Polysialic acid (polySia) emerges as a novel regulator of microglia activity. We recently identified polysialylated proteins in the Golgi compartment of murine microglia that are released in response to inflammatory stimulation. Since exogenously added polySia is able to attenuate the inflammatory response, we proposed that the release of polysialylated proteins constitutes a mechanism for negative feedback regulation of microglia activation. Here, we demonstrate that translocation of polySia from the Golgi to the cell surface can be induced by calcium depletion of the Golgi compartment and that polysialylated proteins are continuously released for at least 24 h after the onset of inflammatory stimulation. The latter was unexpected, because polySia signals detected by immunocytochemistry are rapidly depleted. However, it indicates that the amount of released polySia is much higher than anticipated based on immunostaining. This may be crucial for microglial responses during traumatic brain injury (TBI), as we detected polySia signals in activated microglia around a stab wound in the adult mouse brain. In BV2 microglia, the putative polySia receptor Siglec-E is internalized during lipopolysaccharide (LPS)-induced activation and in response to polySia exposure, indicating interaction. Correspondingly, CRISPR/Cas9-mediated Siglec-E knockout prevents inhibition of pro inflammatory activation by exogenously added polySia and leads to a strong increase of the LPS response. A comparable increase of LPS-induced activation has been observed in microglia with abolished polySia synthesis. Together, these results indicate that the release of the microglia-intrinsic polySia pool, as implicated in TBI, inhibits the inflammatory response by acting as a trans-activating ligand of Siglec-E.

聚唾液酸（polySia）作为小胶质细胞活性的新型调节剂出现。我们最近在鼠小神经胶质细胞的高尔基体中发现了多唾液酸化蛋白，这些蛋白会响应炎症刺激而释放。由于外源添加的polySia能够减弱炎症反应，因此我们提出多唾液酸化蛋白的释放构成了小胶质细胞激活的负反馈调节机制。在这里，我们证明了polySia从高尔基体到细胞表面的移位可以通过高尔基体隔室的钙耗竭来诱导，并且多唾液酸化蛋白在炎症刺激发作后至少24 h连续释放。后者是出乎意料的，因为通过免疫细胞化学检测到的polySia信号会很快耗尽。然而，它表明释放的polySia的数量远高于基于免疫染色的预期。这可能是创伤性脑损伤（TBI）期间小胶质细胞反应的关键，因为我们在成年小鼠脑部刺伤周围的活化小胶质细胞中检测到polySia信号。在BV2小胶质细胞中，推定的polySia受体Siglec-E在脂多糖（LPS）诱导的激活过程中以及响应polySia暴露而被内化，表明相互作用。相应地，CRISPR / Cas9介导的Siglec-E敲除可防止外源添加的polySia对促炎性激活的抑制，并导致LPS反应的强烈增加。在已消除的polySia合成的小胶质细胞中，观察到了LPS诱导的激活的可比增加。一起，Siglec-E的反式激活配体。