The genetics of essential metal homeostasis during development

Abstract

The essential metals copper, zinc, and iron play key roles in embryonic, fetal, and postnatal development in higher eukaryotes. Recent advances in our understanding of the molecules involved in the intricate control of the homeostasis of these metals and the availability of natural mutations and targeted mutations in many of the genes involved have allowed for elucidation of the diverse roles of these metals during development. Evidence suggests that the ability of the embryo to control the homeostasis of these metals becomes essential at the blastocyst stage and during early morphogenesis. However, these metals play unique roles throughout development and exert pleiotropic, metal-specific, and often cell-specific effects on morphogenesis, growth, and differentiation. Herein, we briefly review the major players known to be involved in the homeostasis of each of these essential metals and their known roles in development.

FIG. 1

Overview of copper homeostasis. Copper is oxidized (Cu²⁺) in the intestinal lumen and in the serum, but is reduced to Cu¹⁺ before transport into cells. The Steap ferric/cupric reductases (Steaps 2, 3, and 4) are localized to endosomes/lysosomes and the plasma membrane and may be essential for the reduction of copper, as well as the reduction of iron (Ohgami et al., 2006). Copper is transported into ENTEROCYTE/ENDODERM cells by unknown mechanisms (endocytosis?). CTR1 is essential for the acquisition of dietary copper but its function in the enterocyte is unknown. In many other cell types (GENERIC CELL), copper is taken up by CTR1 localized to the plasma membrane and perhaps also by CTR2 (Slc31a2). Copper taken up by ENTEROCYTE/ENDODERM is exported into portal blood/conceptus, respectively, by ATP7A (Menkes disease protein, MNK) which is localized to vesicles trafficking toward the basolateral membrane and to the basolateral membrane. Copper exported to portal blood is taken up into the liver, the primary organ that regulates copper homeostasis. In the HEPATOCYTE, ATP7B (Wilson’s disease protein, WND) effluxes excess copper into the bile and puts copper into the trans-Golgi network (TGN) where it is loaded into ceruloplasmin, a ferroxidase that is the primary copper binding protein in serum. Inside the cell (GENERIC CELL), copper is distributed to cytoplasmic copper chaperones (COX17/19, ATOX1, CCS) which, in turn, deliver copper to mitochondrial inner membrane and ultimately cytochrome C oxidase (CCO) or to ATP7A in the TGN, and cytoplasmic SOD1, respectively. The assembly of copper into CCO is an active area of investigation. It is thought that copper in COX17/19 (and probably COX23/MTCP1) is first transferred to both COX11 and SCO1/2 and ultimately to CCO. ATP7A transports copper into the TGN and activates copper containing secretory and membrane-bound proteins [lysyl oxidase, tyrosinase, peptidylglycine α-amidating monooxygenase (PAM)]. It should be noted that the cellular localization and abundance of CTR1, ATP7A, and ATP7B are dynamically regulated by copper availability which is not reflected in this static cartoon.

FIG. 2

Overview of zinc homeostasis. Twenty four different genes encode proteins that may be involved in the uptake (Zip genes) or efflux (ZnT genes) of this metal in a cell-specific, developmentally regulated, and zinc-regulated manner. The functions of many of these genes remain to be determined. Therefore, this cartoon provides only a superficial and static view of zinc homeostasis. ZIP4 plays a critical role in the absorption of dietary zinc by ENTEROCYTE/ENDODERM cells when zinc is limiting, but other transporters must also play important roles. Zinc is thought to be exported into portal blood or into the conceptus by ZnT1 localized on the basolateral membrane. Other ZnT proteins (i.e. ZnT4) also likely play a role in this process. ZIP5 is localized to the basolateral membranes of enterocytes, endoderm cells and pancreatic acinar cells where it may serve to remove zinc from the blood when zinc is replete. In peripheral tissues (GENERIC CELL), zinc is probably taken up by various ZIP transporters localized on the plasma membrane. To date ZIPs1, 2, 3, 6, 8, 10, and 14 have each been shown to have zinc transport activity in transfection or oocyte injection studies, and most show tissue-specific patterns of expression. Inside the cell, free zinc levels are kept low and zinc can be bound to MT or transported into secretory vesicles, endosomes/lysosomes or zincosomes by ZnT2 and ZnT4. Zinc activates the zinc-sensing transcription factor MTF-1 which regulates transcription of the mouse Mt-I/II and Znt1 genes and represses expression of Zip10 in an effort to control excess zinc. ZnTs2–7 participate in the delivery of zinc into the secretory pathway, whereas ZIP7 may transport zinc out of the Golgi apparatus into the cytoplasm. ZnT3 transports zinc into glutamate containing vesicles in the brain whereas ZnT8 transports zinc into pancreatic β-cell insulin secretory granules. ZIP4 is expressed in β-cells, but its localization in those cells has not been reported.

FIG. 3

Overview of iron homeostasis. Iron is oxidized (Fe³⁺) in the intestinal lumen and in the serum, but is reduced (Fe²⁺) before transport into cells. Iron (Fe²⁺) is transported into ENTEROCYTES by DMT1 after extracellular reduction perhaps by DCytb. Iron uptake by visceral ENDODERM cells is apparently not dependent on DMT1 and could require the transferrin uptake system. Iron is transported out of cells by Ferroportin and is thought to be subsequently oxidized (Fe³⁺) by membrane bound Hephaestin and Ceruloplasmin. Iron, inflammation and hypoxia regulate Hepcidin, a polypeptide hormone made by the HEPATOCYTE. Hepcidin binds to Ferroportin resulting in degradation of the transporter. In the serum, Fe³⁺ binds to Transferrin. Developing ERYTHROBLASTS (heme is the major pool of iron) and many other cell types take up iron via the transferrin-mediated endocytic pathway (see GENERIC CELL). After binding to the transferrin receptor, the iron–transferrin complex is internalized and iron is released from transferrin in the acidified endosome, reduced by a Steap metalloreductase and then transported into the cytoplasm by DMT1. Apo-transferrin and the transferrin receptor are returned to circulation and plasma membrane, respectively, via the recycling endosome pathway. All nonerythroid cells store iron complexed with the ferritin heavy and light chains (Ferritin) which can store up to 4,000 atoms of iron per molecule. Cytoplasmic iron is transported into mitochondria by mitoferrin where it is loaded into Fe-S clusters in a process that requires Frataxin. A substrate essential for Fe-S assembly in the cytoplasm is transported out of the mitochondria by the ATP-binding cassette transporter ABCB7 (see GENERIC CELL). In the erythroblast, mitochondrial iron is primarily utilized for heme synthesis. A major fraction of iron is recycled by the reticulo-endothelial MACROPHAGE which phagocytoses aged red blood cells (RBC). The iron is solubilized in the lysosomes, reduced by a Steap metalloreductase and exported out of the lysosome by NRAMP1. Iron is released from the macrophage by ferroportin, subsequently reoxidized by Ceruloplasmin, and conveyed again via transferrin through the systemic circulation.