Human serum albumin (HSA) has a long circulatory half-life owing, in part, to interaction with the neonatal Fc receptor (FcRn or FCGRT) in acidic endosomes and recycling of internalised albumin. Vascular endothelial and innate immune cells are considered the most relevant cells for FcRn-mediated albumin homeostasis in vivo. However, little is known about endocytic trafficking of FcRn-albumin complexes in primary human endothelial cells. To investigate FcRn-albumin trafficking in physiologically relevant endothelial cells, we generated primary human vascular endothelial cell lines from blood endothelial precursors, known as blood outgrowth endothelial cells (BOECs). We mapped the endosomal system in BOECs and showed that BOECs efficiently internalise fluorescently labelled HSA predominantly by fluid-phase macropinocytosis. Pulse-chase studies revealed that intracellular HSA molecules co-localised with FcRn in acidic endosomal structures and that the wildtype HSA, but not the non-FcRn-binding HSAH464Q mutant, was excluded from late endosomes and/or lysosomes. Live imaging revealed that HSA is partitioned into FcRn-positive tubules derived from maturing macropinosomes, which are then transported towards the plasma membrane. These findings identify the FcRn-albumin trafficking pathway in primary vascular endothelial cells, relevant to albumin homeostasis.

中文翻译：

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原代人血管内皮细胞的内体系统和白蛋白-FcRn 运输。

人血清白蛋白 (HSA) 具有较长的循环半衰期，部分原因是与酸性内涵体中的新生儿 Fc 受体（FcRn 或 FCGRT）相互作用以及内化白蛋白的再循环。血管内皮细胞和先天免疫细胞被认为是体内 FcRn 介导的白蛋白稳态最相关的细胞。然而，人们对原代人内皮细胞中 FcRn-白蛋白复合物的内吞运输知之甚少。为了研究生理相关内皮细胞中 FcRn-白蛋白的运输，我们从血液内皮前体细胞（称为血液生长内皮细胞（BOEC））中生成了原代人血管内皮细胞系。我们绘制了 BOEC 中的内体系统，并表明 BOEC 主要通过液相巨胞饮作用有效内化荧光标记的 HSA。脉冲追踪研究表明，细胞内 HSA 分子与 FcRn 共定位于酸性内体结构中，并且野生型 HSA（而非非 FcRn 结合的 HSAH464Q 突变体）被排除在晚期内体和/或溶酶体之外。实时成像显示，HSA 被分配到源自成熟巨胞质体的 FcRn 阳性小管中，然后被转运至质膜。这些发现确定了原代血管内皮细胞中与白蛋白稳态相关的 FcRn-白蛋白运输途径。