

Why Does Acute Hyperglycemia Worsen the Outcome of Transient Focal Cerebral Ischemia?

Background and Purpose— Hyperglycemia adversely affects the outcome of stroke. Global ischemia data support that the harmful effect of hyperglycemia is mediated by glucose-induced elevated plasma glucocorticoids. Here we sought to evaluate the negative effects of hyperglycemia on transient focal ischemia in the rat, and to test whether these could be prevented by inhibition of either corticosteroid production or neutrophil infiltration.

Methods— Sprague-Dawley rats (n=217) were used. Ischemia was induced by 1 hour middle cerebral artery occlusion (n=196). Acute hyperglycemia was induced by IP injection of dextrose 30 minutes before ischemia. Neutrophil infiltration was blocked by neutropenia with vinblastine. Corticosterone synthesis was inhibited by chemical adrenalectomy with metyrapone. We measured MRI lesion and tissue infarct volumes, evaluated the neurological function, brain myeloperoxidase and matrix metalloproteinase-9 activities, and protein O-glycosylation.

Results— Hyperglycemia significantly enhanced MRI diffusion-weighted imaging alterations, increased cortical, but not subcortical, infarct volume, worsened neurological score, and enhanced brain myeloperoxidase and matrix metalloproteinase-9 activities. Metyrapone did not prevent hyperglycemic brain damage despite successful reduction of plasma corticosterone. Yet, metyrapone tended to reduce cortical infarction and apparent diffusion coefficient lesion volume, indicating some negative contribution of corticosterone. Blocking neutrophil infiltration was also ineffective to prevent the harmful effect of hyperglycemia. A new finding was that O-linked glycosylation of cerebral proteins was increased under hyperglycemia.

Conclusions— In transient middle cerebral artery occlusion, the hyperglycemia-exacerbated brain damage cannot be fully explained by the negative effects of plasma corticosteroids or neutrophil infiltration. The contribution of other intrinsic effects of high glucose, such as brain protein O-glycosylation, deserves further investigation.

Role of N- and O-glycans in polarized biosynthetic sorting

The maintenance of proper epithelial function requires efficient sorting of newly synthesized and recycling proteins to the apical and basolateral surfaces of differentiated cells. Whereas basolateral protein sorting signals are generally confined to their cytoplasmic regions, apical targeting signals have been identified that localize to luminal, transmembrane, and cytoplasmic aspects of proteins. In the past few years, both N- and O-linked glycans have been identified as apical sorting determinants. Glycan structures are extraordinarily diverse and have tremendous information potential. Moreover, because the oligosaccharides added to a given protein can change depending on cell type and developmental stage, the potential exists for altering sorting pathways by modulation of the expression pattern of enzymes involved in glycan synthesis. In this review, we discuss the evidence for glycan-mediated apical sorting along the biosynthetic pathway and present possible mechanisms by which these common and heterogeneous posttranslational modifications might function as specific sorting signals.

Mucin granule intraluminal organization in living mucous/goblet cells. Roles of protein post-translational modifications and secretion

Recent studies suggest that the mucin granule lumen consists of a matrix meshwork embedded in a fluid phase. Secretory products can both diffuse, although very slowly, through the meshwork pores and interact noncovalently with the matrix. Using a green fluorescent protein-mucin fusion protein (SHGFP-MUC5AC/CK) as a FRAP (fluorescence recovery after photobleaching) probe, we have assessed in living mucous cells the relative importance of different protein post-translational modifications on the intragranular organization. Long term inhibition of mucin-type O-glycosylation, sialylation, or sulfation altered SHGFP-MUC5AC/CK characteristic diffusion time (t(1/2)), whereas all but sulfation diminished its mobile fraction. Reduction of protein disulfide bonds with tris(hydroxypropyl)phosphine resulted in virtually complete immobilization of the SHGFP-MUC5AC/CK intragranular pool. However, when activity of the vacuolar H+-ATPase was also inhibited, disulfide reduction decreased SHGFP-MUC5AC/CK t((1/2)) while diminishing its intraluminal concentration. Similar FRAP profiles were observed in granules that remained in the cells after the addition of a mucin secretagogue. Taken together these results suggest that: (a) the relative content of O-glycans and intragranular anionic groups is crucial for protein diffusion through the intragranular meshwork; (b) protein-protein, rather than carbohydrate-mediated, interactions are responsible for binding of SHGFP-MUC5AC/CK to the immobile fraction, although the degree of matrix O-glycosylation and sialylation affects such interactions; (c) intragranular organization does not depend on covalent multimerization of mucins or the presence of native disulfide bonds in the intragranular mucin/proteins, but rather on specific protein-mediated interactions that are important during the early stages of mucin matrix condensation; (d) alterations of the intragranular matrix precede granule discharge, which can be partial and, accordingly, does not necessarily involve the disappearance of the granule.

The chemistry and biology of mucin-type O-linked glycosylation

Mucin-type O-linked glycosylation is a fundamental post-translational modification that is involved in a variety of important biological processes. However, the lack of chemical tools to study mucin-type O-linked glycosylation has hindered our molecular understanding of O-linked glycans in many biological contexts. The review discusses the significance of mucin-type O-linked glycosylation initiated by the polypeptide N-acetylgalactosaminyltransferases in biology and development of chemical tools to study these enzymes and their substrates.

