Evaluating Roles of Sialic Acid in Host-Pathogen Interactions
Creators: McCord, Kelli A
Publisher: name: University of Alberta Library
Year: 2024

<p>The surface of every cell in the human body is coated in a dense layer of sugars, technically referred to as glycans, which constitute a region of the cell known as the glycocalyx. Within this glycocalyx, there are nine fundamental monosaccharide building blocks that link together to form diverse glycan structures. The three most abundant types of glycan structures are N-glycans, O-glycans, and glycolipids, and one commonality among these major classes of glycans is that they often contain a sialic acid monosaccharide at the end of their glycan chain. Sialic acid is a very abundant, negatively charged sugar found in humans. Given its prevalence, negative charge, and terminal location on glycan chains, it serves many functions such as protein stabilization, cell-cell interactions, and host-pathogen interactions. This thesis delves into two very different functional roles of sialic acid in host-pathogen interactions, with a particular focus on the role of sialic acid in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) viral infection and immune cell regulation.<br>
SARS-CoV-2 was responsible for the global pandemic of 2019. Using mass spectrometry techniques, several sialic acid-containing glycans were found to bind to the virus. To evaluate how these sialylated glycans influenced SARS-CoV-2 infection, a pseudoviral assay was developed, and infection was monitored via flow cytometry. By altering sialic acid-containing glycans in the glycocalyx, differences in SARS-CoV-2 pseudoviral infection were observed. This revealed that sialic acid-containing glycolipids and N-glycans contribute to SARS-CoV-2 infection. Gaining a better understanding of SARS-CoV-2 infection marks the first step towards developing improved therapeutics. Given the role of sialic acid in infection, it emerges as a potential target for treatments aimed at mitigating the severity or transmissibility of the virus.<br>
The innate immune system acts as the initial defense against foreign pathogens, responding swiftly to infections. However, inappropriate responses can lead to autoimmune and inflammatory diseases. To balance this, immune cells express a range of activatory and inhibitory receptors. Among the inhibitory receptors, sialic acid-binding immunoglobulin-type lectins (Siglecs) are prominent. Using liposomal nanoparticles, Siglecs were co-ligated with various activatory receptors and immune cell activation was monitored by flow cytometry or Western blotting. More specifically, activation of cells given liposomes displaying only a ligand for an activatory receptor was compared to cells given liposomes displaying both a ligand for an activatory receptor and a ligand for the Siglec. This platform allowed us to determine that Siglecs can inhibit Fc receptors, toll-like receptor 4, and tumor necrosis factor receptor 1, however it was unable to stimulate G-protein coupled receptors. Using liposomal nanoparticles displaying a ligand for the G-protein coupled receptor, formyl peptide receptor 1, Siglec co-engagement did not abrogate formyl peptide receptor activation. These findings provide insight into the natural role of sialic acid in regulating immune cell activation through Siglecs. Moreover, the ability of these liposomes to shut down immune cell activation through Fc receptors, toll-like receptors, and tumor necrosis factor receptor 1, has therapeutic potential for diseases where these receptors are overactive.<br>
Overall, the work described in this thesis helps us to better understand the role of sialic acid in viral infection and immune cell regulation. </p>