Iron is an essential cofactor for several metabolic processes, including the generation of ATP in mitochondria, which is required for axonal function and regeneration. However, it is not known how mitochondria in long axons, such as those in sciatic nerves, acquire iron in vivo. Because of their close proximity to axons, Schwann cells are a likely source of iron for axonal mitochondria in the PNS. Here we demonstrate the critical role of iron in promoting neurite growth in vitro using iron chelation. We also show that Schwann cells express the molecular machinery to release iron, namely, the iron exporter, ferroportin (Fpn) and the ferroxidase ceruloplasmin (Cp). In Cp KO mice, Schwann cells accumulate iron because Fpn requires to partner with Cp to export iron. Axons and Schwann cells also express the iron importer transferrin receptor 1 (TfR1), indicating their ability for iron uptake. In teased nerve fibers, Fpn and TfR1 are predominantly localized at the nodes of Ranvier and Schmidt-Lanterman incisures, axonal sites that are in close contact with Schwann cell cytoplasm. We also show that lack of iron export from Schwann cells in Cp KO mice reduces mitochondrial iron in axons as detected by reduction in mitochondrial ferritin, affects localization of axonal mitochondria at the nodes of Ranvier and Schmidt-Lanterman incisures, and impairs axonal regeneration following sciatic nerve injury. These finding suggest that Schwann cells contribute to the delivery of iron to axonal mitochondria, required for proper nerve repair.

SIGNIFICANCE STATEMENT This work addresses how and where mitochondria in long axons in peripheral nerves acquire iron. We show that Schwann cells are a likely source as they express the molecular machinery to import iron (transferrin receptor 1), and to export iron (ferroportin and ceruloplasmin [Cp]) to the axonal compartment at the nodes of Ranvier and Schmidt-Lanterman incisures. Cp KO mice, which cannot export iron from Schwann cells, show reduced iron content in axonal mitochondria, along with increased localization of axonal mitochondria at Schmidt-Lanterman incisures and nodes of Ranvier, and impaired sciatic nerve regeneration. Iron chelation in vitro also drastically reduces neurite growth. These data suggest that Schwann cells are likely to contribute iron to axonal mitochondria needed for axon growth and regeneration.

铁是多种代谢过程的重要辅助因子，包括线粒体中 ATP 的产生，这是轴突功能和再生所必需的。然而，目前尚不清楚长轴突（例如坐骨神经中的轴突）中的线粒体如何在体内获取铁。由于它们靠近轴突，雪旺细胞很可能是 PNS 中轴突线粒体的铁来源。在这里，我们证明了铁在体外使用铁螯合促进神经突生长的关键作用。我们还表明施万细胞表达释放铁的分子机制，即铁输出蛋白、铁转运蛋白 (Fpn) 和铁氧化酶铜蓝蛋白 (Cp)。在CpKO 小鼠，施万细胞积累铁，因为 Fpn 需要与 Cp 合作输出铁。轴突和雪旺氏细胞也表达铁输入转铁蛋白受体 1 (TfR1)，表明它们具有铁摄取能力。在被梳理的神经纤维中，Fpn 和 TfR1 主要位于 Ranvier 和 Schmidt-Lanterman 切口的节点，轴突部位与施万细胞质密切接触。我们还表明Cp 中雪旺细胞缺乏铁输出通过线粒体铁蛋白减少检测到，KO 小鼠减少轴突中的线粒体铁，影响 Ranvier 和 Schmidt-Lanterman 切口节点处轴突线粒体的定位，并损害坐骨神经损伤后的轴突再生。这些发现表明雪旺氏细胞有助于将铁输送到轴突线粒体，这是正确神经修复所必需的。

重要性声明这项工作解决了周围神经长轴突中线粒体获取铁的方式和位置。我们表明雪旺细胞是一个可能的来源，因为它们表达了输入铁（转铁蛋白受体 1）和输出铁（铁转运蛋白和铜蓝蛋白 [Cp]）到 Ranvier 和 Schmidt-Lanterman 切口节点处的轴突室的分子机制. Cp KO 小鼠不能从 Schwann 细胞输出铁，显示轴突线粒体中的铁含量降低，同时轴突线粒体在 Schmidt-Lanterman 切口和 Ranvier 节点的定位增加，坐骨神经再生受损。体外铁螯合也大大减少了神经突的生长。这些数据表明，雪旺氏细胞可能为轴突生长和再生所需的轴突线粒体提供铁。