脑内的铁,转铁蛋白及转铁蛋白受体

脑铁异常增高可能参与脑神经变性疾病的发生发展。这一发现使得脑铁代谢成为近年广为关注和研究较为广泛的领域。本文综述了这一领域某些方面的目前认识。包括：（１）脑铁分布及功能;（２）铁转铁蛋白及转铁蛋白受体在脑内的合成与分布;（３）脑铁摄取和运输。此外,对铁与某些金属离子,转的蛋白和转铁蛋白受体与脑神经变性疾病的关系,以及转铁蛋白受体内吞在生物大分子跨血脑屏障运输中的作用也作了简要讨论。     

24p3/NGAL介导的铁转运途径

生物体内存在另一转铁途径。脂笼蛋白(lipocalin)家族的成员24p3／NGAL介导铁向细胞内转运,并在具有酸性环境的核内体(endosome)中与铁解离,进而调节铁蛋白(ferritin,Fn)基因和转铁蛋白受体-1(transferrin receptor-1,TfR-1)基因的表达。24p3／NGA转铁途径在亚细胞水平上与转铁蛋白(tansferrin,Tf)类似但相互独立。在胚肾发育过程中,24p3／NGAL与Tf介导的铁转运途径为不同时期的原始肾上皮细胞生长与分化所必需。深入研究24p3／NGAL转铁途径的分子机制及与Tf的异同有十分重要的意义。     

脑铁代谢及调控

随着研究的深入,脑铁代谢相关分子突变引起的疾病越来越多的被人们所认识。脑铁代谢紊乱可能是神经退行性疾病的发病原因之一。对脑铁代谢机理的认识将为预防和治疗脑铁代谢紊乱相关疾病提供重要的理论根据。对脑铁代谢的过程,脑铁代谢的相关分子以及这些分子对脑内铁稳态的调控作用作一介绍。     

低氧与铁代谢相关蛋白

氧和铁这两种元素对生命活动十分重要. 低氧诱导因子(hypoxia-inducible factors, HIFs)作为转录因子,参与一系列靶基因的表达调控以适应低氧. 铁参与 DNA合成、氧气运输、代谢反应等多种细胞活动,过量游离铁会通过Haber-Weiss或 Fenton反应产生毒性自由基. 细胞通过与铁吸收、存储和利用有关的多种铁代谢相 关蛋白之间的协同作用来维持铁稳态. 与铁稳态相关的一些基因是HIFs的靶基因或 者间接受低氧调控,包括转铁蛋白、转铁蛋白受体、二价金属转运体1、铁调素、膜 铁转运蛋白、血浆铜蓝蛋白、铁蛋白等,而胞内铁浓度的改变能影响HIFs的表达. 本文就低氧与铁代谢相关蛋白的关系,尤其是低氧对铁代谢相关蛋白的调节作一综 述.     

低氧诱导因子对铁代谢的调节（英文）

低氧诱导因子(hypoxia-inducible factors,HIFs)是一类介导细胞内低氧反应的核转录复合体。HIF-α和HIF-β形成有功能的异质二聚体。哺乳动物中有HIF-1α、HIF-2α和HIF-3α。HIFs在铁代谢中发挥重要作用。受HIFs调节的铁代谢相关蛋白主要有二价金属转运蛋白1(divalent metal transporter 1,DMT1)、铁转出蛋白(ferroportin 1,FPN1)、十二指肠铁细胞色素b(duodenal cytochrome b,Dcytb)和转铁蛋白受体(transferrin receptor,Tf R)。铁调素(hepcidin)和铁调节蛋白(iron regulatory proteins,IRPs)是调节机体与细胞内铁代谢、维持铁稳态的重要因子,同样受到HIFs的调节。本文综述了HIFs对上述铁代谢相关蛋白的调节作用,以期为治疗铁代谢相关疾病提供可能的靶点。     

综述:金属离子代谢平衡失调与阿尔茨海默病早期发病机制

研究表明,脑内金属离子代谢失衡与阿尔茨海默病(AD)有关,但其机理尚需深入探讨．结合本实验室研究结果,作者对金属离子代谢紊乱与氧化应激,金属离子代谢紊乱与β-淀粉样蛋白、转铁蛋白和转铁蛋白受体、铁调节蛋白、二价金属离子转运体以及天然抗氧化剂通过调节金属离子代谢平衡缓解β-淀粉样蛋白的毒性和保护细胞的作用进行探讨．提出：铁、铜等金属离子缺乏可能主要与AD早期关系密切,而铁、铜等金属离子过载可能主要与AD后期损伤关系密切的学术观点．     

铁代谢蛋白在肾脏的表达与功能

最近的研究证实,肾小管细胞具有能力表达包括转铁蛋白受体1(transferrin receptor-1,TfR1)、二价金属离子转运蛋白1(divalent metal transporter-1,DMT1)、膜铁转运蛋白1(ferroportin-1,FPN1)、铁调节蛋白(iron regulatory protein,IRP)和铁调素(hepcidin,Hepc)在内的几乎所有铁代谢蛋白.这些蛋白质的存在以及相关研究显示肾脏可能具有排出多余铁的功能,因此对体铁平衡起有十分重要的作用.     

