Regulation of Iron Homeostasis by BMP Signaling

Hepcidin是一种关键的铁调节激素，控制铁输出蛋白铁转运蛋白的表达，在需要支持红细胞生成和其他基本功能时增加铁供应，但防止铁过量的毒性。异常低的hepcidin表达导致铁超载紊乱遗传性血色素沉着症，并有助于铁负荷贫血，如b-地中海贫血。过量的hepcidin有助于铁限制红细胞生成和贫血在许多慢性疾病。一个悬而未决的关键问题是肝脏如何感知体内的铁水平，以适当地协调hepcidin的表达。我们之前发现骨形态发生蛋白（BMP）6-SMAD信号通路是铁反应中hepcidin转录的中心调节因子。此外，在动物模型中，BMP6-SMAD通路的调节剂调节hepcidin的表达以治疗慢性疾病血色素沉着症和贫血。这些研究已经对血色素沉着病的病理生理学产生了重要的见解，并确定了BMP6-SMAD途径作为铁疾病的可行治疗靶点。然而，肝脏如何感知铁来调节BMP6-SMAD信号，从而调节hepcidin的产生，在很大程度上仍然是未知的。在上一个资助期内，我们发现肝内皮细胞是调控hepcidin产生的BMP6的关键来源。我们还建立了原代肝内皮细胞培养模型，证明肝内皮细胞中的BMP6转录受细胞内铁含量的控制。最后，我们将这种细胞培养模型与定量蛋白质组学和RNA-seq筛选相结合，确定了铁转运蛋白和转录因子途径，我们假设这些途径在铁响应中介导BMP6的产生，以控制hepcidin的表达和全身铁稳态中发挥关键作用。在Specific Aim I中，我们将使用原代肝内皮培养和遗传小鼠模型来确定特异性铁转运蛋白在控制肝内皮细胞铁含量和铁调节途径中控制BMP6表达的作用。在Specific Aim II中，我们将使用原代肝内皮细胞培养、染色质免疫沉淀、报告者测定、药理学方法和内皮条件敲除小鼠模型来确定候选转录因子途径的作用及其在铁响应中控制BMP6产生的激活机制。本项目的长期目标是了解BMP信号通路如何通过不同的信号调节hepcidin的表达和全身铁平衡，深入了解健康和疾病中铁稳态的生理和病理生理学，并最终开发新的治疗铁代谢紊乱的治疗策略。

Hepcidin is a key iron regulatory hormone that controls expression of the iron exporter ferroportin to increase the iron supply when needed to support erythropoiesis and other essential functions, but to prevent the toxicity of iron excess. Abnormally low hepcidin expression leads to the iron overload disorder hereditary hemochromatosis and contributes to iron loading anemias such as b-thalassemia. Excess hepcidin contributes to iron restricted erythropoiesis and anemia in a number of chronic diseases. A key unanswered question is how the liver senses iron levels in the body to appropriately coordinate hepcidin expression. We previously discovered that the bone morphogenetic protein (BMP)6-SMAD signaling pathway is a central regulator of hepcidin transcription in response to iron. Moreover, modulators of the BMP6-SMAD pathway regulate hepcidin expression to treat hemochromatosis and anemia of chronic disease in animal models. These studies have already yielded important insights into the pathophysiology of hemochromatosis and have identified the BMP6-SMAD pathway as a viable therapeutic target for iron disorders. However, it remains largely unknown how iron is sensed by the liver to regulate BMP6-SMAD signaling and thereby hepcidin production. In the last funding period, we discovered that liver endothelial cells are a key source for BMP6 in the regulation of hepcidin production. We also established a primary liver endothelial cell culture model and demonstrated that BMP6 transcription in liver endothelial cells is governed by intracellular iron content. Finally, we used this cell culture model in conjunction with quantitative proteomics and RNA-seq screens to identify iron transporters and transcription factor pathways that we hypothesize play a key role in mediating BMP6 production in response to iron to control hepcidin expression and systemic iron homeostasis. In Specific Aim I, we will use primary liver endothelial cultures and genetic mouse models to establish the role of specific iron transporters in controlling liver endothelial cell iron content and iron-regulated pathways to govern BMP6 expression. In Specific Aim II, we will use primary liver endothelial cell cultures, chromatin immunoprecipitation, reporter assays, pharmacologic approaches, and endothelial conditional knockout mouse models to determine the role of candidate transcription factor pathways and their mechanism of activation in controlling BMP6 production in response to iron. The long-term goals of this project are to understand how the BMP signaling pathway is modulated by different signals to regulate hepcidin expression and systemic iron balance, to gain insights into the physiology and pathophysiology of iron homeostasis in health and disease, and ultimately to develop new therapeutic strategies for treating disorders of iron metabolism.