Department of Biochemistry, the Vanderbilt Brain Institute, and the Kennedy Center for Human Development, Vanderbilt University School of Medicine, Nashville, Tennessee 37232, USA.
J Neurosci. 2012 Sep 19;32(38):13022-31. doi: 10.1523/JNEUROSCI.6350-11.2012.
During the development of the peripheral nervous system there is extensive apoptosis, and these neuronal corpses need to be cleared to prevent an inflammatory response. Recently, Jedi-1 and MEGF10, both expressed in glial precursor cells, were identified in mouse as having an essential role in this phagocytosis (Wu et al., 2009); however, the mechanisms by which they promote engulfment remained unknown. Both Jedi-1 and MEGF10 are homologous to the Drosophila melanogaster receptor Draper, which mediates engulfment through activation of the tyrosine kinase Shark. Here, we identify Syk, the mammalian homolog of Shark, as a signal transducer for both Jedi-1 and MEGF10. Syk interacted with each receptor independently through the immunoreceptor tyrosine-based activation motifs (ITAMs) in their intracellular domains. The interaction was enhanced by phosphorylation of the tyrosines in the ITAMs by Src family kinases (SFKs). Jedi association with Syk and activation of the kinase was also induced by exposure to dead cells. Expression of either Jedi-1 or MEGF10 in HeLa cells facilitated engulfment of carboxylated microspheres to a similar extent, and there was no additive effect when they were coexpressed. Mutation of the ITAM tyrosines of Jedi-1 and MEGF10 prevented engulfment. The SFK inhibitor PP2 or a selective Syk inhibitor (BAY 61-3606) also blocked engulfment. Similarly, in cocultures of glial precursors and dying sensory neurons from embryonic mice, addition of PP2 or knock down of endogenous Syk decreased the phagocytosis of apoptotic neurons. These results indicate that both Jedi-1 and MEGF10 can mediate phagocytosis independently through the recruitment of Syk.
在周围神经系统的发育过程中，存在广泛的细胞凋亡，这些神经元残骸需要被清除，以防止炎症反应。最近，在小鼠中发现，胶质前体细胞表达的 Jedi-1 和 MEGF10 在吞噬作用中具有重要作用（Wu 等人，2009 年）；然而，它们促进吞噬的机制仍不清楚。Jedi-1 和 MEGF10 都与果蝇 melanogaster 受体 Draper 同源，后者通过激活酪氨酸激酶 Shark 来介导吞噬作用。在这里，我们确定了 Shark 的哺乳动物同源物 Syk，作为 Jedi-1 和 MEGF10 的信号转导物。Syk 通过其细胞内结构域中的免疫受体酪氨酸基激活基序（ITAMs）与每个受体独立相互作用。Src 家族激酶（SFKs）对 ITAMs 中酪氨酸的磷酸化增强了这种相互作用。Jedi 与 Syk 的相互作用和激酶的激活也被暴露于死细胞所诱导。在 HeLa 细胞中表达 Jedi-1 或 MEGF10 都能促进羧基化微球的吞噬作用，并且当它们共表达时没有累加效应。Jedi-1 和 MEGF10 的 ITAM 酪氨酸突变阻止了吞噬作用。SFK 抑制剂 PP2 或选择性 Syk 抑制剂（BAY 61-3606）也阻断了吞噬作用。同样，在胚胎小鼠的胶质前体和死亡感觉神经元的共培养物中，添加 PP2 或敲低内源性 Syk 会减少凋亡神经元的吞噬作用。这些结果表明，Jedi-1 和 MEGF10 都可以通过招募 Syk 独立地介导吞噬作用。