转铁蛋白受体及其在药物运输中的作用

血脑屏障的存在阻止了中枢神经系统疾病许多潜在治疗药物的通过.近年来主要利用脑毛细血管内皮细胞膜中的转运蛋白,如转铁蛋白受体、胰岛素受体等,将外源药物与这些受体的特异性抗体相连,通过受体介导的内吞作用将药物转运到脑组织中.转铁蛋白受体在抗癌药物定向运输及恶性肿瘤细胞基因治疗中的研究已经处于临床阶段.     

AMPA受体转运及其调控的分子机制

α-氨-3-羟基-5-甲基-4-异恶唑丙酸受体（α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors,AMPA receptors）介导中枢神经系统（CNS）绝大多数快兴奋性突触传递,在学习、记忆和认知等方面具有重要功能. 突触AMPA受体的数量、分布和亚基组成是调节突触传递强度的一个主要机制,与AMPA受体转运密切相关. 最新研究显示,异常的AMPA受体转运与阿尔茨海默病（Alzheimer’s disease,AD）、脆性X综合征(fragile X syndrome, FXS)等神经疾病有关. 本文主要针对AMPA受体转运及其调控的分子机制做一综述,以期为AD、FXS等神经疾病提供新的治疗靶点和途径.     

铁转运刺激因子研究进展

铁转运刺激因子 (stimulatorofFetransport,SFT)是近年新发现的一个重要的铁代谢蛋白。SFT是一种跨膜糖蛋白 ,含 6个跨膜区域 ,在第一个细胞内环中含有功能上十分重要的REIHE序列。它广泛分布于各组识 ,其主要功能是促进转铁蛋白结合铁和非转铁蛋白结合铁的转运。SFT的基因表达和功能发挥受铁的调控。遗传性血色素沉着病人的肝脏内SFTmRNA的表达显著增加 ,因而SFT超表达可能与遗传性血色素沉着病的形成有关     

脑铁代谢和神经变性性疾病

最近关于脑铁代谢研究的新成果,尤其是与脑铁转运、储存、调节相关的某些突变基因的发现,足以得出以下结论,即异常增高的脑铁至少是部份神经变性疾病的起始原因。研究显示,脑铁过量积聚主要是由于遗传性和非遗传性因素所引起的某些服铁代谢蛋白功能异常或表达失控。正是异常增高的脑铁触发一系列病理反应,最终导致神经为性性疾病病人服神经元死亡。本文简要叙述了目前对服铁分布、功能和脑铁代谢蛋白的认识,讨论了内铁转运机制以及服铁和神经变性性疾病之间的关系研究的新进展。     

铁自噬与铁死亡及其相关疾病

铁是血红素、线粒体呼吸链复合体和各种生物酶的重要辅助因子,参与氧气运输、氧化还原反应和代谢物合成等生物过程。铁蛋白(ferritin)是一种铁存储蛋白质,通过储存和释放铁来维持机体内铁平衡。铁自噬(ferritinophagy)作为一种选择性自噬方式,介导铁蛋白降解释放游离铁,参与细胞内铁含量的调控。适度铁自噬维持细胞内铁含量稳定,但铁自噬过度会释放出大量游离铁。通过芬顿 (Fenton)反应催化产生大量的活性氧(reactive oxygen species, ROS),发生脂质过氧化造成细胞受损。因此,铁自噬在维持细胞生理性铁稳态中发挥至关重要的作用。核受体共激活因子4 (nuclear receptor co-activator 4, NCOA4)被认为是铁自噬的关键调节因子,与铁蛋白靶向结合,并传递至溶酶体中降解释放游离铁,其介导的铁自噬构成了铁代谢的重要组成部分。最新研究表明,NCOA4受体内铁含量、自噬、溶酶体和低氧等因素的调控。NCOA4介导的铁蛋白降解与铁死亡(ferroptosis)有关。铁死亡是自噬性细胞死亡过程。铁自噬通过调节细胞铁稳态和细胞ROS生成,成为诱导铁死亡的上游机制,与贫血、神经退行性疾病、癌症、缺血/再灌注损伤与疾病的发生发展密切相关。本文针对NCOA4介导的铁自噬通路在铁死亡中的功能特征,探讨NCOA4在这些疾病中的作用,可能为相关疾病的治疗提供启示。     

