Expedient assembly of Oligo-LacNAcs by a sugar nucleotide regeneration system: Finding the role of tandem LacNAc and sialic acid position towards siglec binding

Review on bioproduction of sialylated human milk oligosaccharides: Synthesis methods, physiologic functions, and applications

Human milk oligosaccharides (HMOs) are crucial for promoting neonatal health, with sialylated oligosaccharides, a significant subclass, offering a variety of health benefits such as prebiotic effects, anti-inflammatory and antimicrobial properties, antiviral defense, and cognitive development support. Among these, 3'-sialyllactose (3'-SL) and 6'-sialyllactose (6'-SL) have received "GRAS" status from the U.S. Food and Drug Administration and approval from the European Food Safety Authority for use as novel food additives in infant formula and supplements. This review focuses on the synthesis methods of sialylated human milk oligosaccharides (SHMOs), their functional properties, downstreaming developments and application technologies. Given the challenges associated with achieving sufficient availability for food and medical applications, the review emphasizes the viability and efficiency of various production strategies. The review also highlights recent research advancements and offers insights for optimizing large-scale production to support future applications in the food and pharmaceutical industries.

Enzyme cascades for nucleotide sugar regeneration in glycoconjugate synthesis

Leloir glycosyltransferases are instrumental in the synthesis of glycoconjugates. Nucleotide sugars as their donor substrates are still considered expensive making preparative enzymatic syntheses economically unattractive. The review highlights the development and advancements of in situ regeneration cycles that utilize nucleotides as byproducts from glycosyltransferase reactions to synthesize respective nucleotide sugars. This approach reduces costs and avoids inhibition of Leloir glycosyltransferases. Regeneration cycles for ten nucleotide sugars are explored emphasizing enzyme cascades from salvage pathways and nucleotide biosynthesis. Additionally, the review highlights advancements involving sucrose synthase for the in situ regeneration of nucleotide sugars from sucrose. Sucrose synthase as the first example of a reversible glycosyltransferase reaction paved the way to establish economic syntheses of glycosylated natural products. Important aspects like enzyme immobilization and protein fusion to optimize processes are discussed. Overall, the review underscores the significance of advanced in situ regeneration cycles for nucleotide sugars for cost-effective access to high-value glycoconjugates. KEY POINTS: • Enzyme cascades for in situ regeneration of nucleotide sugars • Effective cycles for large-scale synthesis of glycoconjugates • Regeneration of nucleotide sugars from sucrose by sucrose synthase.

Comprehensive Modular Synthesis of Ganglioside Glycans and Evaluation of their Binding Affinities to Siglec-7 and Siglec-9

In the present work, bacterial glycosyltransferases are utilized to construct ganglioside glycans in a convergent approach via a sugar‒nucleotide regeneration system and one-pot multienzyme reactions. Starting from β-lactoside enables the diversification of both the glycan moieties and the linkages in the lower α-arm and upper β-arm. Overall, a comprehensive panel of 24 natural a-series (GM3, GM2, GM1a, GD1a, GT1a, and fucosyl-GM1), b-series (GD3, GD2, GD1b, GT1b, and GQ1b), c-series (GT3, GT2, GT1c, GQ1c, and GP1c), α-series (GM1α, GD1aα, and GT1aα), and o-series (GA2, GA1, GM1b, GalNAc-GM1b, and GD1c) ganglioside glycans are prepared, which are suitable for biological studies and further applications. Moreover, a microarray is constructed with these synthesized ganglioside glycans to investigate their binding specificity with recombinant Fc-fused Siglec-7 and Siglec-9, which are immune checkpoint-like glycan recognition proteins on natural killer cells. The microarray binding results reveal that GD3 and GT1aα are specific ligands for Siglec-7 and Siglec-9, respectively, and this discovery can lead to the identification of appropriate ligands for investigating the roles of these Siglecs in immunomodulation.

