Heme trafficking and recycling in iron metabolism

该提案的长期目标是确定铁代谢中血红素的运输和再循环。缺铁是世界上最常见的营养失调症。缺铁最常见的临床表现是由于红细胞（RBC）的血红蛋白化受损而导致的贫血。这是因为人体中超过70%的铁在血红蛋白生产中用作血红素。在健康的成年人中，大约90%的身体铁来自回收的血红素铁。每天，网状内皮系统（RES）巨噬细胞通过每秒摄取近500万个衰老或受损的红细胞来回收血红素铁;骨髓利用这种回收的铁来产生新的红细胞。因此，巨噬细胞铁再循环的遗传缺陷导致贫血。尽管血红素在铁循环中的重要性，但负责RES巨噬细胞中血红素运输和贩运的途径仍然知之甚少。在红细胞吞噬作用之后，铁通过胞质溶胶中的血红素加氧酶（HMOX 1）从血红素酶促释放。这种铁可以储存在铁蛋白（FTH 1）中，也可以通过膜铁转运蛋白（FPN 1）从细胞中输出，以重新用于骨髓中新的RBC生产。因此，HMOX 1，FPN 1和FTH 1的遗传破坏-血红素铁循环途径中的步骤-导致小鼠胚胎死亡。我们确定了第一个真核血红素进口商/转运，HRG 1，并表明它是必不可少的运输血红素从巨噬细胞吞噬溶酶体进入细胞质中的巨噬细胞。我们最近的研究表明，HRG 1缺陷的小鼠是可行的，即使他们不能回收血红素铁和积累10倍过量血红素巨噬细胞内。小鼠对血红素毒性具有耐受性，因为它们通过形成疟原虫色素将血红素隔离在溶酶体内，溶酶体变得比正常大10-100倍，疟原虫色素是迄今为止仅在吸血生物体中鉴定的大的多聚体血红素晶体，用于解毒血红素。血红素耐受性需要一个完全可操作的血红素降解途径，因为HMOX 1的单倍不足与HRG 1缺乏结合会导致围产期致死，这表明了一种合成致死相互作用。我们令人兴奋的结果表明，哺乳动物中存在一种以前未预料到的血红素解毒和耐受途径。本研究旨在揭示HRG 1介导的血红素耐受性的分子基础。我们将测试的假设，血红素耐受性是由血红素运输的HRG 1和它的遗传和物理相互作用与血红素降解和血红素贩运机制的其他组件的协调调节。我们将利用（a）遗传学方法来阐明疟原虫色素形成和血红素耐受性的调节机制;和（B）细胞生物学方法来确定HRG 1和HMOX 1之间的合成致死相互作用的机制。我们的目标是深入了解血红素运输和回收的分子基础及其在哺乳动物血红素耐受性中的作用。

The long-term goals of this proposal are to define heme transport and recycling in iron metabolism. Iron deficiency is the most common nutritional disorder in the world. The most common clinical manifestation of iron deficiency is anemia due to impaired hemoglobinization of red blood cells (RBCs). This is because over 70% of iron in the human body is used as heme in hemoglobin production. In healthy adults, about 90% of body iron is derived from recycled heme-iron. Each day, reticuloendothelial system (RES) macrophages recycle heme-iron by ingesting almost 5 million senescent or damaged RBCs per second; the bone marrow utilizes this recycled iron to produce new RBCs. As a consequence genetic defects in macrophage iron recycling result in anemia. Despite the importance of heme in iron recycling, the pathways responsible for heme transport and trafficking in RES macrophages remain poorly understood. After erythrophagocytosis, the iron is enzymatically released from heme by heme oxygenases (HMOX1) in the cytosol. This iron can either be stored in ferritin (FTH1) or exported out of the cell by ferroportin (FPN1) to be reutilized for new RBC production in the bone marrow. Consequently, genetic disruption in HMOX1, FPN1, and FTH1 – steps in the heme-iron recycling pathway – causes embryonic lethality in mice. We identified the first eukaryotic heme importer/transporter, HRG1 and showed that it is essential for transporting heme from the macrophage phagolysosome into the cytoplasm in macrophages. Our recent studies show that HRG1-deficient mice are viable even though they are incapable of recycling heme-iron and accumulate 10-fold excess heme within macrophages. The mice are tolerant to heme toxicity because they sequester heme inside lysosomes, which become 10-100 times larger than normal, by forming hemozoin - large multimeric heme crystals heretofore only identified in blood-feeding organisms to detoxify heme. Heme tolerance requires a fully-operational heme degradation pathway as haploinsufficiency of HMOX1 combined with HRG1 deficiency causes perinatal lethality, demonstrating a synthetic lethal interaction. Our exciting results suggest the existence of a previously unanticipated pathway for heme detoxification and tolerance in mammals. The studies in this proposal are designed to uncover the molecular basis of HRG1-mediated heme tolerance. We will test the hypotheses that heme tolerance is conferred by the coordinated regulation of heme transport by HRG1 and its genetic and physical interactions with heme degradation and additional components of the heme trafficking machinery. We will utilize (a) a genetic approach to elucidate the regulatory mechanisms of hemozoin formation and heme tolerance; and (b) a cell biological approach to identify the mechanisms for the synthetic lethal interactions between HRG1 and HMOX1. Our goals are to acquire a deep understanding for the molecular basis for heme trafficking and recycling and its role in heme tolerance in mammals.