Lactoferrin receptors in Gram-negative bacteria: Insights into the iron acquisition process

One component of the anti-microbial function of lactoferrin (Lf) is its ability to sequester iron from potential pathogens. To overcome this iron limitation, a number of gram-negative bacterial pathogens have developed a mechanism for acquiring iron directly from this host glycoprotein. This mechanism involves surface receptors capable of specifically binding Lf from the host, removing iron and transporting it across the outer membrane. The iron is then bound by a periplasmic iron-binding protein, FbpA, and transported into the cell via an inner membrane complex comprised of FbpB and FbpC. The receptor has been shown to consist of two proteins, LbpA and LbpB. LbpB is bilobed lipoprotein anchored to the outer membrane via fatty acyl groups attached to the N-terminal cysteine. LbpA is a homologue of siderophore receptors, which consist of an N-terminal plug and a C-terminal beta-barrel region. We propose that the receptor proteins, LbpA and LbpB, induce conformational changes in human Lf (hLf) that lower its affinity for iron that binding by FbpA can drive the transport across the outer membrane, a mechanism shared with transferrin (Tf) receptors. The interaction between the receptor proteins and Lf is quite extensive and has been previously studied by using chimeric proteins comprised of Lf & Tf. In an attempt to evaluate the role of FbpA in the transport process, a series of site-directed mutants of FbpA were prepared and used to replace the wild-type protein in the iron acquisition pathway. The mutations were made in the iron-binding and anion-binding ligands of FbpA and were designed to result in altered binding properties. Protein crystallography of the iron-bound form of the Q58L mutant protein revealed that it was in the open conformation with iron coordinated by Y195 and Y196 from the C-terminal domain but not by the other iron-liganding amino acids from the N-terminal domain, H9 and E57. Replacement of the native FbpA in Neisseria meningitidis with wild-type or mutant Haemophilus influenzae FbpAs resulted in a defect in growth on Tf or Lf, suggesting that there may be a barrier to functional expression of H. influenzae FbpAs in Neisseria meningitidis. Thus mutants of the N. meningitidis FbpA are being prepared to replace wild-type protein in order to test their ability to mediate transport from hLf.     

Receptor-mediated transcytosis of lactoferrin through the blood-brain barrier

Lactoferrin (Lf) is an iron-binding protein involved in host defense against infection and severe inflammation; it accumulates in the brain during neurodegenerative disorders. Before determining Lf function in brain tissue, we investigated its origin and demonstrate here that it crosses the blood-brain barrier. An in vitro model of the blood-brain barrier was used to examine the mechanism of Lf transport to the brain. We report that differentiated bovine brain capillary endothelial cells exhibited specific high (Kd = 37.5 nM; n = 90,000/cell) and low (Kd = 2 microM; n = 900,000 sites/cell) affinity binding sites. Only the latter were present on nondifferentiated cells. The surface-bound Lf was internalized only by the differentiated cell population leading to the conclusion that Lf receptors were acquired during cell differentiation. A specific unidirectional transport then occurred via a receptor-mediated process with no apparent intraendothelial degradation. We further report that iron may cross the bovine brain capillary endothelial cells as a complex with Lf. Finally, we show that the low density lipoprotein receptor-related protein might be involved in this process because its specific antagonist, the receptor-associated protein, inhibits 70% of Lf transport.     

Brain Iron Toxicity: Differential Responses of Astrocytes,Neurons, and Endothelial Cells

Iron accumulation or iron overload in brain is commonly associated with neurodegenerative disorders such as Parkinson’s and Alzheimer’s diseases, and also plays a role in cellular damage following hemorrhagic stroke and traumatic brain injury. Despite the brain’s highly regulated system for iron utilization and metabolism, these disorders often present following disruptions within iron metabolic pathways. Such dysregulation allows saturation of proteins involved in iron transport and storage, and may cause an increase in free ferrous iron within brain leading to oxidative damage. Not only do astrocytes, neurons, and brain endothelial cells serve unique purposes within the brain, but their individual cell types are equipped with distinct protective mechanisms against iron-induced injury. This review evaluates iron metabolism within the brain under homeostatic and pathological conditions and focuses on the mechanism(s) of brain cellular iron toxicity and differential responses of astrocytes, neurons, and brain vascular endothelial cells to excessive free iron. Special issue dedicated to Dr. Moussa Youdim. An erratum to this article can be found at     

