Roles of Neogenin and Matriptase-2 in Hemojuvelin-Mediated Hepcidin Expression

 描述(由申请人提供)：这项建议针对铁超载紊乱和铁限制贫血的分子基础，这两种疾病是世界上最常见的血液疾病之一。铁稳态或其紊乱的分子机制还不是很清楚。最近的研究发现，铁调节激素海普西丁是调节全身铁稳态的关键分子。海普西丁主要在肝细胞中表达，其表达受体内铁负荷的正向调节。血凝素(HJV)和基质金属蛋白酶-2(MT2)是海普西丁表达的一对关键调控因子。HJV是一种强大的诱导剂，而MT2通过将HJV裂解成非活性形式而成为一种基本的抑制因子。人类缺乏功能性HJV显著降低了海普西丁的表达，并导致幼年血色素沉着症，这是最严重的铁超载障碍形式。MT2基因突变不适当地增加了海普西丁的表达，并导致铁耐受性缺铁性贫血。HJV和MT2都在肝细胞中表达，并与新生素相互作用，新生蛋白是普遍表达的蛋白质。在转基因细胞中的研究强烈表明，新生素是HJV和MT2功能所必需的。然而，新生素在铁稳态中的作用尚不清楚。更重要的是，肝细胞对铁的敏感机制还知之甚少，它仍然是铁代谢领域的一个关键问题。长期目标是更好地了解全身铁稳态的机制。这一特殊应用的目的是表征新生素、HJV和MT2在调节海普西丁表达中的协调作用。我们的中心假设是，新生素和HJV构成了一个轴，以设定肝细胞中海普西丁的基础表达水平，而体内铁负荷负向调节MT2和肝细胞硫酸乙酰肝素蛋白多糖(HSPG)的脱落，从而间接地将海普西丁的表达调节到适当的水平。这一假设是根据申请者和其他实验室提供的数据提出的。这项拟议研究的基本原理是，了解铁对海普西丁表达的调节，有可能开发出治疗铁超载障碍和铁限制性贫血的新疗法。在强大的初步数据的指导下，这一假说将通过追求两个特定的目标来检验：1)确定肝细胞新生蛋白在海普西丁表达中的关键作用；2)确定MT2的铁调节是否作为关键的铁传感器来调节海普西丁的诱导表达。这种方法是创新的，因为它专注于在分子、细胞和系统水平上研究新生素、HJV、MT2和HSPG在铁调控的海普西丁表达中的机制。这项拟议的研究具有重要意义，因为它有望为药物策略的发展提供基础。这些研究的成功完成不仅将增加我们对全身性铁平衡机制的理解，也将为将这些进展转化为铁紊乱患者的切实好处奠定基础。

 DESCRIPTION (provided by applicant): This proposal targets the molecular basis of iron overload disorders and iron-restricted anemias, which are among the most common hematological diseases worldwide. The molecular mechanisms of iron homeostasis or its disorders are not well understood. Recent studies have identified the iron-regulatory hormone hepcidin as the key molecule responsible for the regulation of systemic iron homeostasis. Hepcidin is expressed predominantly in hepatocytes, and its expression is positively regulated by bodily iron load. Hemojuvelin (HJV) and matriptase-2 (MT2) are a pair of pivotal regulators for hepcidin expression. HJV is a robust inducer, whereas MT2 is an essential suppressor by cleaving HJV into inactive forms. Lack of functional HJV in humans markedly decreases hepcidin expression and causes juvenile hemochromatosis, the most severe form of iron overload disorders. Mutations of MT2 inappropriately increase hepcidin expression and lead to iron-refractory iron-deficiency anemia. Both HJV and MT2 are expressed in hepatocytes, and interact with neogenin, a ubiquitously expressed protein. Studies in transfected cells strongly indicate that neogenin is required for the function of HJV and MT2. However, the role of neogenin in iron homeostasis is not known. More importantly, the iron-sensing mechanism in hepatocytes is poorly understood, and it remains to be a key issue in the field of iron metabolism. The long-term goal is to better understand the mechanism of systemic iron homeostasis. The objective of this particular application is to characterize the coordination of neogenin, HJV, and MT2 in the regulation of hepcidin expression. Our central hypothesis is that neogenin and HJV constitute an axis to set the basal level of hepcidin expression in hepatocytes, and that bodily iron load negatively regulates MT2 and the shedding of hepatocyte heparan sulfate proteoglycans (HSPG) to indirectly adjust hepcidin expression to an appropriate level. This hypothesis has been formulated on the basis of the data produced by the applicants' and other laboratories. The rationale for the proposed research is that understanding the regulation of hepcidin expression by iron has the potential to develop new therapies for iron overload disorders and iron-restricted anemias. Guided by strong preliminary data, this hypothesis will be tested by pursuing two specific aims: 1) Determine the essential role of hepatocyte neogenin in hepcidin expression; 2) determine whether iron regulation of MT2 acts as a key iron sensor to modulate the induction of hepcidin expression. The approach is innovative, because it focuses on the mechanistic studies of neogenin, HJV, MT2, and HSPG in iron-regulated hepcidin expression at molecular, cellular and systemic levels. The proposed research is significant, because it is expected to provide the basis for the development of pharmacologic strategies. Successful completion of these studies will not only increase our understanding of systemic iron homeostatic mechanism but also lay the foundation for translating these advances into tangible benefits for patients with iron disorders.