Integrated chemoenzymatic synthesis of a comprehensive sulfated ganglioside glycan library to decipher functional sulfoglycomics and sialoglycomics.

Ganglioside glycans are ubiquitous and complex biomolecules that are involved in a wide range of biological functions and disease processes. Variations in sialylation and sulfation render the structural complexity and diversity of ganglioside glycans, and influence protein-carbohydrate interactions. Structural and functional insights into the biological roles of these glycans are impeded due to the limited accessibility of well-defined structures. Here we report an integrated chemoenzymatic strategy for expeditious and systematic synthesis of a comprehensive 65-membered ganglioside glycan library covering all possible patterns of sulfation and sialylation. This strategy relies on the streamlined modular assembly of three common sialylated precursors by highly stereoselective iterative sialylation, modular site-specific sulfation through flexible orthogonal protecting-group manipulations and enzymatic-catalysed diversification using three sialyltransferase modules and a galactosidase module. These diverse ganglioside glycans enable exploration into their structure-function relationships using high-throughput glycan microarray technology, which reveals that different patterns of sulfation and sialylation on these glycans mediate their unique binding specificities.

A universal glycoenzyme biosynthesis pipeline that enables efficient cell-free remodeling of glycans.

The ability to reconstitute natural glycosylation pathways or prototype entirely new ones from scratch is hampered by the limited availability of functional glycoenzymes, many of which are membrane proteins that fail to express in heterologous hosts. Here, we describe a strategy for topologically converting membrane-bound glycosyltransferases (GTs) into water soluble biocatalysts, which are expressed at high levels in the cytoplasm of living cells with retention of biological activity. We demonstrate the universality of the approach through facile production of 98 difficult-to-express GTs, predominantly of human origin, across several commonly used expression platforms. Using a subset of these water-soluble enzymes, we perform structural remodeling of both free and protein-linked glycans including those found on the monoclonal antibody therapeutic trastuzumab. Overall, our strategy for rationally redesigning GTs provides an effective and versatile biosynthetic route to large quantities of diverse, enzymatically active GTs, which should find use in structure-function studies as well as in biochemical and biomedical applications involving complex glycomolecules.

Site-selective sulfation of N-glycans by human GlcNAc-6-O-sulfotransferase 1 (CHST2) and chemoenzymatic synthesis of sulfated antibody glycoforms.

Sulfation is a common modification of glycans and glycoproteins. Sulfated N-glycans have been identified in various glycoproteins and implicated for biological functions, but in vitro synthesis of structurally well-defined full length sulfated N-glycans remains to be described. We report here the first in vitro enzymatic sulfation of biantennary complex type N-glycans by recombinant human CHST2 (GlcNAc-6-O-sulfotransferase 1, GlcNAc6ST-1). We found that the sulfotransferase showed high antennary preference and could selectively sulfate the GlcNAc moiety located on the Manα1,3Man arm of the biantennary N-glycan. The glycan chain was further elongated by bacterial ß1,4 galactosyltransferase from Neiserria meningitidis and human ß1,4 galactosyltransferase IV（B4GALT4）, which led to the formation of different sulfated N-glycans. Using rituximab as a model IgG antibody, we further demonstrated that the sulfated N-glycans could be efficiently transferred to an intact antibody by using a chemoenzymatic Fc glycan remodeling method, providing homogeneous sulfated glycoforms of antibodies. Preliminary binding analysis indicated that sulfation did not affect the apparent affinity of the antibody for FcγIIIa receptor.

Extracellular sialyltransferase st6gal1 in breast tumor cell growth and invasiveness.

The sialyltransferase ST6GAL1 that adds α2-6 linked sialic acids to N-glycans of cell surface and secreted glycoproteins is prominently associated with many human cancers. Tumor-native ST6GAL1 promotes tumor cell behaviors such as invasion and resistance to cell stress and chemo- and radio-treatments. Canonically, ST6GAL1 resides in the intracellular secretory apparatus and glycosylates nascent glycoproteins in biosynthetic transit. However, ST6GAL1 is also released into the extracellular milieu and extracellularly remodels cell surface and secreted glycans. The impact of this non-canonical extrinsic mechanism of ST6GAL1 on tumor cell pathobiology is not known. We hypothesize that ST6GAL1 action is the combined effect of natively expressed sialyltransferase acting cell-autonomously within the ER-Golgi complex and sialyltransferase from extracellular origins acting extrinsically to remodel cell-surface glycans. We found that shRNA knockdown of intrinsic ST6GAL1 expression resulted in decreased ST6GAL1 cargo in the exosome-like vesicles as well as decreased breast tumor cell growth and invasive behavior in 3D in vitro cultures. Extracellular ST6GAL1, present in cancer exosomes or the freely soluble recombinant sialyltransferase, compensates for insufficient intrinsic ST6GAL1 by boosting cancer cell proliferation and increasing invasiveness. Moreover, we present evidence supporting the existence novel but yet uncharacterized cofactors in the exosome-like particles that potently amplify extrinsic ST6GAL1 action, highlighting a previously unknown mechanism linking this enzyme and cancer pathobiology. Our data indicate that extracellular ST6GAL1 from remote sources can compensate for cellular ST6GAL1-mediated aggressive tumor cell proliferation and invasive behavior and has great clinical potential for extracellular ST6GAL1 as these molecules are in the extracellular space should be easily accessible targets.

