BMP Ligands in Hepcidin Regulation

铁是一种必需的营养素，但过量的铁是有毒的。全身性铁稳态的主要调节剂是激素铁调素，其由肝脏分泌并诱导铁输出体膜铁转运蛋白的降解以抑制来自饮食的铁吸收和来自身体储存的铁释放。铁调素的产生受铁、红细胞生成驱动和炎症的调节，以提供足够的铁用于红细胞生成和其他基本功能，但限制铁过量的毒性。我们已经发现，骨形态发生蛋白（BMP）信号传导途径，通过配体BMP 6和BMP 2，是铁调素转录响应于大多数已知的信号的中央调节器。BMP配体是最初作为骨诱导因子发现的二聚体蛋白，但现在已知在许多器官中从胚胎发生到成体组织稳态的许多生物过程中发挥关键作用。BMP被制成由前结构域和通过蛋白水解切割释放的配体结构域组成的无活性前体。尽管前结构域缺乏信号传导活性，但相关BMP/TGF-β家族成员越来越认识到前结构域在配体折叠和二聚化中起关键作用，并且还可调节配体/受体相互作用。值得注意的是，BMP 6前结构域突变与人类铁调素调节和铁过载的改变有关。我们将展示最近发表的数据，BMP 2和BMP 6功能在铁调素和铁稳态调节;异二聚体BMP 4/7和BMP 2/7，而不是同源二聚体配体，是哺乳动物胚胎发生的主要介质; BMP前结构域在BMP 4/7异二聚体和同源二聚体的形成和功能中发挥重要作用。因此，我们假设BMP 2/6异二聚体是铁调素和铁稳态调节的关键功能配体，并且前结构域在BMP 2/6成熟和功能中具有关键作用。在目的I中，我们将使用我们的非洲爪蟾系统、原代肝细胞、小鼠模型和新的ELISA测定来测试BMP 2/6异二聚体是否存在并在体内受铁调节，并阐明BMP 6和BMP 2是如何蛋白水解加工的，它们的前结构域如何有助于异二聚体和/或同二聚体的形成以及调节铁调素的功能，以及BMP 6前结构域突变如何影响这些过程导致铁过载。在目的II中，我们将显示初步数据，铁和红细胞生成驱动器仍然调节hepcidin在BMP 6和/或BMP 2的情况下，我们将确定其他两个BMP配体参与hepcidin调节。我们将使用遗传小鼠模型来建立这两种BMP配体在体内铁调素和铁稳态调节中的功能作用。该项目的长期目标是了解BMP信号传导如何调节以控制铁调素表达和系统性铁稳态;这一过程如何在铁紊乱中受到干扰，如遗传性血色素沉着症，炎症性贫血和b-地中海贫血;并最终为这些铁紊乱开发新的治疗方法。我们还旨在获得BMP配体成熟和前结构域功能的基本见解，这将与BMP信号传导重要的许多其他领域相关。

Iron is an essential nutrient, but excess iron is toxic. The master regulator of systemic iron homeostasis is the hormone hepcidin, which is secreted by the liver and induces degradation of the iron exporter ferroportin to inhibit iron absorption from the diet and iron release form body stores. Hepcidin production is regulated by iron, erythropoietic drive, and inflammation to provide adequate iron for erythropoiesis and other essential functions, but to limit the toxicity of iron excess. We have discovered that the bone morphogenetic protein (BMP) signaling pathway, via the ligands BMP6 and BMP2, is a central regulator of hepcidin transcription in response to most known signals. BMP ligands are dimeric proteins that were initially discovered as bone inducing factors, but are now known to play critical roles in many biologic processes from embryogenesis to adult tissue homeostasis in many organs. BMPs are made as inactive precursors comprised of a prodomain and a ligand domain that is released by proteolytic cleavage. Although prodomains lack signaling activity, there is increasing recognition from related BMP/TGF-b family members that prodomains play critical roles in ligand folding and dimerization, and may also regulate ligand/receptor interactions. Notably, BMP6 prodomain mutations have been linked to altered hepcidin regulation and iron overload in humans. We will show recently published data that BMP2 and BMP6 function together in hepcidin and iron homeostasis regulation; heterodimeric BMP4/7 and BMP2/7, rather than homodimeric ligands, are the major mediators mammalian embryogenesis; and BMP prodomains play essential roles in BMP4/7 heterodimer and homodimer formation and function. We therefore hypothesize that BMP2/6 heterodimers are a key functional ligand for hepcidin and iron homeostasis regulation and that prodomains have critical roles in BMP2/6 maturation and function. In Aim I, we will use our Xenopus system, primary liver cells, mouse models, and novel ELISA assay to test whether BMP2/6 heterodimers are present and regulated by iron in vivo, and to elucidate how BMP6 and BMP2 are proteolytically processed, how their prodomains contribute to heterodimer and/or homodimer formation and function to regulate hepcidin, and how BMP6 prodomain mutations impact these processes to cause iron overload. In Aim II, we will show preliminary data that both iron and erythropoietic drive still regulate hepcidin in the absence of BMP6 and/or BMP2, and we will identify two other BMP ligands that participate in hepcidin regulation. We will use genetic mouse models to establish the functional role of these two BMP ligands in hepcidin and iron homeostasis regulation in vivo. The long-term goals of this project are to understand how BMP signaling is regulated to control hepcidin expression and systemic iron homeostasis; how this process is perturbed in iron disorders such as hereditary hemochromatosis, anemia of inflammation, and b-thalassemia; and ultimately to develop new treatments for these iron disorders. We also aim to gain fundamental insights into BMP ligand maturation and prodomain function that will be relevant for many other fields where BMP signaling is important.