Controllable Enzymatic Synthesis of Natural Asymmetric Human Milk Oligosaccharides

Among human milk oligosaccharides (HMOs), linear HMOs are synthesized through mature but varied routes. Although branched HMOs can be synthesized by chemical, enzymatic, or chemoenzymatic methods, these methods cannot be easily applied to the synthesis of asymmetric multiantennary oligosaccharides. Herein, we developed a controllable method to synthesize asymmetric biantennary HMOs. In our synthetic route, GlcNAcβ1,3(GlcN3β1,6)Glaβ1,4Glc was first chemically synthesized as the core tetrasaccharide, which contains β1,6GlcN3 as the "stop" sugar in transferase-catalyzed glycosylation. The desired sugars at the GlcNAcβ1-3Gal arm can be assembled using galactosyltransferase, N-acetylglucosaminyltransferase, and fucosyltransferase. Then, the Staudinger reduction and acetylation were used to transform GlcN3 to GlcNAc and assemble sugars by initiating the "go" process. By manipulating transferase-catalyzed glycosylations, 22 natural asymmetric biantennary oligosaccharides were synthesized.

Siglec 15 as a biomarker or a druggable molecule for non-small cell lung cancer

Lung cancer has been the main cause of cancer mortality worldwide. Furthermore, lung cancer rates of new cases per year evidenced a large incidence of this neoplasm in both men and women. Because there is no biomarker for early detection, it is frequently detected late, at an advanced state. The introduction of multiple lines of tyrosine kinase inhibitors in patients with EGFR, ALK, ROS1, and NTRK mutations has modified the therapy of lung cancer. Immunotherapy advances have resulted in substantial improvements in overall survival and disease-free survival, making immune checkpoint inhibitors (ICIs) a potential option for lung cancer treatment. Current PD-1/PD-L1/CTLA-4 immunotherapies have resulted in important response and survival rates. However, existing medicines only function in around 20% of unselected, advanced NSCLC patients, and primary and acquired resistance remain unsolved obstacles. Therefore, precise predictive indicators must be identified to choose the best patients for ICI treatment. Thus, Sialic acid-binding immunoglobulin-like lectin 15 (Siglec-15) stands out as a potential tumor biomarker, with distinctive expression in normal tissues, in tumor immune involvement, and a high structural similarity to PD-L1. Understanding the tumor immune response and the search for new therapeutic targets leads to the improvement of therapeutic pathways directed at the tumor microenvironment. The present review aims to analyze Siglec-15 potential as a diagnostic, prognostic, and response biomarker in lung cancer, considering its results evidenced in the current literature.

Biocatalytic Approaches to Building Blocks for Enzymatic and Chemical Glycan Synthesis

While the field of biocatalysis has bloomed over the past 20-30 years, advances in the understanding and improvement of carbohydrate-active enzymes, in particular, the sugar nucleotides involved in glycan building block biosynthesis, have progressed relatively more slowly. This perspective highlights the need for further insight into substrate promiscuity and the use of biocatalysis fundamentals (rational design, directed evolution, immobilization) to expand substrate scopes toward such carbohydrate building block syntheses and/or to improve enzyme stability, kinetics, or turnover. Further, it explores the growing premise of using biocatalysis to provide simple, cost-effective access to stereochemically defined carbohydrate materials, which can undergo late-stage chemical functionalization or automated glycan synthesis/polymerization.

Combining CuAAC reaction enables sialylated Bi- and triantennary pseudo mannose N-glycans for investigating Siglec-7 interactions

Naturally occurring N-glycans display much diversity in modifications, linkages, and peripheral presentation of the oligosaccharide chain. Despite continued advancements in oligosaccharide synthesis, synthetic access to these natural glycans remains challenging. Biologically relevant complex N-glycan mimetics with various natural and unnatural modifications are an alternate way for investigating glycan-protein interactions. Further supporting this pattern, we report here a new class of sialylated bi- and triantennary pseudo mannose N-glycans reproducing orientation of the underlying glycan chain and branching patterns and replacing the two inner mannopyranosyl units with 1,2,3-triazole rings. Such mimetics are straightforwardly generated by implementing multiple intermolecular Cu(I)-catalyzed azide-alkyne cycloaddition between chemoenzymatically synthesized azido sialosides and rationally designed C-3 and C-6 di-O- or C-2, C-3, and C-6 tri-O-alkynylated mannoside. Human recombinant Siglec-7-Fc fusion protein recognizes almost all sialylated pseudo mannose N-glycans in the microarray. However, a differential Sia-binding pattern was also observed. Given the library size, comparison of pairwise mannose N-glycan combinations showed that biantennary linear α(2,3)α(2,8)- and α(2,6)α(2,8)- or branched α(2,3)α(2,6)-, and triantennary branched α(2,3)α(2,6)-sialyl pseudo N-glycans possess similar binding capabilities and affinity to recombinant Siglec-7-Fc. While the full range of topological mannose arms remain elusive, the bi- and triantennary mimics are simpler structures for interrogating Siglec interactions.

