Chemoenzymatic Assembly of Bacterial Glycoconjugates for Site-Specific Orthogonal Labeling

Surface-modified bacteria: synthesis, functionalization and biomedical applications

The past decade has witnessed a great leap forward in bacteria-based living agents, including imageable probes, diagnostic reagents, and therapeutics, by virtue of their unique characteristics, such as genetic manipulation, rapid proliferation, colonization capability, and disease site targeting specificity. However, successful translation of bacterial bioagents to clinical applications remains challenging, due largely to their inherent susceptibility to environmental insults, unavoidable toxic side effects, and limited accumulation at the sites of interest. Cell surface components, which play critical roles in shaping bacterial behaviors, provide an opportunity to chemically modify bacteria and introduce different exogenous functions that are naturally unachievable. With the help of surface modification, a wide range of functionalized bacteria have been prepared over the past years and exhibit great potential in various biomedical applications. In this article, we mainly review the synthesis, functionalization, and biomedical applications of surface-modified bacteria. We first introduce the approaches of chemical modification based on the bacterial surface structure and then highlight several advanced functions achieved by modifying specific components on the surface. We also summarize the advantages as well as limitations of surface chemically modified bacteria in the applications of bioimaging, diagnosis, and therapy and further discuss the current challenges and possible solutions in the future. This work will inspire innovative design thinking for the development of chemical strategies for preparing next-generation biomedical bacterial agents.

Selective Exoenzymatic Labeling of Lipooligosaccharides of Neisseria gonorrhoeae with α2,6-Sialoside Analogues

The interactions between bacteria and their host often rely on recognition processes that involve host or bacterial glycans. Glycoengineering techniques make it possible to modify and study the glycans on the host's eukaryotic cells, but only a few are available for the study of bacterial glycans. Here, we have adapted selective exoenzymatic labeling (SEEL), a chemical reporter strategy, to label the lipooligosaccharides of the bacterial pathogen Neisseria gonorrhoeae, using the recombinant glycosyltransferase ST6Gal1, and three synthetic CMP-sialic acid derivatives. We show that SEEL treatment does not affect cell viability and can introduce an α2,6-linked sialic acid with a reporter group on the lipooligosaccharides by Western blot, flow cytometry and fluorescent microscopy. This new bacterial glycoengineering technique allows for the precise modification, here with α2,6-sialoside derivatives, and direct detection of specific surface glycans on live bacteria, which will aid in further unravelling the precise biological functions of bacterial glycans.

Recent Chemical and Chemoenzymatic Strategies to Complex-Type N-Glycans

Glycosylation is one of the major forms of protein post-translational modification. N-glycans attached to proteins by covalent bonds play an indispensable role in intercellular interaction and immune function. In human bodies, most of the cell surface glycoproteins and secreted glycopeptides are modified with complex-type N-glycans. Thus, for analytical or medicinal purposes, efficient and universal methods to provide homogeneous complex-type N-glycans have been an urgent need. Despite the extremely complicated structures, tremendous progress in the synthesis of N-glycans has been achieved. On one hand, chemical strategies are shown to be effective to prepare core oligosaccharides of N-glycans by focusing on stereoselective glycosylations such as β-mannosylation and α-sialylation, as well as the methodology of the N-glycan assembly. On the other hand, chemoenzymatic strategies have also become increasingly powerful in recent years. This review attempts to highlight the very recent advancements in chemical and chemoenzymatic strategies for eukaryotic complex-type N-glycans.

Sweet impersonators: Molecular mimicry of host glycans by bacteria

All bacteria display surface-exposed glycans that can play an important role in their interaction with the host and in select cases mimic the glycans found on host cells, an event called molecular or glycan mimicry. In this review, we highlight the key bacteria that display human glycan mimicry and provide an overview of the involved glycan structures. We also discuss the general trends and outstanding questions associated with human glycan mimicry by bacteria. Finally, we provide an overview of several techniques that have emerged from the discipline of chemical glycobiology, which can aid in the study of the composition, variability, interaction and functional role of these mimicking glycans.

Chemical Reporters for Exploring Microbiology and Microbiota Mechanisms

The advances made in bioorthogonal chemistry and the development of chemical reporters have afforded new strategies to explore the targets and functions of specific metabolites in biology. These metabolite chemical reporters have been applied to diverse classes of bacteria including Gram-negative, Gram-positive, mycobacteria, and more complex microbiota communities. Herein we summarize the development and application of metabolite chemical reporters to study fundamental pathways in bacteria as well as microbiota mechanisms in health and disease.

Chemoenzymatic Methods for the Synthesis of Glycoproteins

Glycosylation is one of the most prevalent posttranslational modifications that profoundly affects the structure and functions of proteins in a wide variety of biological recognition events. However, the structural complexity and heterogeneity of glycoproteins, usually resulting from the variations of glycan components and/or the sites of glycosylation, often complicates detailed structure-function relationship studies and hampers the therapeutic applications of glycoproteins. To address these challenges, various chemical and biological strategies have been developed for producing glycan-defined homogeneous glycoproteins. This review highlights recent advances in the development of chemoenzymatic methods for synthesizing homogeneous glycoproteins, including the generation of various glycosynthases for synthetic purposes, endoglycosidase-catalyzed glycoprotein synthesis and glycan remodeling, and direct enzymatic glycosylation of polypeptides and proteins. The scope, limitation, and future directions of each method are discussed.