Integrated Chemoenzymatic Approach to Streamline the Assembly of Complex Glycopeptides in the Liquid Phase.

Glycosylation of proteins is a complicated post-translational modification. Despite the significant progress in glycoproteomics, accurate functions of glycoproteins are still ambiguous owing to the difficulty in obtaining homogeneous glycopeptides or glycoproteins. Here, we describe a streamlined chemoenzymatic method to prepare complex glycopeptides by integrating hydrophobic tag-supported chemical synthesis and enzymatic glycosylations. The hydrophobic tag is utilized to facilitate peptide chain elongation in the liquid phase and expeditious product separation. After removal of the tag, a series of glycans are installed on the peptides via efficient glycosyltransferase-catalyzed reactions. The general applicability and robustness of this approach are exemplified by efficient preparation of 16 well-defined SARS-CoV-2 O-glycopeptides, 4 complex MUC1 glycopeptides, and a 31-mer glycosylated glucagon-like peptide-1. Our developed approach will open up a new range of easy access to various complex glycopeptides of biological importance.

Modulation of Siglec-7 Signaling Via In Situ-Created High-Affinity cis-Ligands.

Sialic acid-binding immunoglobulin-like lectins, also known as Siglecs, have recently been designated as glyco-immune checkpoints. Through their interactions with sialylated glycan ligands overexpressed on tumor cells, inhibitory Siglecs on innate and adaptive immune cells modulate signaling cascades to restrain anti-tumor immune responses. However, the elucidation of the mechanisms underlying these processes is just beginning. We find that when human natural killer (NK) cells attack tumor cells, glycan remodeling occurs on the target cells at the immunological synapse. This remodeling occurs through both the transfer of sialylated glycans from NK cells to target tumor cells and the accumulation of de novo synthesized sialosides on the tumor cells. The functionalization of NK cells with a high-affinity ligand of Siglec-7 leads to multifaceted consequences in modulating a Siglec-7-regulated NK-activation. At high levels of ligand, an enzymatically added Siglec-7 ligand suppresses NK cytotoxicity through the recruitment of Siglec-7 to an immune synapse, whereas at low levels of ligand an enzymatically added Siglec-7 ligand triggers the release of Siglec-7 from the cell surface into the culture medium, preventing a Siglec-7-mediated inhibition of NK cytotoxicity. These results suggest that a glycan engineering of NK cells may provide a means to boost NK effector functions for related applications.

Glycoengineering of NK Cells with Glycan Ligands of CD22 and Selectins for B-Cell Lymphoma Therapy.

CD22, a member of Siglec family of sialic acid binding proteins, has restricted expression on B cells. Antibody-based agents targeting CD22 or CD20 on B lymphoma and leukemia cells exhibit clinical efficacy for treating these malignancies, but also attack normal B cells leading to immune deficiency. Here, we report a chemoenzymatic glycocalyx editing strategy to introduce high-affinity and specific CD22 ligands onto NK-92MI and cytokine-induced natural killer cells to achieve tumor-specific CD22 targeting. These CD22-ligand modified cells exhibited significantly enhanced tumor cell binding and killing in vitro without harming healthy B cells. For effective lymphoma cell killing in vivo, we further functionalized CD22 ligand-modified NK-92MI cells with the E-selectin ligand sialyl Lewis X to promote trafficking to bone marrow. The dual-functionalized cells resulted in the efficient suppression of B lymphoma in a xenograft model. Our results suggest that natural killer cells modified with glycan ligands to CD22 and selectins promote both targeted killing of B lymphoma cells and improved trafficking to sites where the cancer cells reside, respectively.

hFUT1-Based Live-Cell Assay To Profile α1-2-Fucoside-Enhanced Influenza Virus A Infection.

