Microcytic anemia, erythropoietic protoporphyria, and neurodegeneration in mice with targeted deletion of iron-regulatory protein 2

Abstract

Iron-regulatory proteins (IRPs) 1 and 2 posttranscriptionally regulate expression of transferrin receptor (TfR), ferritin, and other iron metabolism proteins. Mice with targeted deletion of IRP2 overexpress ferritin and express abnormally low TfR levels in multiple tissues. Despite this misregulation, there are no apparent pathologic consequences in tissues such as the liver and kidney. However, in the central nervous system, evidence of abnormal iron metabolism in IRP2 ؊/؊ mice precedes the development of adult-onset progressive neurodegeneration, characterized by widespread axonal degeneration and neuronal loss. Here, we report that ablation of IRP2 results in ironlimited erythropoiesis. TfR expression in erythroid precursors of IRP2 ؊/؊ mice is reduced, and bone marrow iron stores are absent, even though transferrin saturation levels are normal. Marked overexpression of 5-aminolevulinic acid synthase 2 (Alas2) results from loss of IRPdependent translational repression, and markedly increased levels of free protoporphyrin IX and zinc protoporphyrin are generated in IRP2 ؊/؊ erythroid cells. IRP2 IntroductionIron functions as an indispensable cofactor for numerous enzymes and proteins in mammals, and regulation of iron uptake and distribution within animals is accordingly highly regulated. 1,2 Intestinal iron absorption and tissue iron storage are optimized to deliver the iron needed for numerous metabolic processes, including heme synthesis. The recently identified peptide hormone, hepcidin, is responsible for appropriately coordinating intestinal iron uptake and macrophage iron release to meet the needs of the organism and to maintain normal serum transferrin saturation levels. 3 In most tissues, the circulating pool of diferric transferrin serves as the major source of iron for individual cells. When diferric transferrin (Tf) binds to transferrin receptors (TfRs), the Tf-TfR complex internalizes in endosomes, where acidification facilitates release of free iron, 4 and the membrane iron transporter divalent metal transporter 1 (DMT1) (SLC11A2) transports iron into the cytosol. 5,6 In the cytosol, iron is incorporated into iron proteins or transported to cellular organelles, and excess cytosolic iron is sequestered and stored by ferritin. 6,7 Cells regulate expression of ferritin and TfR to optimize cytosolic iron levels. When cells are iron depleted, they increase TfR expression and uptake of transferrinbound iron, while they simultaneously decrease expression of ferritin and iron sequestration. Proteins known as iron regulatory proteins (IRPs) coordinately regulate expression of TfR, ferritin, and numerous other iron metabolism proteins. IRP1 and IRP2 are homologous genes that monitor cytosolic iron levels. When cells are iron depleted, IRPs bind to RNA motifs known as iron-responsive elements (IREs) within transcripts that encode iron metabolism proteins (reviewed in Rouault and Klausner 1 ; and Hentze et al 2 ). IREs are found in numerous transcripts, including ferritin H-and L-chains, TfR1, 7 erythrocyti...

Renal Iron Metabolism

“…Animal studies have revealed that IRP2 has a very significant role in regulation of tissue iron metabolism, and complete loss of IRP2 results in microcytic anemia, elevated serum ferritin, and late-onset neurodegeneration (34 -36). The anemia results from decreased TfR expression in erythroid precursor cells (34). Interstitial fibroblasts in the kidney respond to hypoxia that results from decreased red cell oxygen-carrying capacity by secreting erythropoietin (37,38), and erythropoietin levels increase three-to fivefold in animals that lack IRP2, as expected in response to anemia (34).…”

Multiple organ toxicity, including hypochromic anemia, following repeated dose oral administration of phenobarbital (PB) in rats

“…CHr is, therefore, a useful early indicator of Feand TIBC and decrease in TSAT were considered to be --eral mutant or gene-targeted mice, which showed severe anemia in spite of a high or normal level of serum Fe. For -in show microcytic, hypochromic anemia through heme et al, 2007) or reduced capacity to effectively use transferrin-bound Fe (Wilkins et al, 2006); precursors has been reported in the mutant mice named nm1054, in which the CHr level is low despite high levels of serum Fe, TIBC, and TSTA (Ohgami et al, 2005); 3) a reduction in transferrin receptor in multiple tissues has been reported in mice with targeted deletion of ironregulatory protein 2 (IRF2), which binds to iron-responsive elements within transcripts encoding Fe metabolism proteins, including transferrin receptor (Cooperman et al, 2005). Considering evidence of characteristic anemia in our case and in these literatures, further studies are required to determine whether PB impairs hemoglobin synthesis, Fe utility, or Fe intake in the erythroid lineage.…”

Human iron regulatory protein 2 is easily cleaved in its specific domain: consequences for the haem binding properties of the protein

“…IRP2 −/− mice develop microcytic anaemia [6][7][8], whereas IRP1 −/− mice are asymptomatic. Inactivation of both IRP1 and IRP2 is lethal at an early stage during embryonic development [9].…”