Chemoenzymatic Synthesis and Applications of Prokaryote-Specific UDP-Sugars

This method describes the chemoenzymatic synthesis of several nucleotide sugars, which are essential substrates in the biosynthesis of prokaryotic N- and O-linked glycoproteins. Protein glycosylation is now known to be widespread in prokaryotes and proceeds via sequential action of several enzymes, utilizing both common and modified prokaryote-specific sugar nucleotides. The latter, which include UDP-hexoses such as UDP-diNAc-bacillosamine (UDP-diNAcBac), UDP-diNAcAlt, and UDP-2,3-diNAcManA, are also important components of other bacterial and archaeal glycoconjugates. The ready availability of these "high-value" intermediates will enable courses of study into inhibitor screening, glycoconjugate biosynthesis pathway discovery, and unnatural carbohydrate incorporation toward metabolic engineering.

Glycan-Based Cell Targeting To Modulate Immune Responses

Glycosylation is an integral post-translational modification present in more than half of all eukaryotic proteins. It affects key protein functions, including folding, stability, and immunogenicity. Glycoengineering approaches, such as the use of bacterial N-glycosylation systems, or expression systems, including yeasts, insect cells, and mammalian cells, have enabled access to defined and homogenous glycoproteins. Given that glycan structures on proteins can be recognized by host lectin receptors, they may facilitate cell-specific targeting and immune modulation. Myeloid C-type lectin receptors (CLRs) expressed by antigen-presenting cells are attractive targets to shape immune responses. Multivalent glycan display on nanoparticles, liposomes, or dendrimers has successfully enabled CLR targeting. In this review, we discuss novel strategies to access defined glycan structures and highlight CLR targeting approaches for immune modulation.

Design and Synthesis of Nanofibers of Self-assembled de novo Glycoconjugates towards Mucosal Lining Restoration and Anti-Inflammatory Drug Delivery

The medical practice for IBD is solely based on anti-inflammatory drugs, but the outcome is far from ideal. Our long-term research goal is to seek a better clinical outcome by combining the anti-inflammatory therapy with physical mucus layer restoration. As the first step towards that objective, we choose to develop self-assembled hydrogels of de novo glycoconjugates that consist of anti-inflammatory drugs and glycopeptides. By covalently linking peptides (e.g., nap-phe-phe-lys), saccharides (e.g., glucosamine), and an anti-inflammatory drug (i.e., olsalazine), we have demonstrated that the obtained molecules self-assemble in water to form hydrogels composed of 3D networks of the nanofibers under acidic conditions. We also confirmed that the resulting glycoconjugates are cell compatible. However, the preliminary assessment of the efficacy of the hydrogels on the murine model is inconclusive, which warrants further investigation and molecular engineering.

Synthesis of a suite of click-compatible sugar analogs for probing carbohydrate metabolism

Metabolic labeling based on the click chemistry between alkynyl and azido groups offers a powerful tool to study the function of carbohydrates in living systems, including plants. Herein, we describe the chemical synthesis of six alkynyl-modified sugars designed as analogs to D-glucose, D-mannose, L-rhamnose and sucrose present in plant cell walls. Among these new alkynyl probes, four of them are the 6-deoxy-alkynyl analogs of the corresponding sugars and do not possess any 6-OH groups. The other two are based on a new structural design, in which an ethynyl group is incorporated at the C-6 position of the sugar and the 6-OH group remains. The synthetic routes for both types of probes share common aldehyde intermediates, which are derived from the corresponding 6-OH precursor with other hydroxy groups protected. The overall synthesis sequence of these probes is efficient, concise, and scalable.

Probing Polytopic Membrane Protein-Substrate Interactions by Luminescence Resonance Energy Transfer

Integral membrane proteins play essential roles in all living systems; however, major technical hurdles challenge analyses of this class of proteins. Biophysical approaches that provide structural information to complement and leverage experimentally determined and computationally predicted structures are urgently needed. Herein we present the application of luminescence resonance energy transfer (LRET) for investigating the interactions of the polytopic membrane-bound oligosaccharyl transferases (OTases) with partner substrates. Monomeric OTases, such as the PglBs from Campylobacter jejuni and Campylobacter lari, catalyze transfer of glycans from membrane-associated undecaprenol diphosphate-linked substrates to proteins in the bacterial periplasm. LRET-based distance measurements are enabled by the inclusion of an encoded N-terminal lanthanide-binding tag (LBT), and LRET between the luminescent (LBT)-Tb(3+) donor complex and fluorescently labeled peptide and glycan substrates provides discrete distance measurements across the span of the membrane. LRET-based measurements of detergent-solubilized PglB from C. lari allowed direct comparison with the distances based on the previously reported the C. lari PglB crystal structure, thereby validating the approach in a defined system. Distance measurements between peptide and glycan substrates and the C. jejuni PglB offer new experimental information on substrate binding to the related, but structurally uncharacterized, eukaryotic OTase.