Host cell surface glycans play critical roles in influenza virus A (IVA) infection ranging from modulation of IVA attachment to membrane fusion and host tropism. Approaches for quick and sensitive profile of viral avidity toward a specific type of host cell glycan can contribute to the understanding of tropism switching among different IVA strains. Here, we developed a method based on chemoenzymatic glycan engineering to investigate the possible involvement of α1-2-fucosides in IVA infections. Using a truncated human fucosyltransferase 1 (hFUT1), we created α1-2-fucosides in situ on host cells to assess their influence on the host cell binding to IVA hemagglutinin and the susceptibility of host cells toward IVA-induced killing. We discovered that the newly created α1-2-fucosides on host cells enhanced the infection of several human pandemic IVA subtypes either directly or indirectly. These findings suggest that glycan epitopes other than sialic acid should also be considered for assessing the human pandemic risk of this viral pathogen.

CUPRA-ZYME: An Assay for Measuring Carbohydrate-Active Enzyme Activities, Pathways, and Substrate Specificities.

Carbohydrate-Active enZymes (CAZymes) are involved in the synthesis, degradation, and modification of carbohydrates. They play critical roles in diverse physiological and pathophysiological processes, have important industrial and biotechnological applications, are important drug targets, and represent promising biomarkers for the diagnosis of a variety of diseases. Measurements of their activities, catalytic pathway, and substrate specificities are essential to a comprehensive understanding of the biological functions of CAZymes and exploiting these enzymes for industrial and biomedical applications. For glycosyl hydrolases a variety of sensitive and quantitative spectrophotometric techniques are available. However, measuring the activity of glycosyltransferases is considerably more challenging. Here, we introduce CUPRA-ZYME, a versatile and quantitative electrospray ionization mass spectrometry (ESI-MS) assay for measuring the kinetic parameters of CAZymes, monitoring reaction pathways, and profiling substrate specificities. The method employs the recently developed competitive universal proxy receptor assay (CUPRA), implemented in a time-resolved manner. Measurements of the hydrolysis kinetics of CUPRA substrates containing ganglioside oligosaccharides by the glycosyl hydrolase human neuraminidase 3 served to validate the reliability of kinetic parameters measured by CUPRA-ZYME and highlight its use in establishing catalytic pathways. Applications to libraries of substrates demonstrate the potential of the assay for quantitative profiling of the substrate specificities glycosidases and glycosyltransferases. Finally, we show how the comparison of the reactivity of CUPRA substrates and glycan substrates present on glycoproteins, measured simultaneously, affords a unique opportunity to quantitatively study how the structure and protein environment of natural glycoconjugate substrates influences CAZyme activity.

An automated platform for the enzyme-mediated assembly of complex oligosaccharides.

An automated platform that can synthesize a wide range of complex carbohydrates will greatly increase their accessibility and should facilitate progress in glycoscience. Here we report a fully automated process for enzyme-mediated oligosaccharide synthesis that can give easy access to different classes of complex glycans including poly-N-acetyllactosamine derivatives, human milk oligosaccharides, gangliosides and N-glycans. Our automated platform uses a catch and release approach in which glycosyltransferase-catalysed reactions are performed in solution and product purification is accomplished by solid phase extraction. We developed a sulfonate tag that can easily be installed and enables highly efficient solid phase extraction and product release using a single set of washing conditions, regardless of the complexity of the glycan. Using this custom-built synthesizer, as many as 15 reaction cycles can be performed in an automated fashion without a need for lyophilization or buffer exchange steps.

Streamlining the chemoenzymatic synthesis of complex N-glycans by a stop and go strategy.

Contemporary chemoenzymatic approaches can provide highly complex multi-antennary N-linked glycans. These procedures are, however, very demanding and typically involve as many as 100 chemical steps to prepare advanced intermediates that can be diversified by glycosyltransferases in a branch-selective manner to give asymmetrical structures commonly found in nature. Only highly specialized laboratories can perform such syntheses, which greatly hampers progress in glycoscience. Here we describe a biomimetic approach in which a readily available bi-antennary glycopeptide can be converted in ten or fewer chemical and enzymatic steps into multi-antennary N-glycans that at each arm can be uniquely extended by glycosyltransferases to give access to highly complex asymmetrically branched N-glycans. A key feature of our approach is the installation of additional branching points using recombinant MGAT4 and MGAT5 in combination with unnatural sugar donors. At an appropriate point in the enzymatic synthesis, the unnatural monosaccharides can be converted into their natural counterpart, allowing each arm to be elaborated into a unique appendage.