Sialosides Containing 7-N-Acetyl Sialic Acid Are Selective Substrates for Neuraminidases from Influenza A Viruses

Synthetic Sialosides Terminated with 8-N-Substituted Sialic Acid as Selective Substrates for Sialidases from Bacteria and Influenza Viruses

Sialosides containing C8-modified sialic acids are challenging synthetic targets but potentially useful probes for diagnostic substrate profiling of sialidases and elucidating the binding specificity of sialic acid-interacting proteins. Here, we demonstrate efficient chemoenzymatic methods for synthesizing para-nitrophenol-tagged α2-3- and α2-6-linked sialyl galactosides containing C8-acetamido, C8-azido, or C8-amino derivatized N-acetylneuraminic acid (Neu5Ac). High-throughput substrate specificity studies showed that the C8-modification of sialic acid significantly changes its recognition by sialidases from humans, various bacteria, and different influenza A and B viruses. Sialosides carrying Neu5Ac with a C8-azido modification were generally well tolerated by all the sialidases we tested, whereas sialosides containing C8-acetamido-modified Neu5Ac were only cleaved by selective bacterial sialidases. In contrast, sialosides with C8-amino-modified Neu5Ac were cleaved by a combination of selective bacterial and influenza A virus sialidases. These results indicate that sialosides terminated with a C8-amino or C8-acetamido-modified sialic acid can be used with other sialosides for diagnostic profiling of disease-causing sialidase-producing pathogens.

Capsid virus-like particle display improves recombinant influenza neuraminidase antigen stability and immunogenicity in mice

Supplementing influenza vaccines with additional protective antigens such as neuraminidase (NA) is a promising strategy for increasing the breadth of the immune response. Here, we improved the immunogenicity and stability of secreted recombinant NA (rNA) tetramers by covalently conjugating them onto the surface of AP205 capsid virus-like particles (cVLPs) using a Tag/Catcher ligation system. cVLP display increased the induction of IgG2a subclass anti-NA antibodies, which exhibited cross-reactivity with an antigenically distant homologous NA. It also reduced the single dose rNA amounts needed for protection against viral challenge in mice, demonstrating a dose-sparing effect. Moreover, effective cVLP-display was achieved across different NA subtypes, even when the conjugation was performed shortly before administration. Notably, the rNA-cVLP immunogenicity was retained upon mixing or co-administering with commercial vaccines. These results highlight the potential of this approach for bolstering the protective immune responses elicited by influenza vaccines.

Enabling Chemoenzymatic Strategies and Enzymes for Synthesizing Sialyl Glycans and Sialyl Glycoconjugates

Sialic acids are fascinating negatively charged nine-carbon monosaccharides. Sialic acid-containing glycans and glycoconjugates are structurally diverse, functionally important, and synthetically challenging molecules. We have developed highly efficient chemoenzymatic strategies that combine the power of chemical synthesis and enzyme catalysis to make sialic acids, sialyl glycans, sialyl glycoconjugates, and their derivatives more accessible, enabling the efforts to explore their functions and applications. The Account starts with a brief description of the structural diversity and the functional importance of naturally occurring sialic acids and sialosides. The development of one-pot multienzyme (OPME) chemoenzymatic sialylation strategies is then introduced, highlighting its advantages in synthesizing structurally diverse sialosides with a sialyltransferase donor substrate engineering tactic. With the strategy, systematic access to sialosides containing different sialic acid forms with modifications at C3/4/5/7/8/9, various internal glycans, and diverse sialyl linkages is now possible. Also briefly described is the combination of the OPME sialylation strategy with bacterial sialidases for synthesizing sialidase inhibitors. With the goal of simplifying the product purification process for enzymatic glycosylation reactions, glycosphingolipids that contain a naturally existing hydrophobic tag are attractive targets for chemoenzymatic total synthesis. A user-friendly highly efficient chemoenzymatic strategy is developed which involves three main processes, including chemical synthesis of lactosyl sphingosine as a water-soluble hydrophobic tag-containing intermediate, OPME enzymatic extension of its glycan component with a single C18-cartridge purification of the product, followed by a facile chemical acylation reaction. The strategy allows the introduction of different sialic acid forms and diverse fatty acyl chains into the products. Gram-scale synthesis has been demonstrated. OPME sialylation has also been demonstrated for the chemoenzymatic synthesis of sialyl glycopeptides and in vitro enzymatic N-glycan processing for the formation of glycoproteins with disialylated biantennary complex-type N-glycans. For synthesizing human milk oligosaccharides (HMOs) which are glycans with a free reducing end, acceptor substrate engineering and process engineering strategies are developed, which involve the design of a hydrophobic tag that can be easily installed into the acceptor substrate to allow facile purification of the product from enzymatic reactions and can be conveniently removed in the final step to produce target molecules. The process engineering involves heat-inactivation of enzymes in the intermediate steps in multistep OPME reactions for the production of long-chain sialoside targets in a single reaction pot and with a single C18-cartridge purification process. In addition, a chemoenzymatic synthon strategy has been developed. It involves the design of a derivative of the sialyltransferase donor substrate precursor, which is tolerated by enzymes in OPME reactions, introduced to enzymatic products, and then chemically converted to the desired target structures in the final step. The chemoenzymatic synthon approach has been used together with the acceptor substrate engineering method in the synthesis of complex bacterial glycans containing sialic acids, legionaminic acids, and derivatives. The biocatalysts characterized and their engineered mutants developed by the Chen group are described, with highlights on synthetically useful enzymes. We anticipate further development of chemoenzymatic strategies and biocatalysts to enable exploration of the sialic acid space.

Isolation by multistep chromatography improves the consistency of secreted recombinant influenza neuraminidase antigens

Recombinant protein-based approaches are ideally suited for producing vaccine antigens that are not overly abundant in viruses, such as influenza neuraminidase (NA). However, obtaining sufficient quantities of recombinant viral surface antigens remains challenging, often resulting in the use of chimeric proteins with affinity tags that can invariably impact the antigen's properties. Here, we developed multistep chromatography approaches for purifying secreted recombinant NA (rNA) antigens that are derived from recent H1N1 and H3N2 viruses and produced using insect cells. Analytical analyses showed that these isolation procedures yielded homogenous tetrameric rNA preparations with consistent specific activities that were not possible from a common immobilized metal affinity chromatography purification procedure. The use of classical chromatography improved the rNA tetramer homogeneity by removing the requirement of the N-terminal poly-histidine affinity tag that was shown to promote higher order rNA oligomer formation. In addition, these procedures reduced the specific activity variation by eliminating the exposure to Ni2+ ions and imidazole, with the latter showing pH and NA subtype dependent effects. Together, these results demonstrate that purification by multistep chromatography improves the homogeneity of secreted rNAs and eliminates the need for affinity tag-based approaches that can potentially alter the properties of these recombinant antigens.