Distribution and Evolutionary History of Sialic Acid Catabolism in the Phylum Actinobacteria

Sialic acids play essential roles in the physiology of humans and other metazoan animals, and microbial sialic acid catabolism (SAC) is one of the processes critical for pathogenesis. To date, microbial SAC is studied mainly in commensals and pathogens, while its distribution in free-living microbes and evolutionary pathway remain largely unexplored.

Explorations in a galaxy of sialic acids: a review of sensing horizons, motivated by emerging biomedical and nutritional relevance

Rationale for nutrition value and biodiagnostic requirements of sialic acids including Neu5Ac.

Insertions in the SARS-CoV-2 Spike N-Terminal Domain May Aid COVID-19 Transmission

Coronavirus disease 2019 (COVID-19) caused by SARS-CoV-2 is an ongoing pandemic that causes significant health/socioeconomic burden. Variants of concern (VOCs) have emerged which may affect transmissibility, disease severity and re-infection risk. Most studies focus on the receptor-binding domain (RBD) of the Spike protein. However, some studies suggest that the Spike N-terminal domain (NTD) may have a role in facilitating virus entry via sialic-acid receptor binding. Furthermore, most VOCs include novel NTD variants. Recent analyses demonstrated that NTD insertions in VOCs tend to lie close to loop regions likely to be involved in binding sialic acids. We extended the structural characterisation of these putative sugar binding pockets and explored whether variants could enhance the binding to sialic acids and therefore to the host membrane, thereby contributing to increased transmissibility. We found that recent NTD insertions in VOCs (i.e., Gamma, Delta and Omicron variants) and emerging variants of interest (VOIs) (i.e., Iota, Lambda, Theta variants) frequently lie close to known and putative sugar-binding pockets. For some variants, including the recent Omicron VOC, we find increases in predicted sialic acid binding energy, compared to the original SARS-CoV-2, which may contribute to increased transmission. We examined the similarity of NTD across a range of related Betacoronaviruses to determine whether the putative sugar-binding pockets are sufficiently similar to be exploited in drug design. Despite global sequence and structure similarity, most sialic-acid binding pockets of NTD vary across related coronaviruses. Typically, SARS-CoV-2 possesses additional loops in these pockets that increase contact with polysaccharides. Our work suggests ongoing evolutionary tuning of the sugar-binding pockets in the virus. Whilst three of the pockets are too structurally variable to be amenable to pan Betacoronavirus drug design, we detected a fourth pocket that is highly structurally conserved and could therefore be investigated in pursuit of a generic drug. Our structure-based analyses help rationalise the effects of VOCs and provide hypotheses for experiments. For example, the Omicron variant, which has increased binding to sialic acids in pocket 3, has a rather unique insertion near pocket 3. Our work suggests a strong need for experimental monitoring of VOC changes in NTD.

Molecular Recognition Insights of Sialic Acid Glycans by Distinct Receptors Unveiled by NMR and Molecular Modeling

All cells are decorated with a highly dense and complex structure of glycan chains, which are mostly attached to proteins and lipids. In this context, sialic acids are a family of nine-carbon acidic monosaccharides typically found at the terminal position of glycan chains, modulating several physiological and pathological processes. Sialic acids have many structural and modulatory roles due to their negative charge and hydrophilicity. In addition, the recognition of sialic acid glycans by mammalian cell lectins, such as siglecs, has been described as an important immunological checkpoint. Furthermore, sialic acid glycans also play a pivotal role in host–pathogen interactions. Various pathogen receptors exposed on the surface of viruses and bacteria are responsible for the binding to sialic acid sugars located on the surface of host cells, becoming a critical point of contact in the infection process. Understanding the molecular mechanism of sialic acid glycans recognition by sialic acid-binding proteins, present on the surface of pathogens or human cells, is essential to realize the biological mechanism of these events and paves the way for the rational development of strategies to modulate sialic acid-protein interactions in diseases. In this perspective, nuclear magnetic resonance (NMR) spectroscopy, assisted with molecular modeling protocols, is a versatile and powerful technique to investigate the structural and dynamic aspects of glycoconjugates and their interactions in solution at the atomic level. NMR provides the corresponding ligand and protein epitopes, essential for designing and developing potential glycan-based therapies. In this review, we critically discuss the current state of knowledge about the structural features behind the molecular recognition of sialic acid glycans by different receptors, naturally present on human cells or pathogens, disclosed by NMR spectroscopy and molecular modeling protocols.

Siglecs as Therapeutic Targets in Cancer

Hypersialylation is a common post-translational modification of protein and lipids found on cancer cell surfaces, which participate in cell-cell interactions and in the regulation of immune responses. Sialic acids are a family of nine-carbon α-keto acids found at the outermost ends of glycans attached to cell surfaces. Given their locations on cell surfaces, tumor cells aberrantly overexpress sialic acids, which are recognized by Siglec receptors found on immune cells to mediate broad immunomodulatory signaling. Enhanced sialylation exposed on cancer cell surfaces is exemplified as “self-associated molecular pattern” (SAMP), which tricks Siglec receptors found on leukocytes to greatly down-regulate immune responsiveness, leading to tumor growth. In this review, we focused on all 15 human Siglecs (including Siglec XII), many of which still remain understudied. We also highlighted strategies that disrupt the course of Siglec-sialic acid interactions, such as antibody-based therapies and sialic acid mimetics leading to tumor cell depletion. Herein, we introduced the central roles of Siglecs in mediating pro-tumor immunity and discussed strategies that target these receptors, which could benefit improved cancer immunotherapy.

