Novel role of endoplasmic reticulum-associated degradation in iron metabolism

我的实验室感兴趣的是内质网(ER)相关降解(ERAD)的(病理)生理意义，ERAD是清除错误折叠的ER蛋白以用于胞浆蛋白酶体降解的主要ER质量控制机制。Sel1L-Hrd1蛋白复合体代表了从酵母到人类在进化上最保守的ERAD机制。在过去的几年里，我们和其他人从细胞类型和底物特异性的角度报道了Sel1L-Hrd1 ERAD在健康和疾病中的生理意义；然而，我们对其生理作用的了解仍然有限。在这项应用的初步数据中，我们进行了无偏倚的蛋白质组学筛选，从而确定铜蓝蛋白(CP)蛋白是肝脏中调节铁稳态的铁氧基酶，作为ERAD底物。我们进一步表明，野生型和疾病突变体CP都容易错误折叠，并被Sel1L-Hrd1 ERAD泛素化和降解。此外，肝细胞特异性Sel1L缺陷小鼠循环中CP活性升高，并对缺铁诱导的低色素性小细胞性贫血具有抵抗力。这些数据表明，肝细胞Sel1L-Hrd1 ERAD在CP的生物发生和全身铁稳态中起着关键作用。这些发现是令人兴奋的，因为CP是铁稳态的重要调节因素，而且因为CP错义突变在人类中会导致一种称为无浆蛋白血症的临床疾病，其特征是铁在器官中异常堆积。然而，内质网中新生CP的生物发生仍未被探索。因此，这一应用的主要假设是，肝细胞中的Sel1L-Hrd1 ERAD通过分别在生理和病理条件下调节野生型和疾病突变型CP蛋白的周转来控制全身铁稳态。我们将完成以下三个目标：(1)确定Sel1L-Hrd1 ERAD在铁代谢中的生理和病理意义；(2)阐明ERAD调控CP生物发生的分子机制；(3)阐明ERAD在无浆蛋白血症发病机制中的病理意义。这些研究的完成不仅将阐明ERAD介导的铁代谢调控的意义和分子机制，而且将为基础和病理条件下铁代谢的调控提供新的见解。与人类健康相关：铁稳态失调影响着全球数百万人，这会导致缺铁性贫血，并在超负荷时增加糖尿病、肝脏和肾脏疾病的风险。结合平行的生理和生化研究，这一应用将在ERAD和铁代谢之间建立直接联系，揭示ERAD和错误折叠相关的蛋白毒性应激的新机制，并促进我们对与蛋白质折叠缺陷相关的疾病发病机制的总体理解。

My laboratory is interested in the (patho-)physiological importance of endoplasmic reticulum (ER)-associated degradation (ERAD), a principal ER quality-control machinery to clear misfolded ER proteins for cytosolic proteasomal degradation. The Sel1L-Hrd1 protein complex represents the most evolutionarily conserved ERAD machinery from yeast to humans. In the past several years, we and others have reported the physiological significance of Sel1L-Hrd1 ERAD in health and disease in a cell-type and substrate-specific manner; however, our understanding of its physiological role remains limited. In the preliminary data of this application, we performed an unbiased proteomics screen that led to the identification of ceruloplasmin (Cp) protein, a ferroxidase regulating iron homeostasis, as an ERAD substrate in the liver. We further showed that both wildtype and a disease mutant Cp are misfolding-prone and are ubiquitinated and degraded by Sel1L- Hrd1 ERAD. Moreover, hepatocyte-specific Sel1L-deficient mice exhibit elevated Cp activity in the circulation and are resistant to iron deficiency-induced hypochromic microcytic anemia. These data point to a critical role of hepatocyte Sel1L-Hrd1 ERAD in Cp biogenesis and systemic iron homeostasis. These findings are exciting because Cp is an essential regulator in iron homeostasis and because Cp missense mutations in humans cause a clinical condition known as aceruloplasminemia, characterized by abnormal iron accumulation in organs. However, the biogenesis of nascent Cp in the ER remains unexplored. Hence, the overarching hypothesis of this application is that Sel1L-Hrd1 ERAD in hepatocytes controls systemic iron homeostasis by regulating the turnover of both wildtype and disease mutant Cp proteins under physiological and pathological conditions, respectively. We will accomplish the following three Aims: (1) Determine the physiological and pathological significance of Sel1L-Hrd1 ERAD in iron metabolism; (2) Delineate the molecular mechanism underlying Cp biogenesis regulated by ERAD; and (3) Delineate the pathological importance of ERAD in the pathogenesis of aceruloplasminemia. Completion of these studies will not only delineate the significance and molecular mechanism underlying ERAD-mediated regulation of iron metabolism, but also provide novel insights into how iron metabolism is regulated under basal and pathological conditions. Relevance to human health: Disorders of iron homeostasis affect millions of individuals worldwide, which cause anemia in deficiency and increase the risk of diabetes, liver and kidney diseases upon overload. This application, with parallel physiological and biochemical studies, will establish a direct link between ERAD and iron metabolism, uncover novel mechanisms underlying ERAD and misfolding-associated proteotoxic stress, and advance our understanding of disease pathogenesis associated with protein folding defects in general.