Regulation of Iron Homeostasis by BMP Signaling

描述(申请人提供)：铁稳态受到严格调控，以提供这一生长和生存的关键元素，但防止铁过量的毒性。我们最近发现，骨形态发生蛋白(BMP)信号通路通过调节铁调节激素海普西丁的表达，在全身铁平衡中发挥重要作用。海普西丁是一种由肝脏分泌的可溶性蛋白质，它通过下调铁出口蛋白铁蛋白在细胞表面的表达来控制饮食中的铁吸收和体内储存的铁的释放。海普西丁缺乏，导致膳食铁的过度吸收和进行性组织铁沉积和功能障碍，是铁超负荷紊乱遗传性血色病的常见致病机制，其原因是编码海普西丁本身、血色沉着蛋白HFE、转铁蛋白受体2和血凝素(HJV)的基因突变。尽管HFe和转铁蛋白受体2调节HepCidin表达的机制(S)尚不清楚，但我们最近发现HJV是骨形态发生蛋白的共同受体，BMP-HJV-SMAD信号调节体内的Hepsidin表达和全身铁平衡。在这里，我们发现HJV与Bmp6结合，Bmp6-/-小鼠具有与Hjv-/-小鼠相似的血色素沉着表型，这表明Bmp6是HJV的关键内源性配体，在体内调节海普西丁的表达和铁的代谢是必要的。我们还发现，膳食铁调节Bmp6的表达与海普西丁的表达是一致的，提示调节Bmp6的表达可能是铁调节海普西丁表达的机制之一。最后，我们发现HFE-/-小鼠适当地增加了Bmp6的水平，但Bmp6下游信号靶标的表达不适当地低，这表明HFE可能与Bmp6-HJV-SMAD信号级联作用来调节海普西丁的表达。在特定的目标I中，我们将使用体外和体内方法来确定铁上调Bmp6表达的机制。在AIM II中，我们将使用碘化蛋白相互作用和Biacore结合分析来剖析允许Bmp6及其辅助受体HJV相互作用的蛋白质相互作用区域，以增强SMAD信号和海普西丁的表达。在目标III中，我们将使用生化分析、细胞培养模型、HFE-/-小鼠和HFE转基因小鼠来研究HFE是否与Bmp6-HJV-SMAD信号级联相互作用来调节海普西丁的表达。该项目的长期目标是了解BMP信号通路在调节全身铁稳态中的作用。希望这项工作能为治疗诸如慢性病贫血和血色素沉着症等铁代谢紊乱提供新的治疗策略。

DESCRIPTION (provided by applicant): Iron homeostasis is tightly regulated to provide this critical element for growth and survival, but to prevent the toxicity of iron excess. We have recently discovered that the bone morphogenetic protein (BMP) signaling pathway plays an important role in systemic iron balance by modulating expression of the iron regulatory hormone hepcidin. A soluble protein secreted by the liver, hepcidin acts by downregulating the cell- surface expression of the iron exporter ferroportin to control iron absorption from the diet and iron release from body stores. Hepcidin deficiency, which causes excessive dietary iron absorption and progressive tissue iron deposition and dysfunction, appears to be the common pathogenic mechanism underlying the iron overload disorder hereditary hemochromatosis due to mutations in the genes encoding hepcidin itself, the hemochromatosis protein HFE, transferrin receptor 2, and hemojuvelin (HJV). Although the mechanism(s) by which HFE and transferrin receptor 2 regulate hepcidin expression remain uncertain, we have recently discovered that HJV is a BMP co-receptor and that BMP-HJV-SMAD signals regulate hepcidin expression and systemic iron balance in vivo. Here, we show that HJV binds BMP6, and that Bmp6-/- mice have a hemochromatosis phenotype resembling Hjv-/- mice, suggesting that BMP6 a key endogenous ligand for HJV that is necessary for regulating hepcidin expression and iron metabolism in vivo. We also show that dietary iron modulates BMP6 expression concordantly with hepcidin expression, suggesting that regulation of BMP6 expression may be one mechanism by which iron modulates hepcidin expression. Finally, we show that Hfe-/- mice have appropriately increased Bmp6 levels, but inappropriately low expression of downstream signaling targets of Bmp6, suggesting that HFE may interact with the BMP6-HJV-SMAD signaling cascade to regulate hepcidin expression. In Specific Aim I, we will use in vitro and in vivo approaches to determine the mechanism by which iron upregulates BMP6 expression. In Aim II, we will use iodinated protein-interaction and Biacore binding assays to dissect the protein-interaction domains that allow BMP6 and its co-receptor HJV to interact to enhance SMAD signaling and hepcidin expression. In Aim III, we will use biochemical assays, cell culture models, Hfe-/- mice, and Hfe transgenic mice to investigate whether HFE interacts with the BMP6-HJV-SMAD signaling cascade to regulate hepcidin expression. The long-term goal of this project is to understand the role of the BMP signaling pathway in regulating systemic iron homeostasis. It is hoped that this work may lead to new therapeutic strategies for treating disorders of iron metabolism such as anemia of chronic disease and hemochromatosis.