The Department of Biochemistry and Pharmacology and Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Victoria 3010, Australia.
Institute of Molecular Medicine and Experimental Immunology (IMMEI), University Clinic Bonn, Rheinische Friedrich-Wilhelms-Universität, Venusberg Campus 1, 53127 Bonn, Germany.
J Cell Sci. 2023 Aug 1;136(15). doi: 10.1242/jcs.260912. Epub 2023 Aug 11.
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.
人血清白蛋白（HSA）由于与酸性内体中的新生 Fc 受体（FcRn 或 FCGRT）相互作用以及内化白蛋白的再循环，其循环半衰期较长。血管内皮细胞和固有免疫细胞被认为是体内 FcRn 介导的白蛋白动态平衡的最相关细胞。然而，对于 FcRn-白蛋白复合物在内皮细胞中的内吞转运过程知之甚少。为了研究生理相关的内皮细胞中 FcRn-白蛋白的转运，我们从血液内皮前体（称为血管外生内皮细胞，BOEC）中生成了原代人血管内皮细胞系。我们对 BOEC 中的内体系统进行了作图，并表明 BOEC 可通过液流型巨胞饮作用有效地内化荧光标记的 HSA。脉冲追踪研究表明，细胞内的 HSA 分子与酸性内体结构中的 FcRn 共定位，野生型 HSA（但不是非 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饮体的 FcRn 阳性小管中，然后这些小管被转运到质膜。这些发现确定了原代血管内皮细胞中 FcRn-白蛋白的转运途径，与白蛋白的动态平衡有关。
