The long-term objective of our studies is to define the cellular and molecular determinants of heme homeostasis in human nutrition. Iron deficiency is the most common nutritional disorder. Dietary heme (iron-protoporphyrin IX) is a significant source of bioavailable iron, but the genes and pathways responsible for heme transport and utilization in human enterocytes remain elusive. In humans, greater than 60% of total body iron is present as heme in hemoglobin. Iron from heme is recycled by phagocytosis of senescent red blood cells and proteolytic digestion of hemoglobin in macrophages. Similar to enterocytes, the pathways for heme transport and utilization across the phagolysosomal membrane in macrophages are unknown. Since heme is a hydrophobic, cytotoxic macrocycle, we assert that heme does not passively diffuse through membranes but is actively transported via specific intra- and inter-cellular pathways that comprise heme uptake, trafficking, and sequestration. We have demonstrated that the roundworm Caenorhabditis elegans is an excellent animal model to identify heme transport pathways because it synthesizes a large number of hemoproteins with human homologs but does not synthesize heme de novo. Worms require dietary heme for growth and reproduction. Thus, the worm model provides a clean genetic background devoid of endogenous heme and the ability to externally manipulate the metabolic flux of intracellular heme. Utilizing C. elegans as a genetic animal model of heme auxotrophy, we identified HRG-1, the first eukaryotic heme importer/ transporter (Nature 2008). HRG-1 is a permease that is conserved in humans and binds and transports heme. The worm and human HRG-1 proteins co-localize to the endo-lysosomal compartment. Knockdown of hrg-1 in zebrafish causes hydrocephalus, yolk tube defects, and anemia - phenotypes that are fully rescued by worm HRG-1. These studies established a conserved model for cellular heme transport and validated C. elegans as a bona fide model for the identification of heme homeostasis pathways. The studies in this proposal are designed to elucidate the precise mechanisms of heme transport by HRG-1 at the molecular, cellular, and organismal levels. We seek to test the hypothesis that heme transport mediated by HRG-1 is an integral component of the heme homeostasis regulatory network in animals. We will utilize a structure-function approach to identify the functional elements of HRG-1 responsible for heme transport, a cell biological approach to establish HRG-1 as the vesicular transporter for heme-iron recycling, and a systems approach to identify the complex regulatory circuit which integrates HRG-1 with organismal heme trafficking. Our goal is to obtain a comprehensive understanding of the pathways which mediate heme homeostasis in mammals that have, heretofore, remained poorly understood.
Iron deficiency is the world's number one nutritional disorder, and heme is the most bioavailable form of iron for human consumption. Identification of how heme is transported for the synthesis of proteins containing iron and heme will permit the design of synthetic heme-based "nutraceuticals" specifically targeted to iron-deficient individuals including pregnant mothers and infants. Identifying the molecular basis of heme-iron recycling to produce hemoglobin will lead to new strategies for the synthesis of artificial blood.
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