State Key Laboratory of Membrane Biology, School of Life Sciences, Tsinghua University, Beijing, 100084, China.
Institute for Immunology, Tsinghua-Peking Center for Life Sciences, School of Medicine, Tsinghua University, Beijing, 100084, China.
BMC Biol. 2022 Jun 2;20(1):126. doi: 10.1186/s12915-022-01332-0.
The heme group constitutes a major functional form of iron, which plays vital roles in various biological processes including oxygen transport and mitochondrial respiration. Heme is an essential nutrient, but its pro-oxidant nature may have toxic cellular effects if present at high levels, and its synthesis is therefore tightly regulated. Deficiency and excess of heme both lead to pathological processes; however, our current understanding of metazoan heme transport is largely limited to work in mammals and the worm Caenorhabditis elegans, while functional analyses of heme transport in the genetically amenable Drosophila melanogaster and other arthropods have not been explored.
We implemented a functional screening in Schneider 2 (S2) cells to identify putative heme transporters of D. melanogaster. A few multidrug resistance-associated protein (MRP) members were found to be induced by hemin and/or involved in heme export. Between the two plasma membrane-resident heme exporters CG4562 and CG7627, the former is responsible for heme transit across the intestinal epithelium. CG4562 knockdown resulted in heme accumulation in the intestine and lethality that could be alleviated by heme synthesis inhibition, human MRP5 (hMRP5) expression, heme oxygenase (HO) expression, or zinc supplement. CG4562 is mainly expressed in the gastric caeca and the anterior part of the midgut, suggesting this is the major site of heme absorption. It thus appears that CG4562 is the functional counterpart of mammalian MRP5. Mutation analyses in the transmembrane and nucleotide binding domains of CG4562 characterized some potential binding sites and conservative ATP binding pockets for the heme transport process. Furthermore, some homologs in Aedes aegypti, including that of CG4562, have also been characterized as heme exporters.
Together, our findings suggest a conserved heme homeostasis mechanism within insects, and between insects and mammals. We propose the fly model may be a good complement to the existing platforms of heme studies.
血红素基团构成了铁的主要功能形式，在包括氧运输和线粒体呼吸在内的各种生物过程中发挥着至关重要的作用。血红素是一种必需的营养物质，但如果其浓度过高，其具有的促氧化剂性质可能会对细胞产生毒性作用，因此其合成受到严格调控。血红素的缺乏和过量都会导致病理过程；然而，我们目前对后生动物血红素转运的理解在很大程度上局限于哺乳动物和线虫 Caenorhabditis elegans 的研究，而在遗传上易于操作的果蝇 Drosophila melanogaster 和其他节肢动物中血红素转运的功能分析尚未得到探索。
我们在 Schneider 2（S2）细胞中进行了功能筛选，以鉴定果蝇 D. melanogaster 的潜在血红素转运蛋白。发现一些多药耐药相关蛋白（MRP）成员被血红素诱导，并且参与血红素输出。在两个质膜驻留的血红素外排蛋白 CG4562 和 CG7627 中，前者负责血红素穿过肠上皮的转运。CG4562 的敲低导致血红素在肠内积累，并导致死亡，血红素合成抑制、人 MRP5（hMRP5）表达、血红素加氧酶（HO）表达或锌补充均可缓解这种情况。CG4562 主要在胃盲囊和中肠的前半部分表达，表明这是血红素吸收的主要部位。因此，CG4562 似乎是哺乳动物 MRP5 的功能对应物。对 CG4562 的跨膜和核苷酸结合结构域进行突变分析，确定了血红素转运过程中的一些潜在结合位点和保守的 ATP 结合口袋。此外，埃及伊蚊中的一些同源物，包括 CG4562，也被鉴定为血红素外排蛋白。
总之，我们的研究结果表明昆虫体内以及昆虫与哺乳动物之间存在保守的血红素稳态机制。我们提出，果蝇模型可能是血红素研究现有平台的良好补充。