Studies and Application of Sialylated Milk Components on Regulating Neonatal Gut Microbiota and Health

Breast milk is rich in sialic acids (SA), which are commonly combined with milk oligosaccharides and glycoconjugates. As a functional nutrient component, SA-containing milk components have received increasing attention in recent years. Sialylated human milk oligosaccharides (HMOs) have been demonstrated to promote the growth and metabolism of beneficial gut microbiota in infants, bringing positive outcomes to intestinal health and immune function. They also exhibit antiviral and bacteriostatic activities in the intestinal mucosa of new-borns, thereby inhibiting the adhesion of pathogens to host cells. These properties play a pivotal role in regulating the intestinal microbial ecosystem and preventing the occurrence of neonatal inflammatory diseases. In addition, some recent studies also support the promoting effects of sialylated HMOs on neonatal bone and brain development. In addition to HMOs, sialylated glycoproteins and glycolipids are abundant in milk, and are also critical to neonatal health. This article reviews the current research progress in the regulation of sialylated milk oligosaccharides and glycoconjugates on neonatal gut microbiota and health.

Genetic Removal of Terminal Sialic Acid Residues from the O-Linked Glycans Adjacent to the HPA-9b Polymorphism of Platelet Membrane Glycoprotein IIb Improves the Binding and Detection of HPA-9b Patient Alloantibodies

Sialic acids occupy the terminal position of glycan chains, and have the potential to influence the antigenicity of glycoproteins. Antibody binding sites on a glycoprotein can be solely protein in nature, or can include or be affected by nearby glycan chains, which may either mask the epitope, or conversely comprise part of the antibody binding site. The polymorphisms responsible for formation of the Human Platelet Alloantigens (HPA)-3 (Ile843Ser) and HPA-9 (Val837Met) are next to each other near the C-terminus of the extracellular domain of platelet membrane glycoprotein (GP)IIb, and are adjacent to sialyl-Core 1 O-glycans that emanate from serines 845 and 847. Previous studies have shown that these O-linked glycans are required to support the binding of some of HPA-3a alloantibodies. Loss of these glycans, especially terminal sialic acid residues, during platelet storage or preparation, can present major difficulties in detecting clinically important anti-HPA-3a alloantibodies in suspected cases of fetal/neonatal alloimmune thrombocytopenia (FNAIT). Similarly, detection and identification of anti-HPA-9b alloantibodies from FNAIT patient sera can also be extremely challenging, resulting in the inability to resolve clinical cases of this bleeding disorder. Whether the nearby O-glycans on serines 845 and 847 of GPIIb affect the antigenicity of HPA-9b, and/or influence the binding of anti-HPA-9b alloantibodies in clinically significant cases of FNAIT is unknown. We previously reported the generation of bioengineered, HLA class I-negative, HPA-9a or -9b allele-specific megakaryocytes (MKs) from human induced pluripotent stem cells (iPSCs) that are suitable for whole-cell flow cytometric detection of anti-HPA-9b alloantibodies (Blood 2019;134(22):e1-e8). Unexpectedly, treatment of these allele-specific MKs with neuraminidase actually enhanced the binding of anti-HPA-9b alloantibodies, suggesting that terminal sialic acids on GPIIb partially mask the HPA-9b epitope. To test the hypothesis that removal of terminal sialic acids on nearby O-glycans, or removal of the entire O-glycan chains emanating from Ser845/847 of GPIIb, might enhance the detection of anti-HPA-9b patient alloantibodies, we created a series of deletion mutants in two major sialidases, ST3GAL1 and ST3GAL2, known to be responsible for transferring terminal sialic acid residues to Core 1 O-glycans, in our HPA-9a and -9b allele-specific iPSCs. Immunoprecipitation/western blot analysis confirmed the complete removal of terminal sialic acids on the O-glycan chains of GPIIb in ST3GAL1/2 knockout (KO) iPSC-derived MKs, as reported by the binding of the lectin PNA to the exposed Core 1 structure on GPIIb. These sialylation-deficient ST3GAL1/2 KO HPA-9b MKs exhibited dramatically increased anti-HPA-9b alloantibody binding, further confirming the notion that HPA-9b epitopes are partially masked by terminal sialic acids on nearby GPIIb O-glycan chains. Finally, allele-specific iPSCs lacking the complete O-glycan chains attached to serines 845 and 847 of GPIIb were generated by mutating those residues to alanines using a similar CRISPR/Cas9 gene editing approach. Interestingly, O-glycan chain-deficient Ala845/847 mutant MKs carrying the HPA-9b polymorphism exhibited slightly to moderately reduced binding of anti-HPA-9b alloantibodies, indicating that the presence of the Core 1 O-glycan chains attached to GPIIb serine residues 845 and 847 contribute to the presentation of the HPA-9b epitope - perhaps by stabilizing the conformation of the glycoprotein in this region. Taken together, these data suggest that detection of anti-HPA-9b alloantibodies may be enhanced through the use of iPSC-derived HPA-9b-specific MKs that have been genetically altered to lack nearby terminal sialic acid residues, but retain the glycan chains to which they are attached. Disclosures Curtis: Rallybio: Consultancy. Newman: Rallybio: Consultancy, Research Funding.