Novel role of endoplasmic reticulum-associated degradation in iron metabolism

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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.