Molecular Basis and Role of Siglec-7 Ligand Expression on Chronic Lymphocytic Leukemia B Cells

Siglec-7 (sialic acid-binding immunoglobulin-like lectin 7) is an immune checkpoint-like glycan recognition protein on natural killer (NK) cells. Cancer cells often upregulate Siglec ligands to subvert immunosurveillance, but the molecular basis of Siglec ligands has been elusive. In this study, we investigated Siglec-7 ligands on chronic lymphocytic leukemia (CLL) B cells. CLL B cells express higher levels of Siglec-7 ligands compared with healthy donor B cells, and enzymatic removal of sialic acids or sialomucins makes them more sensitive to NK cell cytotoxicity. Gene knockout experiments have revealed that the sialyltransferase ST6GalNAc-IV is responsible for the biosynthesis of disialyl-T (Neu5Acα2-3Galβ1-3[Neu5Acα2-6]GalNAcα1-), which is the glycotope recognized by Siglec-7, and that CD162 and CD45 are the major carriers of this glycotope on CLL B cells. Analysis of public transcriptomic datasets indicated that the low expression of GCNT1 (encoding core 2 GlcNAc transferase, an enzyme that competes against ST6GalNAc-IV) and high expression of ST6GALNAC4 (encoding ST6GalNAc-IV) in CLL B cells, together enhancing the expression of the disialyl-T glycotope, are associated with poor patient prognosis. Taken together, our results determined the molecular basis of Siglec-7 ligand overexpression that protects CLL B cells from NK cell cytotoxicity and identified disialyl-T as a potential prognostic marker of CLL.

Identification and functional characterization of a Siglec-7 counter-receptor on K562 cells

Sialic acid (Sia)-binding immunoglobulin-like lectin 7 (Siglec-7) is an inhibitory receptor primarily expressed on natural killer (NK) cells and monocytes. Siglec-7 is known to negatively regulate the innate immune system through Sia binding to distinguish self and nonself; however, a counter-receptor bearing its natural ligand remains largely unclear. Here, we identified a counter-receptor of Siglec-7 using K562 hematopoietic carcinoma cells presenting cell surface ligands for Siglec-7. We affinity-purified the ligands using Fc-ligated recombinant Siglec-7 and diSia-dextran polymer, a strong inhibitor for Siglec-7. We then confirmed the counter-receptor for Siglec-7 as leukosialin (CD43) through mass spectrometry, immunoprecipitation, and proximity labeling. Additionally, we demonstrated that the cytotoxicity of NK cells toward K562 cells was suppressed by overexpression of leukosialin in a Siglec-7-dependent manner. Taken together, our data suggest that leukosialin on K562 is a counter-receptor for Siglec-7 on NK cells and that a cluster of the Sia-containing glycan epitope on leukosialin is key as trans-ligand for unmasking the cis-ligand.

Recent Advances in the Chemical Biology of N-Glycans

Asparagine-linked N-glycans on proteins have diverse structures, and their functions vary according to their structures. In recent years, it has become possible to obtain high quantities of N-glycans via isolation and chemical/enzymatic/chemoenzymatic synthesis. This has allowed for progress in the elucidation of N-glycan functions at the molecular level. Interaction analyses with lectins by glycan arrays or nuclear magnetic resonance (NMR) using various N-glycans have revealed the molecular basis for the recognition of complex structures of N-glycans. Preparation of proteins modified with homogeneous N-glycans revealed the influence of N-glycan modifications on protein functions. Furthermore, N-glycans have potential applications in drug development. This review discusses recent advances in the chemical biology of N-glycans.

