Regulation of Iron Homeostasis by BMP Signaling

海普西丁是一种关键的铁调节激素，它控制铁出口蛋白的表达，以在需要时增加铁的供应，以支持红细胞生成和其他基本功能，但防止铁过量的毒性。海普西丁的异常低表达会导致铁超载紊乱，遗传性血色素沉着症，并导致铁负荷贫血，如b-地中海贫血。在许多慢性病中，过量的海普西丁会导致铁限制的红细胞生成和贫血。一个关键的悬而未决的问题是，肝脏如何感知体内的铁水平，以适当地协调海普西丁的表达。我们先前发现，骨形态发生蛋白(BMP)6-SMAD信号通路是铁诱导的海普西丁转录的中心调节因子。此外，Bmp6-SMAD途径的调节物调节海普西丁的表达，以治疗动物模型中的血色素沉着症和慢性病贫血。这些研究已经对血色素沉着症的病理生理学产生了重要的见解，并确认Bmp6-SMAD通路是铁紊乱的一个可行的治疗靶点。然而，目前尚不清楚铁是如何被肝脏感知以调节Bmp6-SMAD信号从而产生海普西丁的。在上一个资助阶段，我们发现肝内皮细胞是Bmp6在调节海普西丁产生过程中的关键来源。我们还建立了原代肝内皮细胞培养模型，并证明了Bmp6在肝内皮细胞中的转录受细胞内铁含量的控制。最后，我们将该细胞培养模型与定量蛋白质组学和RNA-seq筛选相结合，以确定铁转运体和转录因子途径，我们假设这些途径在调节铁响应Bmp6的产生以控制海普西丁的表达和系统铁的动态平衡方面发挥关键作用。在具体目标I中，我们将使用原代肝内皮细胞培养和遗传小鼠模型来建立特定的铁转运蛋白在控制肝内皮细胞铁含量中的作用以及铁调控途径来调控Bmp6的表达。在特定的目标II中，我们将使用原代肝内皮细胞培养、染色质免疫沉淀、报告分析、药理学方法和内皮条件基因敲除小鼠模型来确定候选转录因子途径在控制Bmp6对铁的反应中的作用及其激活机制。该项目的长期目标是了解BMP信号通路如何被不同的信号调节以调节海普西丁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学和病理生理学，并最终开发治疗铁代谢障碍的新的治疗策略。

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.