Murine Models of Heme Metabolism and Iron Recycling

描述(申请人提供)：人体内近三分之二的铁是以血红蛋白的形式存在的，每个红细胞中含有超过十亿个以血红素形式存在的铁原子。因此，不足为奇的是，遗传性或获得性的血红蛋白合成缺陷，包括地中海贫血、血红蛋白病、缺铁和炎症贫血，都是最常见的人类疾病。重要的是，每天在~3600亿红细胞中合成血红蛋白所需的~25毫克铁中的大部分(500万/秒)来自于网状内皮系统(RES)巨噬细胞分解老化的红细胞中的血红素铁的循环利用；在稳定状态下，饮食中的血红素和非血红素铁的吸收每天只对全身铁经济做出1-2毫克的贡献，其中高达三分之二被吸收为血红素铁。然而，尽管Res铁循环和肠道血红素吸收的重要性，巨噬细胞和肠道上皮细胞中血红素跨膜运输的分子途径在很大程度上仍然未知。这在很大程度上是由于识别哺乳动物细胞中的血红素特异性转运蛋白的技术困难，以及无法将这些发现转化为整个生物体。最近，我们的合作者利用线虫，绕过了哺乳动物细胞带来的几个技术障碍，确定了一种蛋白质，即血红素反应基因1(HRG-1)，它具有血红素转运体的功能。我们在体外和体内的初步数据有力地表明，哺乳动物HRG-1的同源基因HRG1参与了巨噬细胞铁循环、肠道血红素吸收以及红系和肝细胞内的血红素代谢。在这里，我们建议通过检测HRG1对巨噬细胞和肠道上皮细胞对血红素和铁的调节，以及对吞噬的红细胞和炎症的调节来研究HRG1在小鼠体内的功能，所有这些都调节全身的铁代谢，并在常见的营养性、获得性和遗传性贫血的发病机制中做出贡献。此外，我们还将直接测试HRG1对于 利用条件性基因打靶技术在小鼠体内实现巨噬细胞铁循环、肠道血红素吸收和红系体内铁代谢。我们希望这些集体成果将使我们对这些迄今鲜为人知的血红素和铁生物学方面有实质性的了解，这些方面对于我们对这些常见疾病的理解和合理治疗至关重要。

DESCRIPTION (provided by applicant): Nearly two-thirds of the body's iron endowment is in the form of hemoglobin and each red blood cell contains more than a billion iron atoms in the form of heme. Consequently, it is not surprising that inherited or acquired defects in hemoglobin synthesis, including the thalassemias, hemoglobinopathies, iron deficiency, and the anemia of inflammation are among the most prevalent human afflictions. Importantly, most of the ~25 mg of iron required to synthesize the hemoglobin in ~360 billion RBCs each day (5 million/sec) derives from recycling of heme-iron from aged RBCs catabolized by reticuloendothelial system (RES) macrophages; at steady state, dietary heme and non-heme iron absorption contribute only 1-2 mg/day to the systemic iron economy, and up to two-thirds of this is absorbed as heme-iron. Nevertheless, despite the importance of RES iron recycling and intestinal heme absorption, the molecular pathways of transmembrane heme transport in macrophages and intestinal epithelial cells have, for the most part, remained unknown. This is in large part due to technical difficulties in identifying heme-specific transporters in mammalian cells and the inability to translate these finding to whole organisms. Recently, using C. elegans, which bypasses several of the technical hurdles presented by mammalian cells, our collaborators identified a protein, heme-responsive gene 1 (hrg-1), which functions a heme transporter. Our preliminary in vitro and in vivo data strongly suggest that the mammalian ortholog of hrg-1, HRG1, is involved in macrophage iron recycling, intestinal heme absorption, as well as intraerythroid and hepatocellular heme metabolism. Here, we propose to investigate the in vivo function of Hrg1 in the mouse by examining its regulation in macrophages and intestinal epithelial cells in response to heme and iron, as phagocytosed red blood cells, as well inflammation, all of which modulate systemic iron metabolism and contribute to the pathogenesis of common nutritional, acquired, and inherited anemias. In addition, we will directly test the hypothesis that Hrg1 is essential for macrophage iron recycling, intestinal heme absorption, and intraerythroid iron metabolism using conditional gene targeting technology in the mouse. We expect that the collective results will yield substantial insight into these hitherto obscure aspects of heme and iron biology that are critical to our understanding and rational treatment of these common diseases.