Nucleotide Sugars in Chemistry and Biology

Nucleotide sugars have essential roles in every living creature. They are the building blocks of the biosynthesis of carbohydrates and their conjugates. They are involved in processes that are targets for drug development, and their analogs are potential inhibitors of these processes. Drug development requires efficient methods for the synthesis of oligosaccharides and nucleotide sugar building blocks as well as of modified structures as potential inhibitors. It requires also understanding the details of biological and chemical processes as well as the reactivity and reactions under different conditions. This article addresses all these issues by giving a broad overview on nucleotide sugars in biological and chemical reactions. As the background for the topic, glycosylation reactions in mammalian and bacterial cells are briefly discussed. In the following sections, structures and biosynthetic routes for nucleotide sugars, as well as the mechanisms of action of nucleotide sugar-utilizing enzymes, are discussed. Chemical topics include the reactivity and chemical synthesis methods. Finally, the enzymatic in vitro synthesis of nucleotide sugars and the utilization of enzyme cascades in the synthesis of nucleotide sugars and oligosaccharides are briefly discussed.

Structural advances of Siglecs: insight into synthetic glycan ligands for immunomodulation

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are transmembrane proteins of the immunoglobulin (Ig) superfamily predominantly expressed on the cells of our immune system. Siglecs recognize sialic acid via their terminal V-set domain. In mammals, sialic acid-terminated glycolipids and glycoproteins are the ligands of Siglecs, and the monomeric affinity of Siglecs for their sialic acid-containing ligands is weak. Significant efforts have been devoted toward the development of chemically modified sialoside ligands to target Siglecs with higher affinity and selectivity. In this review we discuss natural and synthetic sialoside ligands for each human Siglec, emphasizing the ligand binding determinants uncovered from recent advances in protein structural information. Potential therapeutic applications of these ligands are also discussed.

Enzymatic modular synthesis and microarray assay of poly-N-acetyllactosamine derivatives

A facile enzymatic modular assembly strategy for the preparative-scale synthesis of poly-N-acetyllactosamine (poly-LacNAc) glycans with varied lengths and designed sialylation and/or fucosylation patterns is described. These glycans were printed as a microarray to investigate their interactions with a panel of glycan binding proteins (GBPs). Binding affinities revealed that the avidity of GBPs could be largely affected by the length and the patterns of sialylation and fucosylation.

Fluorescence "Turn-on" Lectin Sensors Fabricated by Ligand-Assisted Labeling Probes for Detecting Protein-Glycoprotein Interactions

Elucidation of protein-protein interactions (PPIs) is often very challenging and yields complex and unclear results. Lectin-glycoprotein interactions are especially difficult to study due to the noncovalent nature of the interactions and inherently low binding affinities of proteins to glycan ligands on glycoproteins. Here, we report a "ligand-directed labeling probe (LLP)"-based approach to fabricate protein probes for elucidating protein-glycoprotein interactions. LLP was designed with dual photoactivatable groups for the introduction of an alkyne handle proximal to the carbohydrate-binding pocket of lectins, Ricinus communis agglutinin 120 (RCA₁₂₀) and recombinant human Siglec-2-Fc. In proof-of-principle studies, alkynylated lectins were conjugated with a photoreactive diazirine cross-linker and an environment-sensitive fluorophore, respectively, by the bioorthogonal click reaction. The modified RCA₁₂₀ or Siglec-2-Fc was used for detecting the interaction with the target glycoprotein in the solution or endogenously expressed glycoproteins on live HeLa cells. We anticipate that the fabrication of these protein probes will accelerate the discovery of novel PPIs.

Siglec-15: a potential regulator of osteoporosis, cancer, and infectious diseases

Siglec-15 is a member of the Siglec family of glycan-recognition proteins, primarily expressed on a subset of myeloid cells. Siglec-15 has been known to be involved in osteoclast differentiation, and is considered to be a potential therapeutic target for osteoporosis. Recent studies revealed unexpected roles of Siglec-15 in microbial infection and the cancer microenvironment, expanding the potential pathophysiological roles of Siglec-15. Chemical biology has advanced our understanding of the nature of Siglec-15 ligands, but the exact nature of Siglec-15 ligand depends on the biological context, leaving plenty of room for further exploration.