Action of a library of O-glycosylation inhibitors on the growth of human colorectal cancer cells in culture

O-glycosylation is thought to play a significant role in the regulation of cell growth. However, only limited information is available, and few specific and selective inhibitors have been found. We have synthesized a library of O-glycosylation inhibitors based on benzyl-O-N-acetyl-D-galactosamine. These inhibitors were tested with an established series of human colorectal cancer cell lines, which model the adenoma-carcinoma sequence. Cancer cells were incubated with the inhibitors, and examined for cell growth patterns, and cellular and subcellular glycosylation using a range of lectins with confocal microscopy. The specificity of O-glycan inhibition was confirmed for the library, relative to other forms of glycosylation. All inhibitors tested resulted in smaller cell yields. However, a differential effect on O-glycosylation was detected using the lectins showing variation of localization at a subcellular level in the various cell lines. Further differential action of the inhibitor library was observed for apoptosis and on the cell cycle with the cell lines tested. This work demonstrates that O-glycosylation is closely involved in the regulation of cell growth in colorectal cancer cells and that the generation of a library of low-molecular-mass inhibitors offers a valuable means of examining this regulation at the molecular level.

Synthesis of O-galactosyl aldoximes as potent LacNAc-mimetic galectin-3 inhibitors

A panel of anomeric oxime ether derivatives of beta-galactose were synthesized via the reaction of O-beta-D-galactopyranosylhydroxylamine with aldehydes. The oxime ethers were evaluated as inhibitors against galectin-3 in a competitive fluorescence polarization assay. The best inhibitor, [E]-O-(beta-D-galactopyranosyl)-indole-3-carbaldoxime (E-52), had a Kd value of 180 microM, which is 24 times better than methyl beta-D-galactopyranoside (Kd=4400 microM) and in the same range as methyl lactoside (Kd=220 microM).

Kinetic analysis of a Golgi UDP-GlcNAc:polypeptide-Thr/Ser N-acetyl-alpha-glucosaminyltransferase from Dictyostelium

Mucin-type O-glycosylation in Dictyostelium is initiated in the Golgi by a UDP-GlcNAc:polypeptide-Thr/Ser N-acetyl-alpha-glucosaminyltransferase (Dd-pp alphaGlcNAcT2) whose sequence is distantly related to the sequences of animal polypeptide-Thr/Ser N-acetyl-alpha-galactosaminyltransferases, such as murine Mm-pp alphaGalNAcT1. To evaluate the significance of this similarity, highly purified Dd-pp alphaGlcNAcT2 was assayed using synthetic peptides derived from known substrates. Dd-pp alphaGlcNAcT2 strongly prefers UDP-GlcNAc over UDP-GalNAc, preferentially modifies the central region of the peptide, and modifies Ser in addition to Thr residues. Initial velocity measurements performed over a matrix of UDP-GlcNAc donor and peptide acceptor concentrations indicate that the substrates bind to the enzyme in ordered fashion before the chemical conversion. Substrate inhibition exerted by a second peptide, and the pattern of product inhibition exerted by UDP, suggest that UDP-GlcNAc binds first and the peptide binds second, consistent with data reported for Mm-pp alphaGalNAcT1. Two selective competitive inhibitors of Mm-pp alphaGalNAcT1, retrieved from a screen of neutral-charge uridine derivatives, also inhibit Dd-pp alphaGlcNAcT1 competitively with only slightly less efficacy. Inhibition is specific for Dd-pp alphaGlcNAcT2 relative to two other Dictyostelium retaining glycosyltransferases. These data support a phylogenetic model in which the alphaGlcNAcT function in unicellular eukaryotes converted to an alphaGalNAcT function in the metazoan ortholog while conserving a similar reaction mechanism and active site architecture.

Cellular addresses; step one in creating a glycocode

In this issue of Chemistry & Biology, a library screening approach reveals at least four types of enzymes that attach galactosamine to build cell surface mucin-type glycoproteins. A better molecular understanding of how these information-carrying oligosaccharides are created sets the stage for designing more selective inhibitors and potential therapeutics.

An Inhibitor of O-Glycosylation Induces Apoptosis in NIH3T3 Cells and Developing Mouse Embryonic Mandibular Tissues

The family of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases (ppGaNTases) is responsible for initiating mucin-type O-linked glycosylation in higher eukaryotes. To begin to examine the biological role of O-linked glycosylation, mammalian cells were treated with a small molecule inhibitor (designated 1-68A, Ref. 15) of ppGaNTase activity. NIH3T3 cells exposed to the inhibitor were shown to undergo a significant reduction in cell surface O-glycosylation as detected by staining with jacalin and peanut agglutinin lectins after 30 min of treatment; no reduction in staining using antibodies to O-linked N-acetylglucosamine or the lectin concanavalin A was detected. Apoptosis was also observed in treated cells after 45 min of exposure, ostensibly following the O-glycosylation reduction. Overexpression of several different ppGaNTase isoforms restored cell surface O-glycosylation and rescued inhibitor-induced apoptosis. Additionally, mouse embryonic mandibular organ cultures exposed to 1-68A developed abnormally, presumably because of epithelial and mesenchymal apoptosis that followed a reduction in jacalin and peanut agglutinin staining. Our studies suggest that mucin-type O-linked glycosylation may be required for normal development and that ppGaNTases may play a role in the regulation of apoptosis.