Regulation of Iron Homeostasis by BMP Signaling

描述（由申请人提供）：铁体内平衡受到严格调节，以提供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hepcidin的表达在全身铁平衡中起重要作用。hepcidin是一种由肝脏分泌的可溶性蛋白，通过下调铁输出蛋白铁转运蛋白的细胞表面表达来控制铁从饮食中的吸收和铁从体内储存的释放。Hepcidin缺乏症，可导致过量的膳食铁吸收和进行性组织铁沉积和功能障碍，似乎是铁超载疾病遗传性血色素沉着症的常见致病机制，这是由于编码Hepcidin本身、血色素沉着蛋白HFE、转铁蛋白受体2和血青蛋白（HJV）的基因突变所致。尽管HFE和转铁蛋白受体2调节hepcidin表达的机制尚不清楚，但我们最近发现HJV是一种BMP共受体，BMP-HJV- smad信号调节hepcidin表达和体内全身铁平衡。在这里，我们发现HJV结合BMP6，并且BMP6 -/-小鼠具有与HJV -/-小鼠相似的血色素沉着症表型，这表明BMP6是HJV的关键内源性配体，是体内调节hepcidin表达和铁代谢所必需的。我们还发现，膳食铁调节BMP6表达与hepcidin表达一致，表明BMP6表达的调节可能是铁调节hepcidin表达的一种机制。最后，我们发现Hfe-/-小鼠Bmp6水平适度升高，但Bmp6下游信号靶点的表达不适当降低，这表明Hfe可能与Bmp6 - hjv - smad信号级联相互作用，调节hepcidin的表达。在Specific Aim 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结合分析来解剖允许BMP6及其共受体HJV相互作用以增强SMAD信号传导和hepcidin表达的蛋白相互作用结构域。在Aim 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信号级联相互作用以调节hepcidin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慢性贫血、血色素沉着症等铁代谢障碍提供新的治疗策略。

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.