Tumor necrosis factor-alpha increases lactoferrin transcytosis through the blood-brain barrier

Lactoferrin (Lf) is an iron-binding protein involved in host defense against infection and severe inflammation, which accumulates in the brain during neurodegenerative disorders. Prior to determining Lf function in pathological brain tissues, we investigated its transport through the blood-brain barrier (BBB) in inflammatory conditions. For this purpose, we used a reconstituted BBB model consisting of the coculture of bovine brain capillary endothelial cells (BBCECs) and astrocytes in the presence of tumor necrosis factor-alpha (TNF-alpha). As TNF-alpha can be either synthesized by brain glial cells or present in circulating blood, BBCECs were exposed to this cytokine at their luminal or abluminal side. We have been able to demonstrate that in the presence of TNF-alpha, whatever the type of exposure, BBCECs were activated and Lf transport through the activated BBCECs was markedly increased. Lf was recovered intact at the abluminal side of the cells, suggesting that increased Lf accumulation may occur in immune-mediated pathophysiology. This process was transient as 20 h later, cells were in a resting state and Lf transendothelial traffic was back to normal. The enhancement of Lf transcytosis seems not to involve the up-regulation of the Lf receptor but rather an increase in the rate of transendothelial transport.     

Bayesian inference for parameter estimation in lactoferrin-mediated iron transport across blood-brain barrier

BackgroundIn neurodegenerative diseases such as Alzheimer's and Parkinson's, excessive irons as well as lactoferrin (Lf), but not transferrin (Tf), have been found in and around the affected regions of the brain. These evidences suggest that lactoferrin plays a critical role during neurodegenerative diseases, although Lf-mediated iron transport across blood-brain barrier (BBB) is negligible compared to that of transferrin in normal condition. However, the kinetics of lactoferrins and lactoferrin-mediated iron transport are still unknown.MethodTo determine the kinetic rate constants of lactoferrin-mediated iron transport through BBB, a mass-action based ordinary differential equation model has been presented. A Bayesian framework is developed to estimate the kinetic rate parameters from posterior probability density functions. The iron transport across BBB is studied by considering both Lf- and Tf-mediated pathways for both normal and pathologic conditions.ResultsUsing the point estimates of kinetic parameters, our model can effectively reproduce the experimental data of iron transport through BBB endothelial cells. The robustness of the model and parameter estimation process are further verified by perturbation of kinetic parameters. Our results show that surge in high-affinity receptor density increases lactoferrin as well as iron in the brain.ConclusionsDue to the lack of a feedback loop such as iron regulatory proteins (IRPs) for lactoferrin, iron can transport to the brain continuously, which might increase brain iron to pathological levels and can contribute to neurodegeneration.General significanceThis study provides an improved understanding of presence of lactoferrin and iron in the brain during neurodegenerative diseases.     

The roles of iron in health and disease

Iron is vital for almost all living organisms by participating in a wide variety of metabolic processes, including oxygen transport, DNA synthesis, and electron transport. However, iron concentrations in body tissues must be tightly regulated because excessive iron leads to tissue damage, as a result of formation of free radicals. Disorders of iron metabolism are among the most common diseases of humans and encompass a broad spectrum of diseases with diverse clinical manifestations, ranging from anemia to iron overload and, possibly, to neurodegenerative diseases. The molecular understanding of iron regulation in the body is critical in identifying the underlying causes for each disease and in providing proper diagnosis and treatments. Recent advances in genetics, molecular biology and biochemistry of iron metabolism have assisted in elucidating the molecular mechanisms of iron homeostasis. The coordinate control of iron uptake and storage is tightly regulated by the feedback system of iron responsive element-containing gene products and iron regulatory proteins that modulate the expression levels of the genes involved in iron metabolism. Recent identification and characterization of the hemochromatosis protein HFE, the iron importer Nramp2, the iron exporter ferroportin1, and the second transferrin-binding and -transport protein transferrin receptor 2, have demonstrated their important roles in maintaining body's iron homeostasis. Functional studies of these gene products have expanded our knowledge at the molecular level about the pathways of iron metabolism and have provided valuable insight into the defects of iron metabolism disorders. In addition, a variety of animal models have implemented the identification of many genetic defects that lead to abnormal iron homeostasis and have provided crucial clinical information about the pathophysiology of iron disorders. In this review, we discuss the latest progress in studies of iron metabolism and our current understanding of the molecular mechanisms of iron absorption, transport, utilization, and storage. Finally, we will discuss the clinical presentations of iron metabolism disorders, including secondary iron disorders that are either associated with or the result of abnormal iron accumulation.