Functional investigation of hemojuvelin in regulation of hepcidin expression

描述（由申请方提供）：遗传性血色病（HH）是一组异质性遗传性铁过载疾病，导致特定器官（包括肝脏、心脏、胸腺和胰腺）中的铁蓄积。受影响组织中过量的铁催化氧化损伤，导致肝硬化、肝癌、心肌病、糖尿病、低促性腺激素性功能减退症和关节炎。青少年血色素沉着症（JH）是HH的最严重形式。它是一种常染色体隐性遗传疾病，具有高遗传率，影响年轻的男女患者，并导致严重的临床并发症，通常发生在青少年和20岁出头。如果不治疗，JH是致命的。最近的一项研究确定了这种名为HFE 2的基因，它是大多数JH患者发病的原因。JH患者HFE 2的序列分析显示了许多纯合或复合杂合突变，包括错义、移码和无义突变。虽然发现氨基酸取代G320 V占约三分之二的病例，但这些突变的零星分布暗示JH是由于HFE 2功能丧失所致。HFE 2在铁稳态中的重要性已通过HFE 2破坏（HFE 2-/-）小鼠的研究得到证实，显示出与JH患者相似的严重铁过载表型。有趣的是，在HFE 2突变相关的JH患者和HHF-/-小鼠中均观察到肝hepcidin表达的严重抑制，表明HFE 2是hepcidin（一种中枢铁调节激素）的关键上游调节因子。铁调素主要在肝细胞中表达。由HFE 2编码的蛋白质血幼素（HJV）与小鼠中的两个排斥性引导分子（RGM）家族成员（RGMa和B）具有高度序列相似性。最近的一项研究发现，HJV是骨形态发生蛋白2和4（BMP 2和BMP 4）的共受体，并增强BMP诱导的铁调素表达。我们的初步结果表明，HJV是GPI锚定蛋白，经历部分自催化裂解，并通过与RGMa受体再生蛋白相互作用增加HEK 293细胞中的铁积累。我们的研究还表明，再生蛋白是所需的HJV从细胞中释放。我们建议：1）。确定再生蛋白在细胞HJV加工中的作用。2）。检查BMP和再生蛋白信号传导在HJV介导的hepcidin表达中的作用。3）。描述血清HJV在体内铁稳态中的作用。 关于再生蛋白和BMP信号传导在HJV介导的肝铁调素表达中的功能的知识可以导致理解身体铁稳态，以及铁过载疾病的新治疗。公共卫生相关性：铁是多种生物化学过程如呼吸、代谢和DNA合成所需的必需营养素，但当其过量时也是有毒的（Bothwell等人，1995年）。遗传性血色素沉着症（HH）是高加索人中最常见的遗传性疾病，是一组异质性的遗传性铁过载疾病（考克斯，1996; Hentze等人，2004年）。阐明HFE 2（青少年HH-引起基因）的作用（Papanikolaou等人，2004），将导致充分了解人体铁稳态的调节，以及更有效地治疗铁超载疾病。

DESCRIPTION (provided by applicant): Hereditary hemochromatosis (HH) is a heterogeneous group of inherited iron overload disorders leading to iron accumulation in specific organs, including the liver, heart, thymus, and pancreas. Excess iron in affected tissues catalyzes oxidative damage, resulting in cirrhosis, hepatoma, cardiomyopathy, diabetes, hypogonadotropic hypogonadism, and arthritis. Juvenile hemochromatosis (JH) is the most severe form of HH. It is an autosomal recessive disease with high penetrance that affects young patients of both sexes and leads to severe clinical complications typically in the teens and early 20s. If untreated, JH is lethal. A recent study identified the gene, named HFE2, which is responsible for the onset of disease in the majority of JH patients. Sequence analysis of HFE2 in JH patients revealed numerous homozygous or compound heterozygous mutations, including missense, frame shift, and nonsense mutations. Although the amino acid substitution G320V was found to account for about two-thirds of the cases, the sporadic distribution of these mutations hints that JH is due to the loss of HFE2 function. The importance of HFE2 in iron homeostasis has been confirmed by studies in HFE2-disrupted (Hjv-/-) mice, showing a severe iron overload phenotype similar to JH patients. Intriguingly, a severe depression of hepatic hepcidin expression was observed both in HFE2 mutation-related JH patients and in Hjv-/- mice, indicating that HFE2 is a key upstream regulator of hepcidin, a central iron regulatory hormone. Hepcidin is mainly expressed in liver hepatocytes. The protein encoded by HFE2, hemojuvelin (HJV), shares high sequence similarity to two repulsive guidance molecule (RGM) family members in mice (RGMa and b). A recent study found that HJV is a co-receptor for bone morphogenetic protein 2 and 4 (BMP2 and BMP4) and enhances BMP-induced hepcidin expression. Our preliminary results showed that HJV is a GPI-anchored protein, undergoes a partial autocatalytic cleavage and increases iron accumulation in HEK293 cells through its interaction with neogenin, a receptor for RGMa. Our study also indicates that neogenin is required for HJV release from the cells. We propose to: 1). Determine the role of neogenin in cellular HJV processing. 2). Examine the role of BMP and neogenin signaling in HJV-mediated hepcidin expression. 3). Characterize the role of serum HJV in body iron homeostasis. Knowledge gained regarding the function of neogenin and BMP signaling in HJV-mediated hepatic hepcidin expression could lead to understanding body iron homeostasis, as well as novel treatments for iron overload disorders. PUBLIC HEALTH RELEVANCE: Iron is an essential nutrient required for a variety of biochemical processes such as respiration, metabolism, and DNA synthesis, but also toxic when it is in excess (Bothwell et al., 1995). Hereditary hemochromatosis (HH), the most common inherited disease in Caucasians, is a heterogeneous group of inherited iron overload disorders (Cox, 1996; Hentze et al., 2004). Elucidation of the role of HFE2, the juvenile HH-causing gene (Papanikolaou et al., 2004), will lead to a full understanding of the regulation of body iron homeostasis as well as more effective cures for iron overload disorders.