Regulation of Iron Homeostasis by BMP Signaling

描述（由申请人提供）：铁稳态受到严格调节，为生长和存活提供这种关键元素，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铁调素的表达在系统性铁平衡中起着重要作用。铁调素是一种由肝脏分泌的可溶性蛋白质，其通过下调铁输出蛋白膜铁转运蛋白的细胞表面表达来控制来自饮食的铁吸收和来自身体储存的铁释放。铁调素缺乏导致过量膳食铁吸收和进行性组织铁沉积和功能障碍，似乎是铁超负荷病症遗传性血色病的常见致病机制，这是由于编码铁调素本身、血色病蛋白HFE、转铁蛋白受体2和血幼素（HJV）的基因突变。尽管HFE和转铁蛋白受体2调节铁调素表达的机制仍不确定，但我们最近发现HJV是BMP共受体，并且BMP-HJV-SMAD信号调节体内铁调素表达和全身铁平衡。在这里，我们表明HJV与BMP 6结合，并且Bmp 6-/-小鼠具有类似于Hjv-/-小鼠的血色素沉着症表型，这表明BMP 6是HJV的关键内源性配体，对于调节体内铁调素表达和铁代谢是必需的。我们还表明，膳食铁调节BMP 6的表达与hepcidin的表达一致，这表明BMP 6的表达调节可能是铁调节hepcidin表达的一种机制。最后，我们发现Hfe-/-小鼠适当地增加了Bmp 6水平，但Bmp 6下游信号靶标的表达不适当地低，这表明HFE可能与BMP 6-HJV-SMAD信号级联相互作用以调节hepcidin表达。在特定目标I中，我们将使用体外和体内方法来确定铁上调BMP 6表达的机制。在目标II中，我们将使用碘化蛋白质相互作用和Biacore结合试验来解剖蛋白质相互作用结构域，这些结构域允许BMP 6及其共受体HJV相互作用以增强SMAD信号传导和铁调素表达。在目的III中，我们将使用生化测定、细胞培养模型、Hfe-/-小鼠和Hfe转基因小鼠来研究HFE是否与BMP 6-HJV-SMAD信号级联相互作用以调节铁调素表达。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。希望这项工作可能导致新的治疗策略，用于治疗铁代谢紊乱，如慢性病贫血和血色病。

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.