Identification of global inhibitors of cellular glycosylation

Small molecule inhibitors of glycosylation enzymes can be valuable tools for dissecting glycan functions and present great potential as drug discovery candidates. Most screening campaigns for inhibitors of glycosyltransferases have been limited to in vitro enzyme assays using recombinant enzymes, which often have led to promising candidates that have been difficult to study further in cells and animals due to issues with permeability and toxicity. Here, we aimed to circumvent these issues by employing a cell-based screening assay using glycoengineered cells expressing tailored reporter glycoproteins, which allowed us to narrow in on specific steps in individual glycosylation pathways. We focused on GalNAc-type O-glycosylation, and initially selected the GalNAc-T11 isoenzyme that selectively initiates O-glycosylation of the endocytic low density lipoprotein receptor (LDLR)-related proteins as target. In our screen with a limited small molecule compound library, we did not identify selective inhibitors of GalNAc-T11 or the family of GalNAc-T isoenzymes, however we identified two potent compounds that broadly inhibited glycosylation processes in a reversible manner. We demonstrate that these compounds inhibit all Golgi-localized glycosylation and suggest that this is mediated by reversible fragmentation of the Golgi system, presumably via interactions with HSP90. Finally, we demonstrate how these compounds can be used in cell model studies to dissect roles of different types of glycosylation by showing the importance of Tn-expression for binding of cancer-specific antibodies and by validating further the importance of cellular heparan sulfate in the binding and infection of SARS-CoV-2