Lipopolysaccharide activates microglia via neuraminidase 1 desialylation of Toll-like Receptor 4

Most cell surface receptors are sialylated, i.e. have sialic acid as the terminal residue of their sugar chains, but can be desialylated by sialidases, such as neuraminidase 1 (Neu1). Desialylation by Neu1 can activate immune cells, such as neutrophils, macrophages and monocytes. We investigated the role of Neu1 in activation of microglia using BV-2 cells (a murine microglial cell line) by cytokine ELISAs, enzyme activity assays, antibody/lectin binding and proximity labelling. We found that lipopolysaccharide (LPS) activation caused an increase in Neu1 protein on the cell surface, and an increase in surface sialidase activity that was prevented by Neu1 knockdown. Moreover, LPS induced interleukin 6 (IL-6) and MCP-1 release, which was reduced by Neu1 knockdown and increased by Neu1 over-expression. Neu1 knockdown also prevented the maintenance of IL-6 release by microglia after LPS was removed. Sialidase treatment of the cells was sufficient to induce IL-6 release, prevented by inhibiting toll-like receptor 4 (TLR4). Neu1 was found in close proximity to TLR4 on the surface of cells, and LPS induced desialylation of TLR4 on the cell surface, prevented by Neu1 knockdown. Sialic acid-binding immunoglobulin-like lectin E was found to bind to TLR4 via sialic acid residues and inhibit IL-6 release by BV-2 cells. We conclude that LPS causes Neu1 to translocate to the cell surface, where it desialylates TLR4, releasing inhibitory sialic acid-binding immunoglobulin-like lectin E, enhancing and maintaining inflammatory activation of the microglia. Thus, sialylation is a potent regulator of microglial activation, and Neu1 may be a target to reduce activation of microglia.