Abstract
Microglia are tissue macrophages and mediators of innate immune responses in the brain. The protein-modifying glycan polysialic acid (polySia) is implicated in modulating microglia activity. Cultured murine microglia maintain a pool of Golgi-confined polySia, which is depleted in response to lipopolysaccharide (LPS)-induced activation. Polysialylated neuropilin-2 (polySia-NRP2) contributes to this pool but further polySia protein carriers have remained elusive. Here, we use organotypic brain slice cultures to demonstrate that injury-induced activation of microglia initiates Golgi-confined polySia expression in situ. An unbiased glycoproteomic approach with stem cell-derived microglia identifies E-selectin ligand-1 (ESL-1) as a novel polySia acceptor. Together with polySia-NRP2, polySia-ESL-1 is also detected in primary cultured microglia, in brain slice cultures and in phorbol ester-induced THP-1 macrophages. Induction of stem cell-derived microglia, activated microglia in brain slice cultures and THP-1 macrophages by LPS, but not interleukin-4, causes polySia depletion and, as shown for stem cell-derived microglia, a metalloproteinase-dependent release of polySia-ESL-1 and polySia-NRP2. Moreover, soluble polySia attenuates LPS-induced production of nitric oxide and proinflammatory cytokines. Thus, shedding of polySia-ESL-1 and polySia-NRP2 after LPS-induced activation of microglia and THP-1 macrophages may constitute a mechanism for negative feedback regulation. GLIA 2016 GLIA 2016;64:1314-1330.